Change of state questions

Change of state syllabus

Questions on change of state can test recall, calculations, explanations, diagrams, data handling and practical skills. You should be able to:

• 5.8P explain why heating a system will change the energy stored within the system and raise its temperature or produce changes of state
• 5.9P describe the changes that occur when a solid melts to form a liquid, and when a liquid evaporates or boils to form a gas
• 5.10P describe the arrangement and motion of particles in solids, liquids and gases
• 5.11P practical: obtain a temperature–time graph to show the constant temperature during a change of state
• 5.12P know that specific heat capacity is the energy required to change the temperature of an object by one degree Celsius per kilogram of mass (J/kg °C)
• 5.13P use the equation: change in thermal energy = mass × specific heat capacity × change in temperature ΔQ = m × c × ΔT
• 5.14P practical: investigate the specific heat capacity of materials including water and some solids

How to answer change of state questions

1. Sketch or describe particle spacing, arrangement and motion before explaining bulk behaviour.
2. Convert volumes and temperatures carefully; gas-law temperatures must be in kelvin.
3. State which variables are fixed when describing pressure-volume or pressure-temperature changes.
4. For practical methods, identify direct measurements, repeated readings and the calculation used.

Change of state example questions and worked answers

These examples show how information in a diagram, graph or experimental context becomes part of a complete exam answer.

Example 1: Measuring the specific heat capacity of concrete

June 2023 · Paper 2P · Question 8(a)

Question 1

Concrete on top of buildings can be used to heat water.
The photograph shows a concrete and water heating system being built into the roof of a house.

(Source: © ejhgndrn/Shutterstock)

A scientist wants to determine the specific heat capacity of concrete.
The diagram shows some of the equipment they could use.

Describe a suitable method to find the specific heat capacity of concrete.

Final answer

Measure the mass of the concrete sample with a balance and place it in the insulating material with the heater and thermometer fitted and the lid in place. Record the initial temperature. Switch on the heater and use a stopwatch to measure the heating time; record the voltmeter and ammeter readings. Find the temperature rise, ΔT = final temperature − initial temperature, and calculate the energy supplied using E = VIt. Then calculate the specific heat capacity using c = E ÷ (mΔT), with the mass in kg. Repeat the experiment and average the results.

Mark scheme points

This is one valid set of five points; the mark scheme awards any five from its list.

1. M1 Measure the concrete’s mass using a balance.
2. M2 Measure the heating time using a stopwatch or timer.
3. M3 Find the temperature change: final temperature − initial temperature.
4. M4 Calculate the energy supplied using the voltmeter reading × ammeter reading × time: E = VIt.
5. M5 Rearrange the specific heat capacity equation: c = E ÷ (m × temperature change).

Explanation

The mass and the temperature rise are needed because specific heat capacity is the energy required to raise the temperature of each kilogram of a material by one degree. The stopwatch gives the time for which electrical energy is supplied. Using the voltmeter and ammeter readings gives the heater’s power, V × I, so the energy supplied is E = VIt.

Subtract the initial temperature from the final temperature to find ΔT, then make c the subject of the equation. The insulation and lid help reduce energy transfer to the surroundings. Repeating the experiment and averaging the results helps make the result more reliable.

Common mistakes

• The examiner report noted that a response could measure the time for a temperature rise and repeat the experiment, yet still lose marks by not explaining how the mass and energy were measured. State that the mass is measured with a balance and calculate energy using E = VIt.
• Do not just quote the specific heat capacity equation: rearrange it to show how to calculate c, and use the temperature change rather than the final temperature alone.

Example 2: Evaporation in a cooling tower

June 2016 · Paper 2P · Question 8(b)

Question 2

A cooling tower is designed to transfer thermal energy away from a power station.

Hot water from the power station is sprayed into the cooling tower, as shown.

As it falls through the air, some of the hot water evaporates.

The rest of the water is collected and returned as cold water to the power station.

Explain how evaporation cools the water.

Final answer

The highest-energy water particles escape from the surface of the liquid as a gas. This reduces the average kinetic energy of the particles remaining in the water. Lower average kinetic energy means a lower temperature, so the remaining water cools.

Mark scheme points

Any three of these four points earn full marks, up to a maximum of 3 marks:

1. M1 Water particles escape from the liquid surface or become a gas.
2. M2 The highest-energy particles leave the liquid.
3. M3 The average kinetic energy of the remaining liquid particles decreases. A decrease in their average speed is also accepted.
4. M4 Link temperature to average kinetic energy: lower average kinetic energy means lower temperature.

Explanation

Evaporation occurs when particles escape from a liquid’s surface. The particles have different speeds and therefore different kinetic energies—the energy they have because they are moving.

The highest-energy particles escape, leaving particles with a lower average kinetic energy. Temperature measures how energetic the particles are on average, so the remaining water becomes cooler.

Common mistakes

• Saying only that particles escape. Explain that the highest-energy particles escape and how this cools the remaining water.
• Saying that fewer particles automatically means a lower temperature. The important change is the lower average kinetic energy, not simply the number of particles.

Practise Change of state questions

Build a focused practice set from this part of the Edexcel IGCSE Physics syllabus.