Kinetic Theory Notes

Oxford AQA IGCSE Physics: Revision notes

Key facts

Kinetic theory explains solids, liquids and gases by particle arrangement, movement and energy.
Specific heat capacity: $E = mc\Delta\theta$ .
Specific latent heat: $E = mL$ , with no change in temperature.
Impurities lower the melting point and raise the boiling point.
Flat sections on a heating curve are changes of state.

States of matter

Particles are always moving; how close they are and how freely they move decides the state.

Kinetic theory models matter as tiny moving particles. Heating gives them kinetic energy, so they move faster, which drives changes of state.

Solid

Liquid

Gas

Particle arrangement in a solid, a liquid and a gas.

Solid Liquid Gas Arrangement Regular, close Close, irregular Far apart, random Movement Vibrate around fixed positions Move around each other Move freely at high speed Energy Least Medium Most

	Solid	Liquid	Gas
Arrangement	Regular, close	Close, irregular	Far apart, random
Movement	Vibrate around fixed positions	Move around each other	Move freely at high speed
Energy	Least	Medium	Most

Solid

Arrangement:: Regular, close
Movement:: Vibrate around fixed positions
Energy:: Least

Liquid

Arrangement:: Close, irregular
Movement:: Move around each other
Energy:: Medium

Gas

Arrangement:: Far apart, random
Movement:: Move freely at high speed
Energy:: Most

Which state has particles far apart, moving randomly at high speed?

Specific heat capacity

It is the energy needed to raise the temperature of 1 kg of a substance by 1 °C.

Specific heat capacity has units J/kg°C. A high value means a lot of energy is needed to change temperature, which is why water is used in heating systems.

Here $E$ is energy in J, $m$ mass in kg, $c$ specific heat capacity and $\Delta\theta$ the temperature change in °C.

Energy to heat 2 kg of water, E = 2 × 4200 × Δθ: a 10 °C rise needs 84 000 J.

Energy change $E = m \times c \times \Delta\theta$

Worked example

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

1
Use $E = m \times c \times \Delta\theta$ .
2
$E = 2 \times 4200 \times 10$ .

What energy heats 0.5 kg of water (c = 4200 J/kg°C) by 20°C?

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 of fusion ( $L_f$ ) is for solid to liquid. Specific latent heat of vaporisation ( $L_v$ ) is for liquid to vapour. During a change of state the energy breaks or forms bonds between particles, so the temperature does not rise.

Heating 0.5 kg of ice at 0 °C steadily: the temperature is flat while melting (167 kJ) and boiling (about 1130 kJ).

Energy for change of state $E = m \times L$

Worked example

The specific latent heat of fusion of ice is 334 000 J/kg. How much energy melts 0.5 kg of ice at 0°C?

1
Use $E = m \times L_f$ .
2
$E = 0.5 \times 334\,000$ .

A pan boils steadily at 100°C. Where does the energy supplied go?

Impurities

Impurities lower the melting point of a solid and raise the boiling point of a liquid.

Impurities change the melting and boiling points of a pure substance. This is why salt is spread on icy roads: it lowers the melting point, so ice melts below 0°C.

Pure solid

With impurities

Impurity particles disrupt the regular pattern of the solid, which lowers its melting point.

Melting point

Lowered

Boiling point

Raised

Why is salt spread on roads in freezing weather?

Heating curves

Heating through changes of state gives sloping parts where the temperature rises and flat parts where the state changes.

There are five parts: the solid warms, a flat part while it melts, the liquid warms, a flat part while it boils, then the gas warms. The flat parts are energy breaking bonds, not raising temperature. Cooling curves for stearic acid are a required practical.

Heating 1 kg of ice from -20°C to steam at 120°C.

On a heating curve, what is happening in a flat section?

Try an exam question

Explain why the temperature of boiling water stays at 100°C although energy is still being supplied.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.