States of Matter and Changes of State (Physics 11)

6.4 States of Matter and Changes of State Glaciers are huge masses of ice and snow that once covered most of Earth’s surface. Many of the world’s glaciers a...

6.4 States of Matter and Changes of State Glaciers are huge masses of ice and snow that once covered most of Earth’s surface. Many of the world’s glaciers are retreating at an alarming rate. This means that they are getting smaller. The Athabasca glacier near Banff, Alberta, has retreated over 1.5 km in the last 120 years (Figure 1(a)). Scientists know that global warming is responsible for the retreat of the glaciers. Warm temperatures cause the ice and snow of a glacier to melt into liquid water (Figure 1(b)). The liquid water flows away from the glacier, forming rivers, lakes, and streams that make their way to the oceans. If (a) the melted ice is not replaced by new ice, the glacier will retreat. States of Matter Water, and all other forms of matter, can exist in three different physical states, or phases: solid, liquid, and gas. The kinetic molecular theory described in Section 6.1 explains the differences between these physical states. In a solid, strong forces of attraction (bonds) hold the particles in fixed positions. The particles of a solid vibrate, (b) but they cannot easily slide past each other or move from place to place. This gives solids their rigidity and allows them to maintain their shape. The particles of a liquid Figure 1 (a) The blue sign shows how are also attracted to each other. However, in liquids, the particles have more kinetic far the Athabasca glacier has retreated since 1925. (b) Glacier ice melting into energy than the particles of a solid. This causes the liquid’s particles to vibrate more liquid water than the particles of a solid, and also to slide past each other and move from place to place. This gives liquids the ability to flow and pour. Like solids and liquids, the par- ticles of a gas are attracted to each other. However, gas particles have much more kin- etic energy than the particles of solids and liquids. The particles of a gas vibrate more vigorously than the particles of solids and liquids, and they move large distances past each other. This gives gases their ability to flow and to fill expandable containers like balloons and tires with great pressure. Changes of State When solids, liquids, or gases absorb or release enough thermal energy, they may change state (Figure 2). For example, a solid can change into a liquid and a liquid can change into a gas. When a substance absorbs thermal energy, the particles of the substance begin to move faster and farther apart. Note that the thermal energy is not really “absorbed”; it is transformed into kinetic energy and potential energy of the substance’s particles. Remember that energy is always conserved. It can be trans- formed from one form into another, but it cannot be destroyed. absorb thermal energy solid liquid gas release thermal energy Figure 2 A change of state requires a change in the thermal energy of the substance. 288   Chapter 6 Thermal Energy and Society NEL  Let us consider what happens to a sample of ice initially at 210 °C when it is continuously heated. When the ice is placed on the hot plate, its initial temperature is 210 °C (Figure 3(a)). As the ice absorbs thermal energy, its particles begin to vibrate more vigorously. This warms up the ice and increases its temperature. As the par- ticles absorb more thermal energy, the forces of attraction are not strong enough to hold the particles in fixed positions. Eventually, the solid reaches its melting point, where the particles begin to slide past each other and move from place to place. At this point, the solid begins to change into a liquid (Figure 3(b)). The melting point of water is 0 °C. This change of state is called melting, or fusion. Eventually, all of fusion the process by which a solid the ice becomes liquid water (Figure 3(c)). Notice that thermal energy continues changes to a liquid to be absorbed during the melting process, but the temperature does not change—it remains at 0 °C until the last ice crystal has melted into liquid water. 10 °C 0 °C 0 °C 100 °C 100 °C (a) (b) (c) (d) (e) Figure 3 (a) Ice warms up. (b) As the ice reaches its melting point of 0 °C, it begins to change into a liquid because of the absorption of thermal energy. (c) As the ice–liquid water mixture is continuously heated, it continues to absorb thermal energy but stays at a temperature of 0 °C until the ice has completely melted. (d) The liquid water continues to absorb thermal energy until its temperature reaches its boiling point of 100 °C. At this point, enough thermal energy has been absorbed for it to begin to change into a gas, in this case water vapour. (e) The temperature remains at 100 °C until all of the liquid has changed to water vapour. As the liquid water continues to absorb thermal energy, its particles move faster and farther apart. This warms up the liquid and increases its temperature. As the speed of the particles increases, the forces of attraction become weaker, and it becomes more difficult for the particles to stay together. Eventually, the liquid reaches its boiling point, at which the particles have enough kinetic energy to completely break away from each other. At the boiling point, the liquid water changes into a gas called water vapour (Figure 3(d)). The boiling point of water is 100 °C. This change of state is called evaporation or vaporization. Eventually, all of the liquid evaporates into a gas (Figure 3(e)). Notice that thermal energy continues to be absorbed during the boiling process, but the temperature does not change—it remains at 100 °C until the last drop of water has changed into water vapour. Changes of state occur when- ever any material is heated from a solid to a liquid to a gas. The only difference is that every material has a different melting point and a different boiling point. Removal of thermal energy reverses this process. In this case, a gas cools down into a liquid and a liquid cools down into a solid. The change of a gas into a liquid is called condensation, and the change of a liquid into a solid is called freezing. In some cases, it is possible for a substance to go directly from a solid to a gas, or vice versa, without ever becoming Ontario a liquid. This special change of state is called sublimation. An example Physics 11 SB of sublimation occurs when ice cubes are kept in a freezer for a long period of time. 0-17-650433-8 The ice cubes become smaller and smaller as they sublimate into water vapour. FN C06-F012a-OP11USB CO Allan Moon Pass nel 3rd pass 6.4 States of Matter and changes of State 289 Approved  Heating and Cooling Graphs The changes in temperature that occur when a substance absorbs thermal energy heating graph a graph that shows the can be shown in a graph called a heating graph. Similarly, the changes in temperature temperature changes that occur while that occur when a substance releases thermal energy can be shown in a graph called thermal energy is absorbed by a substance a cooling graph. In both graphs, the vertical axis (y-axis) represents temperature and cooling graph a graph that shows the the horizontal axis (x-axis) represents the amount of thermal energy absorbed or temperature changes that occur while released. Figure 4(a) is the heating graph for water, and Figure 4(b) is the cooling thermal energy is being removed from a graph for water. An artist’s illustration of the state of the water particles is shown substance above each graph. Heating Graph for Water Cooling Graph for Water 110 110 100 100 water liquid water water water 90 vapour 90 vapour vapour boiling 80 warming 80 cooling condensing liquid water liquid water up down 70 into liquid Temperature °C 70 Temperature °C warming up cooling down water 60 60 50 50 40 40 30 30 liquid 20 20 water ice ice ice freezing 10 warming melting 10 into ice cooling up 0 down 0 –10 –10 (a) Thermal energy absorbed (b) Thermal energy released Figure 4 (a) A heating graph and (b) a cooling graph for water. The angled parts on each graph indicate a change in temperature and occur when only one state is present. The flat parts on each graph occur when more than one state is present. The flat parts indicate a constant temperature because one state is changing into another. Notice the following key aspects of each graph: On the heating graph, thermal energy is being absorbed by the water molecules throughout the heating process. On the cooling graph, thermal energy is being released by the water molecules throughout the cooling process. Temperature changes occur only when one state is present. These changes are represented by the angled parts of each graph. The temperature remains constant during a change of state because thermal energy is being used to change the potential energy of the substance’s particles, Ontario Physics not 11 U their kinetic energy. On the heating graph, this occurs when a solid is 0176504338 changing into a liquid and when a liquid is changing into a gas. On the cooling 3a-OP11USB FN graph, this occurs when a gas is changing into a liquid and when a liquid is C06-F13b-OP11USB Art Group CO changing intoGroup CrowleArt a solid. This is represented by the flat parts of both graphs. h Crowle The heating Crowle Deborah graph shows a distinct melting point and boiling point. The cooling graph shows a distinct condensation point and freezing point. The melting and s Pass 3rd pass Investigation 6.4.1 freezing points are both 0 °C, and the boiling and condensation points are both Approved 100 °C. In general, melting and freezing occur at the same temperature, and Heating Graph of Water (p. 305) Not Approved boiling and condensation occur at the same temperature. In this activity, you will observe the temperature change as ice changes You may believe that the temperature of a substance should change while it is into water and create your own absorbing or releasing thermal energy because thermal energy affects the kinetic heating graph of water. energy of particles. But why does the temperature not change during melting, freezing, boiling, or condensation? 290   Chapter 6 Thermal Energy and Society NEL  During melting and boiling, thermal energy must be absorbed by the particles of a substance. This absorbed energy is needed to break the bonds that hold the particles together. However, during condensation and freezing, thermal energy must be released by the particles of a substance. The released energy now allows the particles to move closer together and become more organized. In each of these situations, the change in thermal energy of the substance results in a change in potential energy of the particles. In other words, when a substance melts, boils, condenses, or freezes, the absorbed or released thermal energy is being trans- formed into potential energy, rather than kinetic energy. Since the kinetic energy of the particles does not change, the temperature of the substance remains constant during a change of state. Latent Heat As you have learned, when a substance absorbs or releases thermal energy during latent heat (Q) the total thermal energy a change of state, its temperature remains constant. This absorbed or released absorbed or released when a substance thermal energy during a change of state is called the latent heat (Q) of the substance. changes state; measured in joules The word “latent” means “hidden” because there is no measurable change in tem- perature. This thermal energy remains “hidden” until the opposite change of state latent heat of fusion the amount of occurs. For example, the thermal energy absorbed when ice melts into liquid water thermal energy required to change a solid remains in the liquid water until it is released when the liquid water freezes back into a liquid or a liquid into a solid into ice. Latent heat is measured in joules. Every substance has a latent heat of fusion and a latent heat of vaporization. The latent heat of fusion is the amount of latent heat of vaporization the amount thermal energy absorbed when a substance melts or released when it freezes. Note of thermal energy required to change a that these energy values are the same for a particular substance because the amount liquid into a gas or a gas into a liquid of energy absorbed when a solid melts into a liquid is the same as the amount of energy released when that liquid freezes back into a solid. We use the term “latent specific latent heat (L) the amount heat of fusion” for both values. of thermal energy required for 1 kg The latent heat of vaporization is the amount of thermal energy absorbed when a of a substance to change from one state substance evaporates or released when it condenses. As with latent heat of fusion, the into another; measured in joules per energy values required to cause a substance to evaporate or condense are the same, kilogram (J/kg) and we use the term “latent heat of vaporization” for both values. specific latent heat of fusion (Lf) the The specific latent heat (L) of a substance is the amount of thermal energy required amount of thermal energy required to melt for 1 kg of a substance to change from one state into another. Every substance has or freeze 1 kg of a substance; measured in a different specific latent heat because every substance is composed of different par- joules per kilogram (J/kg) ticles (atoms or molecules). The specific latent heat of fusion (Lf) is the thermal energy specific latent heat of vaporization (Lv) required for 1 kg of a substance to melt or freeze (Table 1). The specific latent heat of the amount of thermal energy required vaporization (Lv) is the thermal energy required for 1 kg of a substance to boil or con- to evaporate or condense 1 kg of a dense (Table 1). The SI unit for specific latent heats is joules per kilogram (J/kg). substance; measured in joules per Table 1 Specific Latent Heats for Various Substances kilogram (J/kg) Specific latent Specific latent heat heat of fusion (Lf) Melting of vaporization (Lv) Boiling Substance (J/kg) point (°C) (J/kg) point (°C) aluminum 6.6 3 105 2519 4.0 3 105 10 900 ethyl alcohol 1.1 3 105 2114 8.6 3 105 78.3 carbon dioxide 1.8 3 105 278 5.7 3 105 257 gold 1.1 3 106 1064 6.4 3 104 2 856 lead 2.5 3 104 327.5 8.7 3 105 1 750 water 3.4 3 105 0 2.3 3 106 100 NEL 6.4 States of Matter and Changes of State    291  Learning Tip To calculate the latent heat (Q) involved during a change of state, you can use the equations Remember Units Q 5 mLf (for substances that are melting or freezing) As with all questions in physics, make sure the units in your calculations Q 5 mLv (for substances that are boiling or condensing) of latent heat match. Mass must be where m is the mass of the substance, Lf is the specific latent heat of fusion, and Lv is expressed in kilograms, and latent heat is expressed in joules since the the specific latent heat of vaporization. units for specific latent heat are joules In the following Tutorial, you will use the specific latent heat of fusion (Lf ) and per kilogram. specific latent heat of vaporization (Lv) to calculate the latent heat (Q), or total amount of thermal energy absorbed or released when a substance changes state. In some of the Sample Problems, you will also determine the amount of thermal energy Investigation 6.4.2 absorbed or released when a substance warms up or cools down but does not change state. In those cases, you will use the quantity of heat equation (Q 5 mcDT) that you Specific Latent Heat of Fusion for Ice (p. 306) learned about in Section 6.3. Note that Q represents both latent heat and quantity of In this investigation, you will use the heat. The reason is that both are measures of the amount of thermal energy absorbed equations Q 5 mcDT and Q 5 mLf or released. The only difference is that latent heat (Q) relates to a substance changing to determine the latent heat of fusion state (temperature remains constant), whereas the quantity of heat (Q) relates to a for melting ice. substance in a particular state (solid or liquid or gas) warming up or cooling down (temperature changes). Tutorial 1 Calculating Latent Heat of Fusion or Vaporization Remember that the latent heat equations Q 5 mLv and Q 5 mLf are used to calculate the amount of thermal energy required for a change of state to occur, whereas the quantity of heat equation Q 5 mcDT is used to calculate the amount of thermal energy absorbed or released when a solid, liquid, or gas warms up or cools down. Sample Problem 1 How much thermal energy is released by 652 g of molten lead when it changes into a solid? Since lead is changing from a liquid to a solid, a change of state is occurring. So, we should use the latent heat equation Q 5 mLf to solve this problem. Given: m 5 652 g 5 0.652 kg; Lf 5 2.5 3 104 J/kg (from Table 1) Required: Q, latent heat of fusion Analysis: Q 5 mLf Solution: Q 5 mLf 5 10.652 kg2 12.5 3 104 J/kg2 Q 5 1.6 3 104 J Statement: The 652 g of lead releases 1.6 3 104 J of thermal energy as it solidifies. Sample Problem 2 Ethyl alcohol is a liquid at room temperature. How much thermal energy is absorbed when 135 g of ethyl alcohol at 21.5 °C is heated until all of it boils and turns into vapour? This is a two-step calculation because the ethyl alcohol will first warm up from 21.5 °C to its boiling point of 78.3 °C (see Table 1 on p. 291) and then change from a liquid into a gas while its temperature remains at 78.3 °C. So there is a warming-up part and a change of state. We will represent the amount of thermal energy absorbed during the warming-up part with Q1, and the amount of thermal energy absorbed during the change of state with Q2. We will use the quantity of heat equation, Q1 5 mcDT, to calculate the amount of thermal energy absorbed during the warming phase. For this calculation, we need to use the specific heat capacity, c, of ethyl alcohol from Table 1 in Section 6.3 on page 281. 292   Chapter 6 Thermal Energy and Society NEL  Then we will use the latent heat equation, Q2 5 mLv, to calculate the amount of thermal energy absorbed during the change of state. For this calculation, we need to use the specific latent heat of vaporization, Lv, of ethyl alcohol from Table 1 in this section. The total amount of thermal energy absorbed in the entire process, Qtotal, is the sum of Q1 and Q2. Given: m 5 135 g 5 0.135 kg; c 5 2.46 3 103 J/ 1kg ~°C); T1 5 21.5 °C; T2 5 78.3 °C; Lv 5 8.6 3 105 J/kg Required: Qtotal, total amount of thermal energy absorbed Analysis: Q1 5 mcDT; Q2 5 mLv; Qtotal 5 Q1 1 Q2 Solution: Q1 5 mcDT 5 10.135 kg2 12.46 3 103 J/ 1kg ~ °C2 2 178.3 °C 2 21.5 °C2 Q1 5 1.886 3 104 J (one extra digit carried) Q2 5 mLv 5 10.135 kg2 18.6 3 105 J/kg) Q2 5 1.161 3 105 J (two extra digits carried) Qtotal 5 1.886 3 104 J 1 1.161 3 105 J Qtotal 5 1.3 3 105 J Statement: Ethyl alcohol absorbs a total of 1.3 3 105 J of energy when a 135 g sample at 21.5 °C is heated until all of it boils and turns into vapour. Practice 1. How much thermal energy is released when 2.0 L of liquid water freezes? T/I [ans: 6.8 3 105 J] 2. How much thermal energy is absorbed when a 350 g bar of gold melts? T/I 5 [ans: 3.9 3 10 J] 3. How much thermal energy is released when 500 g of steam at 100 °C condenses into liquid water and then cools to 50 °C? T/I [ans: 1.3 3 106 J] Water: A Special Liquid Most solids sink in their respective liquids. For example, solid iron sinks in liquid iron. This occurs because the particles of the solid are more closely packed than the particles of the liquid, making the solid denser. However, water is different. Ice floats on water because water is one of the few substances whose solid is less dense than its liquid (Figure 5). This is based on the water molecule’s chemical structure. Water molecules are V-shaped and have two hydrogen atoms attached to one oxygen atom (Figure 6).  O H  H  Figure 5 Most solids are more dense Figure 6 A water molecule has two hydrogen atoms than their liquids. Ice, however, floats on with a slight positive charge and one oxygen atom liquid water. with a slight negative charge (d means “partial charge”). NEL 6.4 States of Matter and Changes of State    293  H H O H H O H O H H H H H H H O O O H H H H O H O H H O H H O H H O H O O represents forces H H H O O H H H of attraction among O H H H OH H H O water molecules O H H O H H O O H O H H H H (a) (b) O H H O H H Figure 7 (a) At temperatures above 4 °C, water molecules are relatively disorganized. (b) As the temperature of the water decreases, the molecules move more slowly, forming a more organized structure, so that molecules of ice take up more space than those of liquid water. The hydrogen atoms in a water molecule have a small positive charge, while the oxygen atom has a small negative charge. The hydrogen atoms of one water molecule are attracted to the oxygen atoms of neighbouring water molecules. This occurs because opposite charges are attracted to one another. However, at temperatures above 4 °C, molecules of water move too fast for these forces of attraction to pull the molecules together. At these temperatures, water mol- ecules are relatively disorganized (Figure 7(a)). As the temperature decreases, the molecules move slowly enough for the forces of attraction to place the molecules into a more organized structure. The more organized molecules of water in ice take up more space than the more disorganized molecules in liquid water, so water expands as it freezes (Figure 7(b)). The expansion of water when it freezes can cause a lot of problems. Pipes in homes or under the street can break under the pressure of the expanding, frozen water. When the water thaws, it flows out of the broken pipes, causing flooding and damage (Figure 8). ario Physics 11 U 6504338 Ontario Physics 11 U C06-F015a-OP11USB 0176504338 NGI FN C06-F015b-OP11USB CO CrowleArt Group 6th pass Deborah Crowle roved Pass 5th pass Approved Approved Not Approved Figure 8 When pipes freeze and burst, it can be a big mess, both above ground on road surfaces and below ground in water and sewer systems. 294   Chapter 6 Thermal Energy and Society NEL  6.4 Summary The three states of matter are solid, liquid, and gas. When thermal energy is released or absorbed a change of state may happen. The change in temperature that occurs as a substance releases or absorbs thermal energy can be shown in a cooling graph or a heating graph. The thermal energy that is absorbed or released during a change of state is called the latent heat of the substance. If the substance is melting or freezing, it is called the latent heat of fusion. If the substance is evaporating or condensing, it is called the latent heat of vaporization. The specific latent heat of fusion (Lf ) is the amount of thermal energy per kilogram needed to melt or freeze a substance. The specific latent heat of vaporization (Lv) is the amount of thermal energy needed per kilogram to evaporate or condense a substance. The equation Q 5 mLf is used to calculate the latent heat of fusion, and Q 5 mLv is used to calculate the latent heat of vaporization. Ice is one of the few solids that floats in its liquid; this is due to the shape of its molecules and the forces of attraction between its molecules. 6.4 Questions 1. (a) Describe each part of the graph in Figure 9 in terms of 3. Describe what would happen if you were to heat liquid the states of matter. water to a temperature of 110 °C. K/U (b) What type of graph is this, a heating graph or a cooling 4. Explain the terms “latent heat of fusion” and “latent heat graph? How can you tell? K/U T/I of vaporization.” K/U 5. To prevent fruit on trees from freezing and becoming inedible, fruit farmers in Ontario often spray their crops with water if they know the temperatures are going to Temperature °C drop below zero. Use your knowledge of latent heat to explain why this will help prevent the fruit from freezing. K/U A 6. Calculate the latent heat of fusion for 2.40 kg of gold as it changes from a molten liquid into a solid bar. T/I 7. How much thermal energy is needed to change 100 g of Thermal energy released ice at 220 °C into steam at 110 °C? T/I Figure 9 8. While forming a 1.50 kg aluminum statue, a metal smith heats the aluminum to 2700 °C, pours it into a mould, and 2. (a) Use Table 2 to graph a heating curve. then cools it to a room temperature of 23.0 °C. Calculate (b) Label the appropriate parts of the graph with the the thermal energy released by the aluminum during following: solid, liquid, gas, melting, evaporation. the process. T/I (c) Determine the melting point and boiling point of the 9. What makes water different from most other substances? substance. T/I Include a description of the physical characteristics in your answer. K/U Table 2 Data Collected during the Heating of a Substance Time (min) 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 Temperature (°C) 37 43 49 55 55 55 56 64 70 80 86 90 90 90 100 U 06-F016-OP11USB rowleArt Group NEL 6.4 States of Matter and Changes of State    295 eborah Crowle  6.5 Heating and Cooling Systems Canadians require heating systems to keep homes, schools, and other buildings com- fortable during the winter. Many buildings are also equipped with cooling systems for the summer. Conventional heating systems use oil or natural gas as fuels, and air con- ditioners typically use electricity. In some cases, electric heaters are used for heating. Scientists and engineers are developing more affordable energy-efficient alternatives that Canadians can use for their heating and cooling needs. Conventional Heating Systems All heating systems have a source of thermal energy, a means of transferring the energy from one place to another, and a device that controls the production and distribution of energy. Conventional heating systems use either electricity or fossil electrical heating system a system that fuels as a source of thermal energy. Electrical heating systems generate thermal energy uses electricity to produce thermal energy by allowing an electric current to pass through metal wires that have a high resist- for heating ance to the current that passes through them. Similar wires are used in the elements of an electric stove and a toaster. The thermal energy is transferred throughout the building by convection. Fossil fuel systems generate thermal energy by burning fossil fuels such as oil or natural gas. Some heating systems use the thermal energy to heat air while others use the forced-air heating system a system that thermal energy to heat water. A forced-air heating system uses a furnace to warm air moves hot air to heat a building to heat a building (Figure 1). In this system, the warm air passes through ducts that eventually reach vents located in the floor or ceiling. The warm air is transferred throughout the building by convection. As warm air enters a room through the vents, colder air is pushed through another set of ducts called the cold air return, providing more air for the furnace to heat. Fresh air is also brought into the system through an intake pipe out of the building. An exhaust system releases gases such as carbon hot water heating system a system that dioxide. A hot water heating system uses a boiler to heat water to heat a building. The uses hot water to heat a building boiler uses electricity or burns fossil fuels to heat the water. The hot water is then pumped through pipes that eventually reach radiators located in various rooms of the building. return return supply ducts Figure 1 In a typical forced-air system, warm air from the furnace is supplied to each room through a network of ducts. exhaust A separate set of ducts returns cooler outside air to the furnace and brings in fresh air air intake from outside to the furnace for heating. furnace 296   Chapter 6 Thermal Energy and Society NEL conventional cooling Systems The physics of conventional air conditioners, refrigerators, and freezers is very similar. All of these appliances use the evaporation of a liquid to absorb thermal energy from air. First, a compressor puts a special type of gas, called a refrigerant, under pressure. Whenever a gas is compressed, its temperature increases. The warm refrigerant gas runs through a coil of tubes, where it starts to release some of its thermal energy. As the refrigerant cools, it changes into a liquid. The liquid refrigerant then passes through an expansion valve, moving from a high-pressure area to a low-pressure career lInK area. At this point, the liquid refrigerant absorbs thermal energy from the sur- rounding air and evaporates back into a gas. The surrounding air cools down as a HVAC (heating, ventilation, air result. The compressor of an air conditioner gets very warm, so air conditioning units conditioning) engineers and are installed outside the building, where the thermal energy can be released into the technicians design and maintain external environment. the heating and cooling systems of In a refrigerator or freezer, the same type of compressor and coil system is used buildings. To learn more, (Figure 2). The only difference is that the cooled air is produced in the refrigerator or go t o n elSon S c i en c e freezer and the warm air is released into the external environment. evaporator capillary tube condenser expansion valve compressor Figure 2 The compressor in a refrigerator puts the refrigerant under pressure to change it into a liquid. When the refrigerant moves through the expansion valve, the pressure decreases and the liquid evaporates as it passes through the evaporator, removing thermal energy from the air. controlling heating and cooling Systems Both heating and cooling systems contain a thermostat to sense the temperature of the environment and turn the furnace or air conditioner on or off accordingly. Conventional thermostats contain a coiled bimetallic strip and a mercury switch mercury (Figure 3). A bimetallic strip consists of two types of metal bonded together back switch to back. Due to the different types of metals, the two sides of the strip expand and contract at different rates when heated or cooled. Expansion and contraction make the coil tighten and loosen as the temperature of the house fluctuates. On top of the bimetallic strip is the mercury switch. Mercury is a liquid metal that flows like water bimetallic and also conducts electricity. When the bimetallic strip coil changes shape again, the strip mercury switch tilts and the mercury moves inside. When the temperature in the building is above or below the temperature that an operator has set on the thermostat, the switch tilts in one direction and causes the mercury to touch two bare wires. This creates a closed circuit and turns the furnace or air conditioner on. As the environ- Figure 3 A bimetallic strip is curled into ment warms up or cools down, the bimetallic coil changes shape again. This causes a coil so it winds up or unwinds as the the switch to tilt in the opposite direction, moving the mercury away from the circuit temperature changes, resulting in the wires. This opens the circuit and turns the furnace or air conditioner off. tilting of the mercury switch on the top. nel 6.5 Heating and cooling Systems 297  Geothermal Systems: An Alternative Method of Heating and Cooling The most popular methods of heating involve the burning of fossil fuels. This burning results in the release of greenhouse gases, such as carbon dioxide, into the atmos- phere, contributing to global warming. The refrigerants used in cooling systems are also greenhouse gases and contribute to global warming when they leak into the environment. Some refrigerants also damage the ozone layer, a layer of ozone gas in the upper atmosphere that helps reduce the amount of ultraviolet radiation from the Sun that reaches Earth’s surface. Damage to the ozone layer contributes to increases in skin cancer caused by excess ultraviolet radiation. To reduce production of green- house gases and protect the ozone layer, alternative forms of heating and cooling are being developed. Several alternatives were introduced in Chapter 5. One of the newest heating and cooling technologies makes use of the thermal energy contained geothermal system a system that within Earth’s crust. Geothermal systems use heat pumps to transfer Earth’s natural transfers thermal energy from under thermal energy for heating and cooling. Earth’s surface into a building to heat Temperatures at a depth of 3 m below the ground remain fairly steady throughout it, and transfers thermal energy from the year. Depending on the latitude, the actual ground temperature can vary. In the building into the ground to cool Ottawa, the temperature remains at about 9 °C, which might seem cold, but it can the building still be used to heat a building in the winter. During the winter, the temperature below the ground is higher than the temper- ature above the ground. In a geothermal system, a liquid antifreeze–water mixture is pumped through a network of plastic pipe that is placed a few metres underground. As the antifreeze–water mixture is pumped through the pipes, the thermal energy from the ground is transferred to the walls of the pipe by conduction and then trans- ferred from the pipe to the liquid by convection. The liquid is then pumped into a heat pump located inside the building, which then transfers the thermal energy from the liquid to the air inside the building (Figure 4(a)). hot cold wa coo rm l wa coo rm l cool warm (a) (b) Figure 4 (a) In a geothermal system, thermal energy from below the ground is transferred to a heating system inside a building during the winter, heating the building. (b) During the summer, thermal energy is transferred from the building to the ground outside, cooling the building. Ontario Physics 11 U 298   Chapter 6 Thermal Energy and Society 0176504338 NEL FN C06-F017_OP11USB  Inside the heat pump, a liquid refrigerant is pumped through a series of coils. Thermal energy is transferred from the antifreeze–water mixture to the refrigerant, changing the refrigerant into a gas. The gas moves through another set of coils where the pressure is increased. As the pressure of the gas increases, its temperature also increases. Air blows over the coils containing the hot refrigerant gas, and the hot air is distributed through ducts to warm the building. During the summer, the heat pump process is reversed. Hot air from the house is pumped over the refrigerant, causing it to warm up. The refrigerant then transfers its thermal energy to the antifreeze–water mixture so that the mixture warms up. Since it is cooler underground than above ground during the summer, the thermal energy is trans- ferred from the antifreeze–water mixture to the ground by conduction (Figure 4(b)). 6.5 Summary Unit taSK BooKmarK You can apply what you have learned All heating systems have a source of thermal energy, a means of transferring about heating and cooling systems to the energy, and a thermostat to control the production and distribution of the Unit Task on page 360. energy. Conventional heating systems use either electricity or fossil fuels as a source of thermal energy. Conventional cooling systems use the evaporation of pressurized refrigerants to absorb thermal energy from the air, resulting in cool air that can then be blown through a duct system. A typical thermostat uses a bimetallic strip and a mercury switch to turn a heating or cooling system on or off as temperatures change. Conventional heating and cooling systems produce greenhouse gases such as carbon dioxide, so they are not considered environmentally friendly. Geothermal systems use Earth’s natural thermal energy for heating and cooling purposes. In the winter thermal energy is transferred from below Earth’s surface into a building to heat it. In the summer, thermal energy is transferred from a building into Earth’s surface to cool it. 6.5 Questions 1. Forced-air heating systems are found in most homes. 6. The specific heat capacity of water is much greater than Brainstorm reasons why they are so popular. K/U A that for air. T/I C A 2. Refrigerants used in air conditioners and refrigerators can (a) Research the specific heat capacity value for air. be a type of chlorofluorocarbon, or CFC. Research other (b) Perform research to compare the efficiency of a typical uses for CFCs. A new forced-air furnace for a home with that of a new hot water boiler. 3. Research programmable thermostats and smart (c) Based on your research, comment on which heating thermostats. How do these thermostats work? Why are system would be more effective, a forced-air system or these thermostats environmentally friendly? A a hot water system. 4. Why can a geothermal system be used as both a heating system and a cooling system? K/U 5. Create a flow chart or other graphic organizer summarizing go to n el son s c i en c e the process of heating and cooling using geothermal energy. K/U A NEL 6.5 Heating and Cooling Systems   299  7.1 Atoms and Isotopes The history of atomic discovery begins with the ancient Greeks, when, around 400 BCE, philosopher Democritus asserted that all material things are composed of extremely small irreducible particles called atoms. His theory was rejected and ignored for almost 2000 years. John Dalton resurrected the atomic theory of matter in the early nineteenth century by characterizing elements by their atomic structure and weight. Over the next hundred years or so, scientists continued to refine their understanding of the atom, until the advances of Niels Bohr and Ernest Rutherford. Bohr–Rutherford Model of the Atom Niels Bohr, a Danish scientist, and Ernest Rutherford, a New Zealand scientist, are cred- ited with a number of discoveries that led to the development of the Bohr–Rutherford atomic model. Rutherford found that when a beam of positively charged particles was fired at a thin gold foil, most particles passed through the foil, as expected, but some were scattered in all directions. To explain this, it was proposed that the atom consists of a dense, positively charged nucleus surrounded by tiny negatively charged electrons and a relatively vast region of empty space. Bohr also discovered that the electrons could only occupy certain energy levels. When these discoveries were com- bined, the Bohr–Rutherford model of the atom was created. This model has the following key features: The dense nucleus contains the atom’s protons and neutrons. The relatively tiny electrons orbit the nucleus. The electrons only occupy certain energy levels. Most of the atom consists of empty space. The model provides a visual method for describing the atomic structure of an ele- ment. The atomic structure refers to the number of protons, neutrons, and electrons in an atom and their organization within the atom. Simplified Bohr–Rutherford diagrams for helium and fluorine are shown in Figure 1. 2p+ 9p+ 2n0 10n0 (a) He (b) F proton a positively charged particle in the Figure 1 Bohr–Rutherford diagrams for (a) helium and (b) fluorine. Notice that a helium atom has nucleus of an atom two protons, two neutrons, and two electrons. A fluorine atom has nine protons, ten neutrons, and neutron an uncharged particle in the nine electrons. nucleus of an atom The nucleus is the centre of the atom and consists of protons and neutrons. A nucleons particles in the nucleus of an proton is a positively charged particle, and a neutron is an uncharged particle. In atom; protons and neutrons nuclear physics, neutrons and protons are often referred to collectively as nucleons. Protons and neutrons have approximately the same mass. An electron is a negatively electron a negatively charged particle charged particle that moves in the space surrounding the nucleus and is extremely found in the space surrounding the nucleus of an atom small compared to nucleons. In general, an atom in its normal state has the same number of electrons as protons. ground state state in which all electrons In a Bohr–Rutherford diagram, electrons are placed in the lower energy levels, or are at their lowest possible energy levels shells, first, until these shells are filled. Atoms that have electrons placed in this way excited state state in which one or more are said to be in their ground state: the electrons are all at the lowest possible energy electrons are at higher energy levels than levels. An atom is said to be in an excited state if it absorbs energy that causes an elec- in the ground state tron to have more energy and move to a higher energy level. An excited atom returns 318 Ontario Physics Chapter 11 U SB 7 Nuclear Energy and Society NEL 0176504338  to its ground state by releasing energy as the electron drops back to its lowest available energy level. Figure 2 shows a hydrogen atom in the ground state and in an excited state. The first energy level is the innermost shell in the Bohr–Rutherford diagram. 1p+ 1p+ (a) H (b) H Figure 2 Hydrogen in (a) an excited state and (b) its ground state According to the Bohr–Rutherford model, each energy level or shell can hold a certain Table 1 Electron Distribution in a number of electrons. Table 1 gives the maximum number of electrons for each shell. Bohr–Rutherford Model Maximum Atomic Number, Mass Number, and the Periodic Table Shell number of The periodic table of elements lists all the elements known today. It can be used to number electrons determine the atomic structure of an element (Figure 3). 1 2 9 atomic number 2 8 F chemical symbol 3 4 18 32 fluorine 19.00 mass number Figure 3 Identifying mass number and atomic number for the element fluorine on the periodic table ntario Physics 11Th UeSBatomic number is the number of protons in an atom of an element. Each atomic number the number of protons in element has a different number of protons. The mass number is equal to the number the nucleus 76504338 of nucleons in an atom. The periodic table entry shown in Figure 3 indicates that mass number the number of protons and C07-F003b-OP11USB fluorine has nine protons and nine electrons. The number of neutrons is determined neutrons in the nucleus O byNGI subtracting the atomic number from the mass number: 19 nucleons 1mass number2 2 9 protons 1atomic number2 5 10 neutrons 12 mass number C ss 4th pass chemical symbol pproved Isotopes ot Approved Carbon-12 consists of six protons and six neutrons (Figure 4). Most naturally occur- 6 atomic number ring carbon has this atomic structure. There is, however, another form of carbon Figure 4 The standard notation for called carbon-14. Carbon-14 has six protons and eight neutrons. Carbon-14 is a dif- carbon-12 ferent isotope than carbon-12. Different isotopes of an element have the same number of protons, but different numbers of neutrons (Figure 5). The mass number of 14 isotope a form of an element that has indicates that an atom of carbon-14 has two more neutrons than an atom of the same atomic number, but a different carbon-12. mass number than all other forms of that element o Physics 11 U number of neutrons 5 mass number 2 atomic number 4338 C07-F004-OP11USB CrowleArt Group n 5 12 2 6 n 5 14 2 6 Deborah Crowle6p+ 6p+ 6n0 0 n56 8n n58 3rd pass ved proved carbon-12 carbon-14 12 14 (a) 6C (b) 6C Ontario Physics 11 U Figure 5 Bohr–Rutherford models for (a) carbon-12 and (b) carbon-14 0176504338 NEL FN C07-F005-OP11USB 7.1 Atoms and Isotopes 319 CO CrowleArt Group  Carbon-14 has some interesting properties that are useful to archaeologists and scientists. A process called carbon dating provides a reasonably accurate method to determine the age of fossils and objects made of things that were once alive. You will learn more about carbon dating in Section 7.3. Most samples of elements consist of a number of different isotopes, some occur- ring naturally and others produced in laboratories. The most common isotope of hydrogen has a nucleus consisting of only one proton. There are, however, two other isotopes of hydrogen. These isotopes are important in nuclear science, so they have been given their own names: deuterium and tritium. Deuterium, which has one proton and one neutron, is a naturally occurring substance. Tritium, which has one proton and two neutrons, is only produced as a by-product of human-made nuclear reactions. The periodic table identifies the most commonly occurring isotopes for each element. A more general table that lists atomic information for all known isotopes is called a chart of the nuclides. In the following Tutorial, you will draw Bohr– Rutherford diagrams for various isotopes. LEarNiNg TIP Tutorial 1 Constructing a Bohr–Rutherford Diagram Mass Numbers The mass numbers of most elements Sample Problem 1 have decimal values associated Draw the Bohr–Rutherford diagram for silicon-31. with them. Carbon-12 has a mass of exactly 12 atomic units because Step 1. Locate silicon on the periodic table. The chemical symbol for silicon is Si. it is the substance to which all other Step 2. The mass of this isotope is given: 31. Use a periodic table to identify the atomic elements are compared by atomic number. The atomic number is 14. mass. The mass number that appears Step 3. Since the atomic number is 14, there are 14 protons and 14 electrons. The in the periodic table for carbon is number of neutrons is found by subtracting the atomic number from the mass slightly higher than 12 because of the small amounts of carbon-14 that number: 31 2 14 5 17 exist in nature. There are 17 neutrons in an atom of silicon. Use this information to draw a Bohr–Rutherford diagram for silicon (Figure 6). 14p+ 17n0 Si Figure 6 Practice 1. Sketch a Bohr–Rutherford diagram for each element. T/I C (a) aluminum, Al (mass number 28) (b) silver, Ag (mass number 110) radioisotope an unstable isotope that (c) two other elements of your choice from the periodic table or Appendix B (page 662) spontaneously changes its nuclear structure and releases energy in the Some isotopes, called radioisotopes, are unstable; that is, they spontaneously change form of radiation their nuclear structure. Radiation is energy released in the form of waves when a radio- radiation energy released when the isotope undergoes a structural change. This radiation can be harmful if not properly nucleus of an unstable isotope undergoes controlled. In some cases, however, these radioisotopes are beneficial. You will a change in structure examine the process by which isotopes spontaneously change later in this chapter. Science Physics 11 320 Chapter 7ISBN: ISBNEnergy Nuclear # 0176504338 and Society NEL FN C07-F007-OP11USB Medical Applications of Radioisotopes Nuclear medical imaging is a diagnostic technique that involves injecting a patient with a small dose of a radioisotope, such as technetium-99m. These materials, some- times called radioactive tracers, emit radiation that can be detected and converted into an image. By comparing radiation patterns of an unhealthy organ to those of a healthy one, doctors are better able to pinpoint a malignancy, or tumour (Figure 7). One of the advantages of nuclear imaging over traditional X-rays is that it provides a detailed account of both hard tissues like bone and softer tissues like the liver and kidneys. X-rays are primarily useful for detecting bone fractures. Figure 7 A radioactive tracer provides a detailed image of a diseased organ. This image is detecting the spread of lung cancer to the skeleton. Research This Technetium-99m SKILLS Skills: Researching, Analyzing, Communicating HANDBOOK A5.1 Technetium-99m is an unusual isotope for which medical C. What is it about Tc-99m that makes it particularly useful in scientists have discovered several important uses. The “m” medicine? T/I in its name identifies it as a meta-stable isotope. D. Discuss some of the applications of Tc-99m in the medical 1. Research Technetium-99m (Tc-99m) on the Internet or at field. A the library. Write a brief report of your findings that includes E. Are there any drawbacks to using nuclear imaging, such as answers to the following questions: health risks, costs, or wait times? A A. What is a meta-stable isotope? T/I go to N ELs oN s c i EN c E B. How is Tc-99m obtained? T/I Medical Treatments One of the earliest medical applications of radioisotopes began in the 1950s, when iodine-131 was used to diagnose and treat thyroid disease. Sufferers of hyper- carEEr lINK thyroidism have an overactive thyroid gland: it releases more thyroid hormone than the body requires. Iodine-131 can be used to both identify a diseased thyroid gland A radiation therapist works with and halt production of the hormone. doctors, other medical staff, and Radionuclide therapy (RNT) is a rapidly growing medical field in which the prop- patients to design and administer erties of certain radioactive substances are used to treat various ailments. RNT is radiation health treatment plans. To currently used to treat certain types of tumours, bone pain, and other conditions. In learn more about careers in radiation cancer treatments, the fundamental idea behind RNT is to bombard rapidly dividing therapy and related fields, harmful cells with radiation. These cells tend to absorb the radiation, which prevents go t o N ELsoN s c i EN c E them from dividing further. NEL 7.1 Atoms and Isotopes 321  7.1 Summary The Bohr–Rutherford model of the atom illustrates the atomic structure of an element. You can identify the number of protons, neutrons, and electrons of an element from its Bohr–Rutherford model. You can identify the mass number and atomic number of an element from the periodic table. Isotopes of an element have the same number of protons but different numbers of neutrons. Radioactive isotopes are unstable and will spontaneously undergo a change in their nuclear structure. Some radioactive isotopes have useful applications, such as medical diagnosis and therapy. 7.1 Questions 1. Draw a Bohr–Rutherford diagram for each isotope. K/U C (b) Describe how each isotope compares with its most (a) oxygen-16 commonly occurring isotope. K/U C (b) potassium-40 6. Identify each isotope shown in Figure 9 given its 2. (a) Draw Bohr–Rutherford diagrams for hydrogen, Bohr–Rutherford diagram. K/U deuterium, and tritium. (b) Identify their similarities and differences. K/U C 3. For each Bohr–Rutherford model shown in Figure 8, (a) determine the atomic number and the mass number 14p+ 10p+ (b) write the chemical name of the isotope K/U 14n0 12n0 (a) (ii) (b) Figure 9 7p+ 13p+ 5n0 13n0 7. An isotope has 16 protons and 22 neutrons. Identify the element. K/U 8. (a) Draw a Bohr–Rutherford model for each isotope of (i) (ii) argon. Figure 8 (i) Ar-40 (ii) Ar-44 (iii) Ar-47 (b) Explain how these isotopes are 4. (a) Draw a Bohr–Rutherford diagram for each isotope of (i) alike beryllium. (ii) different K/U C (i) 74 Be 9. Neon has three stable isotopes: Ne-20, Ne-21, and (ii) 94 Be Ne-22. K/U C (iii) 114 Be (a) Draw a Bohr–Rutherford model for each (b) Explain the similarities and differences between these isotope. models. (b) How are these models alike? How are they (c) Which isotope of beryllium is the most common in different? nature? Explain how you know. K/U C 10. (a) Draw Bohr–Rutherford models for lithium-10, 5. (a) Draw a Bohr–Rutherford model for each isotope. beryllium-10, and boron-10. (i) lithium-5, 53Li (b) How are these models alike? How are they (ii) oxygen-20, 208O different? K/U C 322 Chapter 7 Nuclear Energy and Society NEL Radioactive Decay 7.2 A fascinating area of scientific inquiry around the turn of the twentieth century was the splitting of the atom. In 1896, Henri Becquerel observed this as a natur- ally occurring event when he found that a sample of uranium left an image when placed on photographic film. This eventually led to the discovery of radioactivity—the radioactivity a process by which the spontaneous disintegration of an atom’s nucleus. The film traces were caused by par- nucleus of an atom spontaneously ticles emitted during this process. disintegrates Becquerel’s accidental discovery encouraged scientists to seek ways to induce sim- ilar reactions using various materials. Ernest Rutherford used high-energy particles to bombard nitrogen and discovered that oxygen was produced. A few years later, James Chadwick performed a similar experiment with beryllium that led to the dis- covery of the neutron. These efforts helped scientists develop a better understanding of atomic structure at the nuclear level and formed the basis for the broad range of nuclear scientific work that followed. As you learned in Chapter 5, a reaction in which the nucleus of an atom is split into smaller pieces is known as nuclear fission. nuclear fission the decomposition of A cyclotron, shown in Figure 1, is a device that can accelerate particles to very large, unstable nuclei into smaller, more high speeds. Many nuclear reactions can only occur when particles are travelling at stable nuclei speeds near to that of light. High-energy physics is the study of such interactions. Figure 1 A cyclotron is a device that accelerates particles to very high speeds approaching the speed of light. The high-energy particles that are produced can be used for research experiments, as well as medical treatments. Chemical Reactions A chemical reaction is the interaction of substances to form new substances. The initial substances, which may be elements or compounds, are called reactants. The substances present at the end of the reaction, which also may be elements or compounds, are called products. For example, the reaction between carbon and oxygen produces carbon dioxide. The reactants are carbon, C, and oxygen, O2. The product is carbon dioxide, CO2. Energy is also released during this chemical reaction. We can represent a chemical reaction as a word equation or a chemical equation as follows: word equation: carbon 1 oxygen → carbon dioxide 1 energy chemical equation: C 1 O2 → CO2 1 energy NEL 7.2 Radioactive Decay   323  In a chemical reaction, the entities do not change. All of the entities that were present in the reactants are present in the products. A balanced chemical reaction clearly shows this. In the previous example, there is one atom of carbon on each side of the arrow. Similarly, there are two atoms of oxygen on either side of the arrow. The identities of the elements do not change; only their organization changes. Chemical reactions obey the law of conservation of mass. A chemical reaction, such as the previous example, that releases energy is exo- thermic. By contrast, a chemical reaction that absorbs energy is endothermic. Nuclear Reactions nuclear reaction the process by which Nuclear reactions involve changes in the nuclei of atoms, sometimes resulting in com- the nucleus of an atom sometimes pletely new elements. The identity of an element is determined by examining the changes number of protons in its nucleus. If the number of protons changes, one or more new elements result. By examining the forces present in a nucleus, it is possible to understand the nature of nuclear reactions and why they occur. Electrostatic Force and the Strong Nuclear Force electrostatic force the force of attraction For over a hundred years, scientists have understood the electrostatic force of attrac- or repulsion due to electric charges tion and repulsion between electrically charged particles. Like charges repel, and opposite charges attract. This explains why the positively charged nucleus of an atom attracts negatively charged electrons. It cannot, however, explain how the nucleus itself is held together. Consider the helium nucleus shown in Figure 2. The neutrons have no electrical charge and the protons are both positively charged. What is holding the nucleus together? n0 p p n0 Figure 2 The nucleons in a helium nucleus A different type of force, not discovered until the 1930s, is responsible for holding strong nuclear force the very strong the nucleus together. Like gravity, and unlike the electrostatic force, the strong nuclear force of attraction between nucleons force is always attractive and helps hold together the neutrons and protons in the nucleus of an atom. The strong nuclear force is much stronger than the electrostatic force. The strong nuclear force is responsible only for holding the nucleus of an atom together. Stable and Unstable Isotopes There is a delicate balance between the repulsive electrostatic force and the attractive strong nuclear force in a nucleus. When these forces are balanced, an atom is said to be stable. Atoms with higher atomic numbers (more protons) experience a greater electro- static force of repulsion among the protons, and the protons become more separated. This separation results in a weakening of the strong nuclear force. Additional neutrons add to the strong nuclear force to balance the increasing electrostatic repulsion. Sometimes the electrostatic forces are great enough to overcome the strong Ontario Physics 11 U nuclear force, and the nucleus spontaneously disintegrates (breaks apart) and releases 0176504338 energy. An unstable atom with a nucleus that can spontaneously disintegrate is said to FN radioactive decay the process by which C07-F010-OP11USB be radioactive. The process by which a radioactive atom spontaneously breaks apart to formGroup nucleus breaks apart CrowleArt a radioactive atom’sCO smaller atoms is called radioactive decay. There are three common forms of and forms different atoms radioactive Deborah Crowle decay: alpha decay, beta decay, and gamma decay. Pass 2nd pass Approved 324   Chapter 7 Nuclear Energy and Society NEL Not Approved  Alpha Decay One of the most common forms of radioactive decay is alpha decay, or a-decay. In alpha (a) decay nuclear reaction in alpha decay, a helium nucleus, consisting of two protons and two neutrons, is spontan- which an alpha particle is emitted eously emitted from the nucleus. An illustration of alpha decay is shown in Figure 3. α particle Pu-240 U-236 94 protons 92 protons 146 neutrons 144 neutrons Figure 3 Alpha decay of plutonium-240 In Figure 3, an atom of plutonium-240 decays into uranium-236, and a helium-4 nucleus is emitted in the process. The helium-4 nucleus is called an alpha particle. The alpha particle a particle emitted during equation for this nuclear reaction is alpha decay; composed of a helium 240 nucleus containing two protons and two 94 Pu S 236 4 92 U 1 2 He neutrons When a substance undergoes alpha decay, the mass number (A) is reduced by four and the atomic number (Z) is reduced by two. Generally, this can be shown as A A24 ZX S Z22 Y 1 42He parent atom the reactant atom in a nuclear reaction In a nuclear reaction, the parent atom (X) is the reactant atom, and the daughter atom (Y) is the product atom. The atomic number changes during alpha decay, so a new atom (ele- daughter atom the product atom in a ment) is formed. When this happens, the nuclear reaction is said to be a transmutation. nuclear reaction In the following Tutorial, you will use your understanding of alpha decay to write transmutation a nuclear decay process the equation for a nuclear reaction. in which daughter atoms are different elements from parent atoms Tutorial 1 Determining the Nuclear Equation for Alpha Decay Physics 11 SB 50433-8 Sample Problem 1 When lead-204 undergoes alpha decay, it produces a stable We can see that the new element has the atomic number C07-F011-OP11USB isotope. Determine the element and its atomic number and mass 80 and mass number 200. NGI number. Write the nuclear reaction equation for this alpha decay. 204 S 200 4 82Pb 80Y 1 2He 4th pass Step 1. Use the periodic table to determine that the atomic The periodic table tells us that the new element is an ed number of lead is 82. isotope of mercury. proved Step 2. Determine the atomic number and mass number of the The daughter atom is mercury-200. new isotope using the equation for alpha decay. Step 3. Write the reaction equation: 204 S 20424 4 82Pb 8222Y 1 2He 204 82Pb S 200 4