Thermal Expansion of Solids, Liquids and Gases PDF

**Thermal expansion of solids, liquids and gases** -------------------------------------------------- When almost all known solids, liquids and gases are heated they expand in size. This is called thermal expansion. This occurs when the surrounding pressure does not change. This occurs because as...

**Thermal expansion of solids, liquids and gases** -------------------------------------------------- When almost all known solids, liquids and gases are heated they expand in size. This is called thermal expansion. This occurs when the surrounding pressure does not change. This occurs because as the particles are heated they gain internal energy and they move more. When they do this, the volume, or overall size, of the object increases. The distances between particles is greatest in the gas, then liquid and lastly solid Figure caption, The distances between particles are smallest in the solid, they are larger in the liquid and larger again in the gas. The particles in gases and liquids move more freely than in solids, so their volume increases when heated. ### **Demonstration of solid expansion** #### **Ball and ring**  If the ball fits through the ring at room temperature, heat the metal ball. It expands and will then not fit through the ring. If the ball does not fit through the ring at room temperature heat the metal ring. The ring expands and the hole in the centre gets bigger. The ball will now fit through the ring. The volume of different solid materials expand at different rates. Pouring hot water on the metal lid of a jam jar can make it easier to open the jar. This is because the metal lid expands more than the glass jar. Rivets hold sheets of metal together. The rivet is heated up and hammered between two sheets of metal. The ends are then hammered flat. As the rivet cools, it contracts in volume and pulls the two sheets of metal tightly together. ### **Demonstration of liquid expansion** Like solids, liquids expand when they are heated, but because the bonds between separate molecules are usually less tight, liquids expand more than solids. Liquid Expansion experiment The water level in the capillary tube is initially at A. When the water is heated the liquid level initially falls to level B, before rising to level C and above. When the glass flask and capillary tube are first heated, they expand. This causes the volume of the flask to increase, and the liquid level falls to B. However, the liquid is quickly heated and starts to expand. It expands more than the solid glass and so the water level rises back to A and then up to, and beyond, C. This shows that: - - This is the principle behind liquid-in-glass thermometers. An increase in temperature results in the expansion of the liquid which means it rises up the glass. The liquid used is usually alcohol or mercury. ### **Demonstration that gases expand** Gases also expand when heated. Molecules within gases are further apart and weakly attracted to each other. Heat causes the molecules to move faster, which means that the volume of a gas increases more than the volume of a solid or liquid.  Once the flask stops being heated it cools and contracts. This causes the liquid to be sucked up the tube and into the flask. **The bimetallic strip** ------------------------ sheet metal with rivets A bimetallic strip is made from two different metals riveted together, one on top of the other. When heated, both metals expand, but one expands more than the other. Since they are riveted together, they cannot slip over one another, and the strip bends. The metal which expands more is at the outside of the curve. The metals used are often brass and steel. Brass expands more than steel and so will be on the outside of the curve.  As the metal cools it straightens out again. It bends in the opposite direction if cooled down beneath room temperature. **A bimetallic fire alarm** --------------------------- Bimetallic fire alarm When there is a fire: - - - - Particles in solids, liquids and gases are held together by forces of attraction between them. The forces of attraction between particles are weakest in gases, then liquids and are strongest in solids. This is the reason why gases expand the most when heated, followed by liquids and solids increase in volume the least. **Specific heat capacity** -------------------------- #### **Heating materials** When materials are heated, the molecules gain kinetic energy and start moving faster. The result is that the material gets hotter. Key fact: Temperature is a measure of the average kinetic energy of the molecules. Different materials require different amounts of energy to change temperature. The amount of energy needed depends on: - - - It takes less energy to raise the temperature of a block of aluminium by 1°C than it does to raise the same amount of water by 1°C. The amount of energy required to change the temperature of a material depends on the specific heat capacity of the material. #### **Heat capacity** Key fact: The specific heat capacity of a material is the energy required to raise one kilogram (kg) of the material by one degree Celsius (°C). The specific heat capacity of water is 4,200 Joules per kilogram per degree Celsius (J/kg°C). This means that it takes 4,200 J to raise the temperature of 1 kg of water by 1°C. Some other examples of specific heat capacities are: **Material** **Specific heat capacity (J/kg°/C)** -------------- -------------------------------------- Brick 840 Copper 385 Lead 129 Lead will warm up and cool down fastest because it doesn't take much energy to change its temperature. Brick will take longer to heat up and cool down. Bricks can be used in storage heaters as they stay warm for a long time. Other heaters are filled with oil (1,800 J/kg°C) or water (4,200 J/kg°C) as these emit a lot of heat energy as they cool down and therefore stay warm for a long time. **Calculating thermal energy changes** -------------------------------------- The amount of thermal energy stored or released as the temperature of a system changes can be calculated using the equation: change in thermal energy = mass × specific heat capacity × temperature change ΔEt=m×c×ΔΘ This is when: - - - - Example To boil 0.25 kg of water it first needs to be heated from 20°C to 100°C. If the specific heat capacity of water is 4,180 J/kg°C, how much thermal energy is needed to get the water up to boiling point? Et=m c Δθ Et=0.25×4,180×(100−20) Et=0.25×4,180×80 Et=83,600 J #### **Practical experiment - measuring specific heat capacity** ----------------------------------------------------------- There are different ways to investigate methods of insulation. In this practical activity, it is important to: - - - Aim of the experiment To measure the specific heat capacity of a sample of material. Method 1. 2. 3. 4. 5. 6. Record results in a suitable table. The example below shows some sample results. **Ammeter reading (A)** **Voltmeter reading (V)** **Initial temperature (°C)** **Final temperature (°C)** ------------------------- --------------------------- ------------------------------ ---------------------------- 3.65 10.80 15 38 Analysis The block has a mass of 1 kg and the heater was running for 10 minutes = 600 seconds. Using the example results: energy transferred = potential difference × current × time E=V×I×t E=10.90×3.65×600 E=23,700 J E=mcΔT c=EmΔT c=23,7001×(23) c=1,030J/kg°C The actual value for the specific heat capacity of aluminium is 900 J/kg°C. The calculated value does not match exactly but it is in the correct order of magnitude. Evaluation - - Hazards and control measures **Hazard** **Consequence** **Control measures** ------------------------------------------ ----------------- ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ Hot immersion heater and sample material Burnt skin Do not touch when switched on. Position away from the edge of the desk. Allow time to cool before packing away equipment. Run any burn under cold running water for at least 10 minutes. **Practical experiment - Measuring the specific heat capacity of water** ------------------------------------------------------------------------ There are different ways to determine the specific heat capacity of water. In this required practical activity it is important to: - - Aim of the experiment To measure the specific heat capacity of water. Method 1. 2. 3. 4. 5. 6. 7. Results Record results in a suitable table. The example below shows some sample results. **Energy supplied (J)** **Initial temperature (°C)** **Final temperature (°C)** ------------------------- ------------------------------ ---------------------------- 100,000 15 38 Analysis The water has a mass of 1 kg and the heater supplied 100,000 J, whilst the temperature rose 23°C. Using the example results: c=ΔQmΔθ c=100,0001×23=4,300 J/kg∘C The actual value for the specific heat capacity of water is 4,200 J/kg°C. The calculated value does not match exactly but it is in the correct order of magnitude. Evaluation - - - Hazards and control measures **Hazard** **Consequence** **Control measure** ------------------------------------------ ----------------- ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ Hot immersion heater and sample material Burn skin Do not touch when switched on. Position away from the edge of the desk. Allow time to cool before packing away equipment. Run any burn under cold running water for at least 10 minutes. **Changes of state** -------------------- ### **Melting, evaporating and boiling**  Energy must be transferred, by heating, to a substance for these changes of state to happen. During these changes the particles gain energy, which is used to: - - During melting and boiling there is not a change in temperature because the energy breaks the bonds or forces of attraction. In evaporation, particles leave a liquid from its surface only. In boiling, bubbles of gas form throughout the liquid. They rise to the surface and escape to the surroundings, forming a gas. Evaporation causes an object to cool. When sweat evaporates from skin it cools it down. The amount of energy needed to change state from solid to liquid, and from liquid to gas, depends on the strength of the forces between the particles of a substance. The stronger the forces of attraction, the more energy is required. Every substance has its own melting point and boiling point. The stronger the forces between particles, the higher its melting and boiling points. The strength of the forces between particles depends on the particles involved. For example, the forces between ions in an ionic solid are stronger than those between molecules in water or hydrogen. This explains the melting and boiling point data in the table. **Substance** **Bonding type** **Melting point** **Boiling point** ----------------- ------------------ ------------------- ------------------- Sodium chloride Ionic 801°C 1413°C Water Small molecules 0°C 100°C Hydrogen Small molecules -259°C -252°C These boiling/melting points are for standard atmospheric pressure and will be different at higher/lower atmospheric pressures. Evaporation can take place below the boiling point of a substance. ### **Condensing and freezing** Energy is transferred from a substance to the surroundings when a substance condenses or freezes. This is because the forces of attraction between the particles get stronger. Condensing occurs at the boiling point of a substance. A gas has particles with high kinetic energy stores. As the gas cools these stores reduce. When the cooling gas lowers to its boiling point, it condenses into a liquid. The particles move closer together. Freezing occurs at the melting point. As a liquid cools down the kinetic energy stores of its particles reduce. When the cooling liquid reaches its melting point, it solidifies (or freezes if water) into a solid. The particles move closer together to form the fixed, regular arrangement seen in solids. #### **Predicting a physical state** The state of a substance at a given temperature can be predicted if its melting point and boiling point are known. The table summarises how to work this out. **Temperature** **Predicted state** --------------------------------------------------------- --------------------- Given temperature \< melting point Solid Given temperature is between melting and boiling points Liquid Given temperature \> boiling point Gas ### **Limitations of the particle model** The particle model assumes that particles are solid spheres with no forces between them. However: - - The table below summarises the differences between boiling and evaporation. **Boiling** **Evaporation** --------------------------------- -------------------------------- Fast Slower Can occur throughout the liquid Occurs from the surface only Produces bubbles Does not produce bubbles Does not result in cooling Results in cooling Occurs at the boiling point Occurs below the boiling point Evaporation occurs at a faster rate when: 1. 2. 3. Unlike boiling, evaporation cools the liquid. When sweat evaporates from skin it cools it down because the particles with the most kinetic energy usually evaporate quickest. As the particle with the highest kinetic energy leave the surface of the skin, the average kinetic energy of the surface of the skin decreases, causing the temperature of the skin to reduce.