# Change of state

> Edexcel International GCSE Physics · 4PH1 2017 Spec
> Source: https://www.owlsprep.com/study/edexcel-igcse-physics-s5-change-of-state/

This guide covers change of state particle models, temperature-time graphs, specific heat capacity definitions, ΔQ = mcΔT calculations, and required practicals for Edexcel IGCSE Physics Paper 2 Higher tier.

**Prerequisites:** [Basic properties of solids, liquids and gases](https://www.owlsprep.com/study/edexcel-igcse-physics-s5-introduction-to-states-of-matter/); [Fundamental energy stores and transfers](https://www.owlsprep.com/study/edexcel-igcse-physics-s2-energy-stores-and-transfers/)

## Learning objectives

- Explain how heating changes system energy stores, temperature or state
- Describe particle arrangement and motion in solids, liquids and gases
- Interpret temperature-time graphs for changes of state
- Define specific heat capacity and use ΔQ = mcΔT for calculations
- Describe required practical methods for state change graphs and SHC measurement

## Particle Model of States & Changes of State

**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. 2. When heated, energy is transferred to the internal store of the ice, breaking some intermolecular bonds.
3. 3. 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.

*Calculator:* allowed

## Temperature-Time Graphs for Changes of State

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.

> **tip**
>
> The length of a plateau is proportional to the amount of energy needed for the change of state, but you do NOT need to calculate latent heat for this specification.

**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. 2. 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. 3. Second sloped rising section: all stearic acid is now liquid, temperature increases again as heat raises liquid particle kinetic energy.

**Check your understanding**

Test your understanding of temperature-time graphs

1. 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

   *Why:* 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.

*Calculator:* allowed

## Specific Heat Capacity & ΔQ = mcΔT Calculations

**Specific Heat Capacity (c)** — 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).

*Notation:* c = \frac{\Delta Q}{m \Delta T}

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: $\Delta Q$ = change in thermal energy (J), $m$ = mass of substance (kg), $c$ = specific heat capacity (J/kg °C), $\Delta T$ = 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, $\Delta T = 70 - 20 = 50$ °C
2. 2. Substitute into the equation $\Delta Q = mc\Delta T$:
3. $$\Delta Q = 2 \times 4200 \times 50$$
4. $$\Delta Q = 420,000 \text{ J} = 420 \text{ kJ}$$

**Exam command terms**

Common command terms for this topic:

- **Show that** — You must show all calculation steps, including unit conversions, to arrive at the given value. No marks are awarded for just writing the final answer. *(Show that the energy needed for the calculation above is ~4 × 10⁵ J.)*

- **Evaluate** — Assess the strengths and weaknesses of a practical method, including sources of error.

> **Exam tip:** Always convert mass to kg before substituting into the equation, and double check units to avoid mark deductions.

*Calculator:* allowed

## Required Practical: Measuring Specific Heat Capacity

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. 1. Measure the mass of the sample (e.g. aluminium block, water) using a balance, record in kg.
2. 2. Insert an electrical heater and thermometer into the sample, insulate the sample to reduce heat loss to the surroundings.
3. 3. Record the initial temperature of the sample.
4. 4. 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.
5. 5. Turn off the heater, record the maximum final temperature of the sample.
6. 6. Calculate $\Delta T$ as final temperature minus initial temperature, then calculate $c$ using $c = \Delta Q/(m\Delta T)$, where $\Delta Q = \text{power} \times \text{time}$ (or $IVt$ for electrical energy).

> **warning**
>
> Common sources of error in this practical include heat loss to the surroundings, incomplete thermal contact between the heater/thermometer and the sample, and uneven heating. You may be asked to suggest improvements like adding insulation or stirring the sample.

*Calculator:* allowed

## Common pitfalls

- **Wrong:** Using mass in grams instead of kg in the ΔQ = mcΔT equation
  - Why it fails: Specific heat capacity units are J/kg °C, so mismatched mass units lead to incorrect numerical answers by a factor of 1000.
  - Correct: Always convert mass from grams to kg by dividing by 1000 before substituting into the equation.
- **Wrong:** Stating that temperature increases during melting or boiling
  - Why it fails: During state changes, energy is used to break bonds, not increase particle kinetic energy, so temperature stays constant.
  - Correct: Explicitly state that changes of state occur at constant temperature, corresponding to plateaus on temperature-time graphs.
- **Wrong:** Describing gas particles as slow moving or close together
  - Why it fails: Gas particles have high kinetic energy, move randomly at high speeds, and are far apart with negligible intermolecular forces.
  - Correct: Link particle motion directly to state: solids vibrate in fixed positions, liquids flow close together, gases move fast and far apart.
- **Wrong:** Using absolute temperature instead of temperature difference for ΔT
  - Why it fails: The equation only calculates the energy needed for a change in temperature, not total internal energy of the substance.
  - Correct: Subtract initial temperature from final temperature to get ΔT for all calculations.
- **Wrong:** Ignoring heat loss errors in SHC practical questions
  - Why it fails: Heat lost to the surroundings means measured temperature rise is lower than expected, leading to an overestimation of SHC.
  - Correct: Identify heat loss as the main source of error, and suggest improvements like adding insulation or lids to reduce this.

## 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 | $\Delta Q = m \times c \times \Delta T$ |
| SHC Practical Error | Heat loss causes lower ΔT, higher calculated c | Insulate samples to reduce error |

## 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.

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