Edexcel IGCSE Physics 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: Using a Diagram

Question 1

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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.

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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, m, of the concrete block using a balance.
• Measure the potential difference, V, across the heater and the current, I, through it. Use a stopwatch to measure the heating time, t.
• Record the initial and final temperatures and calculate the temperature change: ΔT = final temperature − initial temperature.
• Calculate the electrical energy supplied: E = V × I × t.
• Calculate the specific heat capacity using c = E ÷ (m × ΔT).
• Repeat the experiment and calculate an average value.

Mark scheme points

Any five of the following points earn the marks:

1. M1 Measure the mass of the concrete using a balance.
2. M2 Measure the heating time using a timer or stopwatch.
3. M3 Calculate ΔT = final temperature − initial temperature.
4. M4 Calculate energy supplied using E = voltmeter reading × ammeter reading × time.
5. M5 Rearrange the equation to give c = energy supplied ÷ (m × temperature change).

Explanation

The mass must be measured because specific heat capacity is defined per kilogram. The electrical power supplied to the heater is V × I, so the energy transferred in time t is:

E = V × I × t
ΔT = Tfinal − Tinitial
c = E ÷ (m × ΔT)

Use the readings from the voltmeter and ammeter rather than quoting only the supply voltage. Repeating the experiment and averaging reduces the effect of random measurement errors.

Common mistakes

• Quoting the specific heat capacity equation without rearranging it to calculate c.
• Omitting the measured mass of the concrete.
• Using only the heater voltage and ignoring the current and heating time when finding energy.
• Failing to state how the temperature change is found.
• Examiners reported that full-mark responses measured the mass, energy and temperature rise and made specific heat capacity the subject of the equation; merely quoting the equation was insufficient.

Example 2: Using a Diagram

Question 2

A student investigates ice, water and steam.

She heats up a sample of ice.

When it has all melted, she carries on heating until the water has all boiled to steam.

Complete the diagram to show how the particles are arranged in ice, water and steam.

One particle in each box has been drawn for you.

Final answer

See diagram.

Mark scheme points

1. M1 Particles in ice are in a regular arrangement.
2. M2 Particles in water are in an irregular arrangement.
3. M3 The water particles are close together, with no gaps large enough to add another particle.
4. M4 Particles in steam are randomly arranged and more widely spaced than those in water.

Explanation

For ice, draw equal-sized particles in neat rows and columns to show a regular arrangement.

For water, keep the particles close together but disturb the rows so that the arrangement is irregular. The spaces between particles must be too small to fit another particle.

For steam, spread the particles throughout the box in a random pattern. The distances between them should be clearly greater than in water.

• Ice: regular.
• Water: irregular and close together.
• Steam: random and widely spaced.

Common mistakes

• Drawing the ice particles randomly instead of in a regular pattern.
• Leaving large gaps between water particles; water must be close-packed.
• Drawing steam particles in neat rows or too close together.
• Adding arrows only: movement arrows are ignored and do not replace the required arrangements.

Practise Change of state questions

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