Change of state

Edexcel International GCSE Physics· Section 5(c) 5.8P-5.14P· 18 min read

1. Particle Model of States & Changes of State★★☆☆☆Higher only⏱ 4 min

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📘 Definition

Change of state

A physical reversible change of a substance from one state of matter to another, caused by heating or cooling, with no change in mass or chemical identity.

When you heat a system, you transfer thermal energy to its internal energy store. This energy either increases the kinetic energy of particles (raising temperature) or breaks intermolecular bonds (causing a change of state, with no temperature change). The two key changes of state you need to describe are melting (solid → liquid) and boiling/evaporation (liquid → gas).

Solid: Particles are close-packed in a fixed regular lattice, vibrating around fixed positions. Strong intermolecular bonds hold particles in place.
Liquid: Particles are still close together, but have enough energy to move past each other and flow. Intermolecular bonds are weaker than in solids.
Gas: Particles are far apart, moving randomly at high speed. Intermolecular bonds are negligible, so gases expand to fill their container.

📐 Worked Example

Describe the changes to particle arrangement and motion when ice melts to form liquid water.

1
1. Ice is a solid: water molecules are fixed in a regular lattice, vibrating around fixed positions held by strong hydrogen bonds.
2
1. When heated, energy is transferred to the internal store of the ice, breaking some intermolecular bonds.
3
1. The molecules gain enough freedom to move past each other, forming a liquid: particles are still close, but no longer held in a fixed lattice, and can flow to take the shape of their container.

Exam tip:

Always link changes of state to particle bonding and energy stores for full marks, rather than only describing macroscopic properties.

2. Temperature-Time Graphs for Changes of State★★★☆☆Higher only⏱ 5 min

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When you heat a solid at a constant rate, a temperature-time graph will show clear plateaus (flat regions) where temperature stays constant, even as you keep adding heat. These plateaus correspond to changes of state.

📐 Worked Example

A student heats a 100g block of stearic acid from room temperature until it fully melts, recording temperature every 30 seconds. Sketch the shape of their temperature-time graph and label the plateau.

1
1. Initial sloped rising section: stearic acid is solid, temperature increases as heat raises particle kinetic energy.
2
1. Flat horizontal plateau: stearic acid melts at its melting point (≈69°C). Temperature stays constant as energy breaks intermolecular bonds instead of raising temperature.
3
1. Second sloped rising section: all stearic acid is now liquid, temperature increases again as heat raises liquid particle kinetic energy.

✓ Quick check

Test your understanding of temperature-time graphs

What does a flat region on a temperature-time graph for a heated substance indicate?
- A. No energy is being transferred to the substance
- B. The substance is undergoing a change of state
- C. The substance has reached its maximum possible temperature
Reveal answer
B —
Correct: Energy is still being added, but it is used for breaking bonds during state change rather than increasing temperature.

For the required practical to obtain a temperature-time graph: heat a solid (e.g. stearic acid) in a water bath, record temperature at regular intervals until fully melted, then plot results. Stir the substance regularly for even heating to get an accurate plateau.

3. Specific Heat Capacity & ΔQ = mcΔT Calculations★★★★☆Higher only⏱ 6 min

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📘 Definition

Specific Heat Capacity (c)

c = \frac{\Delta Q}{m \Delta T}

The amount of thermal energy required to raise the temperature of 1 kilogram of a substance by 1 degree Celsius. Units are joules per kilogram per degree Celsius (J/kg °C).

You must recall the equation for change in thermal energy, as it is not provided in the exam:

\Delta Q = m \times c \times \Delta T

Where: $Δ Q$ = change in thermal energy (J), $m$ = mass of substance (kg), $c$ = specific heat capacity (J/kg °C), $Δ T$ = change in temperature (°C, equal to change in Kelvin for temperature differences).

📐 Worked Example

Calculate the thermal energy needed to raise the temperature of 2kg of water from 20°C to 70°C. The specific heat capacity of water is 4200 J/kg °C.

1
1. List the known values: $m = 2$ kg, $c = 4200$ J/kg °C, $Δ T = 70 - 20 = 50$ °C
2
1. Substitute into the equation $Δ Q = m c Δ T$ :
3
$\Delta Q = 2 \times 4200 \times 50$
4
$\Delta Q = 420,000 \text{ J} = 420 \text{ kJ}$

Exam tip:

Always convert mass to kg before substituting into the equation, and double check units to avoid mark deductions.

4. Required Practical: Measuring Specific Heat Capacity★★★☆☆Higher only⏱ 3 min

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The required practical for SHC uses an electrical heater to supply known amounts of energy to a solid or liquid sample, while measuring temperature rise.

1. Measure the mass of the sample (e.g. aluminium block, water) using a balance, record in kg.
1. Insert an electrical heater and thermometer into the sample, insulate the sample to reduce heat loss to the surroundings.
1. Record the initial temperature of the sample.
1. Turn on the heater, record the power of the heater (or measure current and voltage) and time how long the heater is switched on for.
1. Turn off the heater, record the maximum final temperature of the sample.
1. Calculate $Δ T$ as final temperature minus initial temperature, then calculate $c$ using $c = Δ Q / (m Δ T)$ , where $Δ Q = power \times time$ (or $I V t$ for electrical energy).

5. Common Pitfalls

Wrong move:

Using mass in grams instead of kg in the ΔQ = mcΔT equation

Why:

Specific heat capacity units are J/kg °C, so mismatched mass units lead to incorrect numerical answers by a factor of 1000.

Correct move:

Always convert mass from grams to kg by dividing by 1000 before substituting into the equation.

Wrong move:

Stating that temperature increases during melting or boiling

Why:

During state changes, energy is used to break bonds, not increase particle kinetic energy, so temperature stays constant.

Correct move:

Explicitly state that changes of state occur at constant temperature, corresponding to plateaus on temperature-time graphs.

Wrong move:

Describing gas particles as slow moving or close together

Why:

Gas particles have high kinetic energy, move randomly at high speeds, and are far apart with negligible intermolecular forces.

Correct move:

Link particle motion directly to state: solids vibrate in fixed positions, liquids flow close together, gases move fast and far apart.

Wrong move:

Using absolute temperature instead of temperature difference for ΔT

Why:

The equation only calculates the energy needed for a change in temperature, not total internal energy of the substance.

Correct move:

Subtract initial temperature from final temperature to get ΔT for all calculations.

Wrong move:

Ignoring heat loss errors in SHC practical questions

Why:

Heat lost to the surroundings means measured temperature rise is lower than expected, leading to an overestimation of SHC.

Correct move:

Identify heat loss as the main source of error, and suggest improvements like adding insulation or lids to reduce this.

6. Quick Reference Cheatsheet

Concept	Key Details	Units/Equation
Change of state	Constant temperature, energy breaks bonds, no mass change	N/A
Solid particles	Close-packed fixed lattice, vibrating around fixed positions	N/A
Liquid particles	Close, free to flow, weaker intermolecular bonds	N/A
Gas particles	Far apart, fast random motion, negligible bonds	N/A
Specific Heat Capacity (c)	Energy to raise 1kg of substance by 1°C	J/kg °C
Thermal energy change	Must recall for exam	$Δ Q = m \times c \times Δ T$
SHC Practical Error	Heat loss causes lower ΔT, higher calculated c	Insulate samples to reduce error

7. Frequently Asked

Do I need to memorise the ΔQ = mcΔT equation for the exam?

Yes! The specific heat capacity equation is not provided in any exam materials for Edexcel IGCSE Physics (4PH1), so you must recall it exactly for Paper 2 questions.

Why does temperature stay constant when a substance melts or boils?

During a change of state, all input thermal energy is used to break intermolecular bonds and increase particle separation (stored as potential energy), rather than increasing the kinetic energy of particles (which raises temperature).

Going deeper

syllabusEdexcel 4PH1 2017 Spec Section 5Refer to pages 32-33 for official topic guidance

What's Next

Now that you have mastered change of state and specific heat capacity, you can move on to the remaining topics in the Solids, Liquids and Gases unit for Edexcel IGCSE Physics. Kinetic theory and gas laws are core topics examined in both Paper 1 and Paper 2, so they are a high priority for your revision. You should also practice past paper questions on SHC calculations and practical method evaluation to build your exam technique, as these questions are common in Paper 2 Higher tier. Make sure you can accurately interpret temperature-time graphs and explain state changes using the particle model, as these are often 3-4 mark extended response questions.