States of Matter & Thermal Capacity Notes

Edexcel GCSE Physics: Revision notes

Key facts

The kinetic theory explains solids, liquids and gases by how their particles are arranged and move.
Density $\rho = \dfrac{m}{V}$ . Gases are far less dense because their particles are far apart.
Changes of state are physical changes: mass is conserved.
$\Delta Q = m \times c \times \Delta\theta$ gives the energy for a temperature change.
$Q = m \times L$ gives the energy for a change of state; the temperature stays constant.

Solid

Liquid

Gas

Particle arrangement in the three states

Solids, liquids and gases

Solid particles vibrate in fixed places, liquid particles slide past each other, and gas particles move fast and far apart.

The kinetic theory models matter as particles in constant motion. Density differs between states because of how closely the particles are packed: solids and liquids are much denser than gases.

Solid

Liquid

Gas

Particle arrangement in the three states

Solid

Regular, closely packed
Vibrate about fixed positions

Liquid

Random, close together
Slide past each other

Gas

Random, far apart
Move fast in random directions

1
Measure mass
Using a balance.
2
Measure volume
By displacement for an irregular solid (measuring cylinder for a liquid).
3
Calculate
$\rho = m \div V$ .

Core practical: density of a solid

Why is a gas much less dense than a solid?

Changes of state

Mass is conserved in every change of state, and the change is reversible.

When a substance melts, freezes, evaporates, boils, condenses or sublimes, mass is conserved. These are physical changes: the material recovers its properties if the change is reversed. Many chemical changes make new substances and cannot be simply reversed.

1
Solid
melts to become a liquid
2
Liquid
boils or evaporates to become a gas
3
Gas
condenses to become a liquid
4
Liquid
freezes to become a solid

Then back to step 1

Changes of state (mass is conserved at every step)

1 kg of ice melts. What is the mass of the water produced?

Specific heat capacity

It is the energy needed to raise 1 kg of a substance by 1 °C, used to find the energy for a temperature change.

Heating transfers energy, which either raises the temperature or changes the state. Specific heat capacity is the energy to raise the temperature of 1 kg of a substance by 1 °C. $\Delta Q$ is in joules, $m$ in kg, $c$ in J/kg°C and $\Delta\theta$ in °C.

Core practical: heat a known mass of water with an immersion heater and compare the energy supplied ( $P \times t$ ) with the temperature rise.

Q = mcΔθ for 2 kg of water (c = 4200 J/kg°C): 10 °C needs 84 000 J. Horizontal axis: temperature rise (°C); vertical axis: energy (J).

Thermal energy change (J) $\Delta Q = m \times c \times \Delta\theta$

Worked example

How much energy heats 2 kg of water (c = 4200 J/kg°C) by 10 °C?

1
Write the equation: $\Delta Q = m \times c \times \Delta\theta$ .
2
Substitute: $\Delta Q = 2 \times 4200 \times 10$ .

What does a high specific heat capacity mean?

Specific latent heat

It is the energy needed to change the state of 1 kg of a substance with no change in temperature.

Specific latent heat is the energy to change the state of 1 kg of a substance without changing its temperature. $Q$ is in joules, $m$ in kg and $L$ in J/kg. During melting or boiling the energy goes into breaking bonds between particles, so the temperature stays constant.

Q = mL for melting ice (L = 334 000 J/kg): 0.5 kg needs 167 000 J. Horizontal axis: mass (kg); vertical axis: energy (J).

Specific heat capacity

Temperature change
No change of state
$\Delta Q = m c \Delta\theta$

Specific latent heat

Change of state
No change in temperature
$Q = m L$

Worked example

How much energy melts 0.5 kg of ice at 0 °C? (L = 334 000 J/kg)

1
Write the equation: $Q = m \times L$ .
2
Substitute: $Q = 0.5 \times 334\,000$ .

What happens to the temperature of water boiling at 100 °C while it is heated?

Try an exam question

Calculate the energy needed to melt 0.5 kg of ice at 0 °C and then heat the water to 20 °C. Specific latent heat of fusion of ice = 334 000 J/kg; specific heat capacity of water = 4200 J/kg°C.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.