Year 9 Physics: Changes of State and Specific Latent Heat

Introduction to Changes of State

In physics, we often talk about how matter can change from one form to another. These changes are called changes of state. There are three common states of matter: solid, liquid, and gas.

What Are Changes of State?

1. Melting: This is when a solid turns into a liquid. For example, when ice (a solid) is heated, it melts into water (a liquid).
2. Freezing: This is the opposite of melting. When a liquid cools down, it can turn into a solid. For example, water freezes into ice.
3. Evaporation: This occurs when a liquid turns into a gas. For example, when water is heated, it can evaporate and become steam (a gas).
4. Condensation: This is when a gas turns back into a liquid. For instance, when steam cools down, it turns back into water.
5. Sublimation: This is when a solid turns directly into a gas without becoming a liquid first. An example is dry ice (solid carbon dioxide) turning into gas.

Key Definitions

• Latent Heat: This is the heat energy that is needed to change the state of a substance without changing its temperature.
• Specific Latent Heat: This is the amount of heat required to change 1 kilogram of a substance from one state to another at a constant temperature.

Examples of Specific Latent Heat

1. Specific Latent Heat of Fusion: This is the heat needed to melt a solid into a liquid. For example, when ice melts, it requires energy to break the bonds holding the molecules in the solid state.
2. Specific Latent Heat of Vaporisation: This is the heat needed to turn a liquid into a gas. For example, when water boils, it requires energy to overcome the forces holding the water molecules together.

Key Rules

• During a change of state, the temperature of the substance remains constant. This means that while the ice is melting, it stays at 0°C until it is completely melted.
• The heat energy added or removed during a change of state is used to change the state, not the temperature.

Tips and Tricks

• To remember the changes of state, think about the water cycle: evaporation (water to gas), condensation (gas to water), freezing (water to ice), and melting (ice to water).
• Use diagrams to visualise the changes of state. Drawing a simple flowchart can help.

Questions

Easy Level Questions

1. What is melting?
2. What happens during freezing?
3. Name the three states of matter.
4. What is evaporation?
5. Give an example of condensation.
6. What is sublimation?
7. What does latent heat refer to?
8. What happens to the temperature during a change of state?
9. What is the specific latent heat of fusion?
10. What is the specific latent heat of vaporisation?
11. Is ice a solid, liquid, or gas?
12. What state is water when it is boiling?
13. What process changes a gas to a liquid?
14. What happens to ice when it melts?
15. Is the temperature of water constant while it is boiling?
16. Can a solid turn directly into a gas?
17. What happens to the molecules in a solid when it melts?
18. What do we call water vapour?
19. What state of matter is steam?
20. How does the energy change during melting?

Medium Level Questions

1. Explain what happens to the temperature during melting.
2. How much heat is needed to change 1 kg of ice at 0°C to water at 0°C?
3. What is the formula to calculate the latent heat?
4. Describe the process of evaporation.
5. How does pressure affect the boiling point of water?
6. When does condensation occur?
7. Name a substance that sublimates.
8. What happens to the molecules during condensation?
9. What is the difference between specific latent heat of fusion and vaporisation?
10. How does latent heat affect weather patterns?
11. What is the significance of latent heat in cooking?
12. Describe an experiment to observe melting.
13. How do you calculate the energy needed to evaporate water?
14. When a gas is cooled, what process occurs?
15. What role does specific latent heat play in refrigeration?
16. How does the state of matter change when energy is added?
17. What is the latent heat of fusion for water?
18. Give an everyday example of sublimation.
19. Why is it important that temperature remains constant during a change of state?
20. Explain why dry ice is used in cooling applications.

Hard Level Questions

1. Derive the formula for calculating latent heat.
2. If 500g of ice is melted, how much energy is required? (Use specific latent heat of fusion for water = $$334,000 , \text{J/kg}$$ )
3. Discuss the role of latent heat in the water cycle.
4. How does latent heat influence the temperature of the environment?
5. Explain the process of boiling in terms of energy transfer.
6. Compare the latent heats of fusion and vaporisation for different substances.
7. Why is the specific latent heat of vaporisation typically higher than that of fusion?
8. How does the concept of latent heat apply to climate change?
9. What happens at a molecular level during sublimation?
10. Investigate how latent heat affects weather phenomena such as thunderstorms.
11. How do different materials vary in their specific latent heat values?
12. Explain how latent heat is used in thermal energy storage systems.
13. Discuss the implications of latent heat in industrial processes.
14. Calculate the energy required to turn 2 kg of water at 100°C into steam.
15. How does the specific latent heat of a substance relate to its intermolecular forces?
16. Explain how the latent heat of fusion is important for ice skating.
17. Discuss the implications of latent heat for climate systems.
18. How can understanding latent heat improve energy efficiency?
19. If a gas is compressed, how does this affect its temperature and state?
20. Explain the importance of understanding latent heat in environmental science.

Answers and Explanations

Easy Level Answers

1. Melting is when a solid turns into a liquid.
2. Freezing is when a liquid turns into a solid.
3. Solid, liquid, gas.
4. Evaporation is when a liquid turns into a gas.
5. An example of condensation is water droplets forming on a cold glass.
6. Sublimation is when a solid turns directly into a gas.
7. Latent heat is the energy needed to change the state of a substance without changing its temperature.
8. The temperature remains constant.
9. Specific latent heat of fusion is the heat needed to melt a solid.
10. Specific latent heat of vaporisation is the heat needed to turn a liquid into a gas.
11. Ice is a solid.
12. Water is a liquid when it is boiling.
13. Condensation occurs when a gas turns into a liquid.
14. Ice absorbs heat and turns into water.
15. No, it stays at 100°C until all the water has boiled.
16. Yes, dry ice is an example.
17. The molecules move faster and break apart.
18. Water vapour is the gas state of water.
19. Steam is a gas.
20. Energy is added to break the bonds in the solid.

Medium Level Answers

1. The temperature remains constant until all the solid has melted.
2. $$334,000 \times 1 = 334,000 , \text{J}$$
3. The formula is: $$Q = m \times L$$ where $$Q$$ is the heat energy, $$m$$ is the mass, and $$L$$ is the specific latent heat.
4. Evaporation is when molecules at the surface of a liquid gain enough energy to become gas.
5. Increasing pressure raises the boiling point of water.
6. Condensation occurs when gas cools down to become a liquid.
7. An example of sublimation is dry ice turning into gas.
8. During condensation, molecules lose energy and move closer together.
9. The specific latent heat of fusion is for melting, while vaporisation is for boiling.
10. Latent heat helps drive weather systems like clouds and rain.
11. In cooking, latent heat helps with melting and boiling processes.
12. You can observe melting by heating ice and measuring the temperature.
13. $$Q = m \times L$$ where $$m$$ is the mass of the water.
14. Cooling gas turns it into a liquid (condensation).
15. It helps keep food cold by absorbing heat without changing temperature.
16. Energy is added to break bonds, causing phase changes.
17. $$334,000 , \text{J/kg}$$ for water.
18. An example is dry ice used for special effects.
19. Temperature remaining constant allows for efficient energy transfer.
20. Dry ice sublimates directly into gas.

Hard Level Answers

1. $$Q = m \times L$$ helps in calculating latent heat.
2. $$Q = 0.5 \times 334,000 = 167,000 , \text{J}$$ .
3. Latent heat helps move water between liquid and vapour states, crucial for rain and clouds.
4. Latent heat can absorb or release energy, impacting weather and temperature.
5. Boiling involves molecules gaining energy and escaping into the gas phase.
6. Different substances have unique latent heat values due to their molecular structures.
7. Vaporisation requires more energy because it breaks stronger intermolecular forces.
8. Climate change can be influenced by changes in latent heat distribution.
9. Molecules gain energy and move apart rapidly in sublimation.
10. Latent heat contributes to storm development and precipitation.
11. Materials differ in specific latent heat, affecting their uses.
12. Thermal storage systems use latent heat to store and release energy efficiently.
13. Industrial processes like cooling and freezing depend on understanding latent heat.
14. $$Q = 2 \times 2,260,000 = 4,520,000 , \text{J}$$ .
15. Strong intermolecular forces require more energy to change the state.
16. Ice skating relies on the latent heat of fusion to create a thin water layer.
17. Understanding latent heat can predict weather patterns and effects on ecosystems.
18. Better energy management can be achieved by applying latent heat principles.
19. Compressing a gas can increase its temperature and might change its state if sufficient heat is removed.
20. Understanding latent heat is crucial for predicting weather and managing water resources.

This structured approach to explaining changes of state and specific latent heat should help Year 9 students grasp these important concepts in physics.