Change in States of Water PDF

# Change in States of Water ## 2.1 Change in States of Water **Flipped classroom:** Watch a video and answer the questions. **Let's begin** The photo shows the mist-spraying machine in the Avenue of Stars. On hot summer days, the machine sprays mists to the air to lower the surrounding tempera...

# Change in States of Water ## 2.1 Change in States of Water **Flipped classroom:** Watch a video and answer the questions. **Let's begin** The photo shows the mist-spraying machine in the Avenue of Stars. On hot summer days, the machine sprays mists to the air to lower the surrounding temperature. **Why can spraying mists to the air lower the surrounding temperature?** The answer to Let's begin is related to one of the processes of the change in states of water. Let us learn about the states of water first. ## A Processes of Change in States of Water Look at the glass of iced water in Fig 2.1. Water exists in three physical states, in this case: - liquid state, we commonly call it water; - solid state, we call it ice; - gas state*, we call it water vapour*. **Key point** Water exists in three physical states. Ice is in the solid state. Water is in the lianid state. Water vapour is in the gas state. ## Fig 2.1 A glass of iced water - water vapour (gas) in air (invisible) - ice (solid) - water (liquid) **Link** All matter (including water) is made up of tiny particles. - Particles in solid state are closely packed and arranged in a regular pattern. - Particles in liquid state are closely packed but not arranged regularly. - Particles in gas state are far apart. You will learn more about particles in Bk 1B, Unit 6. ## **Change in States of Water** ## Fig 2.2 A glass of iced water is left at room temperature If we leave the glass of iced water in Fig 2.1 at room temperature for some time, the ice will gradually disappear and water droplets will be formed on the outside of the glass (Fig 2.2). These observed changes are related to the change in states of water. Let us study how water changes states in this section. ## 1 Melting - a process of ice changing into water. Melting is the change in state from a solid to a liquid. When ice is heated to 0 °C, it melts and changes into water. This temperature (0 °C) is called the melting point of ice. ## Fig 2.3 Ice melts and changes into water - Ice (solid) - melting (at melting point, 0 °C) - Water (liquid) ## 2 Freezing - a process from a liquid to a solid. Freezing is the change in state from a liquid to a solid. When water is cooled to 0 °C, it freezes and changes into ice. This temperature (0 °C) is called the freezing point of water. ## Fig 2.4 Water freezes and changes into ice - Water (liquid) - freezing (at freezing point, 0 °C) - Ice (solid) **Link** You will learn about the rate of evaporation of water in the next section. ## 3 Boiling - a process that in form a liquid to gas. Boiling is the change in state from a liquid to a gas. When water is heated to 100 °C, it boils and changes into steam (steam is the gas state of water at or above 100 °C). This temperature (100 °C) is called the boiling point of water. ## Fig 2.5 Water boils and changes into steam. - Water (liquid) - boiling (at boiling point, 100 °C) - Steam (gas) ## 4 Evaporation Water can also change from a liquid to a gas through evaporation*. Unlike boiling, evaporation of water takes place at temperatures below its boiling point (i.e. below 100 °C). When water evaporates, it changes into water vapour. Evaporation of water takes place in many aspects in our everyday life. Fig 2.6 shows an example. After raining, the water particles in the rainwater on the road absorb energy from the sun and move more quickly. They finally escape from the water surface to become water vapour. ## Fig 2.6 After raining, the wet ground becomes dry after all water has evaporated - Water (liquid) - evaporation (at temperatures below boiling point) - Water vapour (gas) ## 5 Condensation Condensation is the change in state from a gas to a liquid. When steam or water vapour cools to or below its boiling point (i.e. at or below 100 °C), it condenses and changes into water. ## Fig 2.7 Steam condenses on the inner surface of the lid - Steam or water vapour (gas) - condensation (at or below boiling point) - Water (liquid) Steam is invisible. The white mist in Fig 2.5 are actually fine water droplets formed when steam cools down in air. **Key point** The following summarizes the change in states of water: - melting - boiling/evaporation - freezing - condensation **Did you know?** **Anti-fog spray for glasses** Our glasses fog up sometimes when we go into the street from an air-conditioned place (Fig 2.8). This is because water vapour in air condenses to form tiny water droplets on the cold glasses. The water droplets form the "fog" that blurs our vision*. ## Fig 2.8 Water vapour in air condenses on a pair of cold glasses - tiny water droplets - lens - before applying anti-fog spray - thin water film anti-fog spray - lens - after applying anti-fog spray ## Fig 2.9 How anti-fog spray prevents glasses from fogging up Some anti-fog sprays contain chemicals that prevent water droplets from forming on glasses. Instead, the water vapour condenses to form a thin water film that does not blur our vision (Fig 2.9). ## **B Temperature change during change of states of water** When water changes its state, how does its temperature change? Let us get some ideas in Practical 2.1. ## Practical 2.1 Heating Ice to Steam **Aim** To study the change in temperature when ice is heated to steam **Apparatus and materials per group** - alcohol thermometer (1) - glass rod (1) - beaker (250 cm³) (1) - stopwatch (1) - stand and clamp (1 set) - Bunsen burner (1) - insulating mat (1) - tripod and wire gauze (1 set) - gas lighter (1) - crushed ice **1 Set up the apparatus as shown.** Make sure the alcohol thermometer does not touch the bottom of the beaker. **Caution** - Wear safety-spectacles! - Do not touch the hot apparatus. **2 Light the Bunsen burner. Start the stopwatch.** **3 Record the thermometer reading every minute in the table on the next page. Observe the changes in the beaker during heating. After the water has boiled for two minutes, turn off the Bunsen burner and stop recording.** - Stir the ice and water with a glass rod before taking thermometer readings. - Refer to Skill reminder on p. 220 for how to take readings. ## **Results** **1** - Time (min): 0, 1, 2 , 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13 - Temperature (°C): -2, -1, 0, 0 , 0, 2, 8, 5, 8, 7, 2, 8, 6, 9, 2, 9, 6, 9, 6, 9, 6, 9, 6 **a State the temperature at which ice starts to melt.** 0°C **b State the temperature at which water starts to boil.** 100°C (96°C) **2 Using the data obtained, draw a line graph of the temperature of ice and / or water against time.** **Skill builder 2.1: Drawing line graphs (p. 86)** **Title: The change in temperature when ice is heated to steam** ## **Discussion** **1 When ice melts, does the temperature change?** No **2 When water boils, does the temperature change?** No ## **Skill builder 2.1 Drawing line graphs** A line graph can show the relationship between two variables. Let us learn how to draw one in the following example. **Example** A beaker of water is heated with a Bunsen flame. The temperature of the water is measured at 1-minute intervals. The results are recorded in the table below. - Time (min): 0, 1, 2, 3, 4, 5 - Water temperature (°C): 24, 35, 44, 53, 62, 70 Draw a line graph of the temperature of water against time. **Steps** 1. Give a title to the graph. 2. Put the independent variable on the horizontal axis (x-axis), and the dependent variable on the vertical axis (y-axis). Write down the units 3. Choose a suitable scale which makes full use of the graph paper. The scales should be equally spaced. 4. Draw a cross to represent each data point. 5. Connect the points with a straight line or a smooth curve. ## **Skill practice 2.1** Some water is cooled and changes into ice. The temperature of the water and / or ice is measured at 5-minute intervals. The results are recorded in the table below. - Time (min): 0, 5, 10, 15, 20, 25, 30, 35, 40, 45 - Water temperature (°C): 21, 16, 12, 7, 3, 0, 0, 0, -2 , -7 **Draw a line graph of the temperature of water and / or ice against time.** **Title: Temperature of water and / or ice against time** ## **Link** You will learn more about energy in Bk 1B, Unit 5. ## **Did you know?** **Does crushed ice or cubed ice melt faster?** Crushed ice melts faster than the same mass of cubed ice. This is because crushed ice has a larger surface area to absorb energy from the surroundings than cubed ice does. ## **Temperature of the ice and / or water against time** The graph in Practical 2.1 (p. 85) would be similar to the graph below. The graph has the following parts: 1. **The temperature of ice rises.** 2. **The ice is melting and changing into water. The temperature remains unchanged at the melting point. During melting, ice absorbs energy from the surroundings.** 3. **The temperature of water rises.** 4. **The water is boiling and changing into steam. The temperature remains unchanged at the boiling point. During boiling, water absorbs energy from the surroundings.** ## **Water may change from one state to another upon heating or cooling** At the moment when water changes states, it either absorbs energy from the surroundings, or releases energy to the surroundings (Fig 2.10). However, its temperature remains unchanged. ## Fig 2.10 Energy is absorbed or released during change of states of water - Ice (solid) - melting - freezing - water absorbs energy from the surroundings - water (liquid) - boiling/evaporation - condensation - water releases energy to the surroundings - Steam or water vapour (gas) **Key point** At the moment when water changes from one state to another, - its temperature remains unchanged (changes / remains unchanged); - it either absorbs energy from the surroundings, or releases energy to the surroundings. **Now, can you explain how the mist-spraying machine in Let's begin (p. 80) works?** When mists are sprayed into the air, they evaporate quickly to form water vapour. During the process, the mists absorb energy from the air. This lowers the surrounding temperature. If the water mists fall on our skin, we feel cool. This is because the water absorbs energy from our body and evaporates. ## **Did you know?** **Why do our hands feel particularly cool when rubbing with alcohol?** When we rub our hands with alcohol*, our hands feel particularly cool. Do you know why? Alcohol has a lower boiling point (82 °C) than water. When it is applied on our hands, it evaporates more quickly and more energy is absorbed from the skin. Thus our hands feel cooler. ## **Fig 2.11 Our hands feel cool when rubbing with alcohol** ## **Link** Many processes in daily life involve energy release or absorption. For example, energy is released during burning and explosion. You will learn more about this in later Units. ## **Section summary 2.1** 1. Water exists in three physical states: solid state (ice), liquid state (water) and gas state (water vapour or steam). 2. The table below summarizes the changes in states of water: | Process | Change in states | Temperature at which the process occurs | Energy absorbed or released | |---|---|---|---| | Melting | solid (ice) -> liquid (water) | 0 °C (melting point)| Absorbed | | Boiling | liquid (water) -> gas (steam) | 100 °C (boiling point) | Absorbed | | Evaporation | liquid (water) -> gas (water vapour)| Temperatures below the boiling point | Absorbed | | Freezing | liquid (water) -> solid (ice) | 0 °C (freezing point) | Released | | Condensation | gas (water vapour / steam) -> liquid (water) | Temperatures at or below the boiling point | Released | ## **Section exercise 2.1** **Level 1** The following cases involve a process of change in states of water. Name the process in each case. (3 marks) 1. Fruit juice turns into ice lollies 2. Wet clothes gradually dry in the air 3. Water forms on the surface of a cold bottle - Freezing - Evaporation - Condensation ## **Level 2** A beaker of water is heated over a Bunsen flame. The graph on the right shows the change in water temperature with time. **a What is the state of water during 0–2 min?** (1 mark) **b i Name the process of change in states of water during 2–4 min.** (1 mark) **ii Is energy absorbed from the surroundings or released to the surroundings in the process in i?** (1 mark) **c Suggest a value for T in the graph.** (1 mark)