OCR GCSE Physics: Matter — Revision Notes

Condensed for the final weeks. For the full explanation, use the Matter study guide.

Density

rho = m / V         kg m^-3 or g cm^-3

Measuring density: for a regular solid, measure dimensions and mass. For an irregular solid, use a displacement can — the volume of water displaced equals the object’s volume. For a liquid, measure the mass of a known volume in a measuring cylinder, remembering to subtract the container’s mass.

Unit conversion: 1 g/cm³ = 1000 kg/m³. Solids and liquids have similar densities, because their particles are close together; gases are roughly a thousand times less dense, because their particles are far apart.

Particle model

State	Arrangement	Movement	Energy
Solid	Regular, close	Vibrate in place	Lowest
Liquid	Close, irregular	Slide past each other	Medium
Gas	Far apart	Fast, random	Highest

Changes of state are physical, not chemical, because no new substance is formed and the change is reversible — the particles themselves are unchanged, only their arrangement and energy.

Internal energy and heating

E = m c delta-theta        specific heat capacity
E = m L                    specific latent heat

Specific heat capacity — energy to raise 1 kg by 1 °C. Specific latent heat of fusion — energy to melt 1 kg. Of vaporisation — energy to boil 1 kg.

Temperature stays constant during a change of state because the energy supplied is used to break the bonds between particles — increasing their potential energy — rather than increasing their kinetic energy. That explanation is examined directly, and “the energy goes into changing state” alone is too vague.

On a heating curve, the flat sections are the changes of state, and the latent heat of vaporisation section is longer than fusion, because boiling must separate the particles completely rather than merely loosening them.

Worked example: two-stage heating

How much energy is needed to turn 0.50 kg of ice at 0 °C into water at 20 °C? (L of fusion = 334,000 J/kg; c of water = 4200 J/kg°C)

Melting:  E = mL = 0.50 x 334,000        = 167,000 J
Heating:  E = mc(delta-theta) = 0.50 x 4200 x 20  = 42,000 J

Total = 209,000 J

Two stages, two equations — melting takes about four times as much energy as the heating that follows, which is the point of the question.

Gas pressure

Gas pressure comes from particles colliding with the container walls, each collision exerting a small force.

Raising temperature at constant volume: particles move faster, so they collide with the walls more frequently and with greater force → pressure rises. Both effects must be stated.

Reducing volume at constant temperature: the same number of particles hit the walls more frequently in a smaller space → pressure rises. Here only the frequency changes, not the force per collision — that distinction separates a full answer from a partial one.

p1 V1 = p2 V2        (constant temperature)

Beyond the specification (not examined on this course): absolute zero (−273 °C) is where particles have minimum energy, not zero energy, and the kelvin scale (0 °C = 273 K) is built on this point. This specification only requires temperature in degrees Celsius and treats the temperature-pressure relationship qualitatively.

Doing work on a gas raises its temperature — which is why a bicycle pump warms up when compressing air. The work done on the gas increases the internal energy.

Exam traps

• Forgetting to subtract the container mass when finding a liquid’s density.
• Saying a change of state is a chemical change.
• Explaining constant temperature during melting without mentioning bonds or potential energy.
• Giving only one effect when temperature rises in a fixed volume.
• Confusing pressure increasing with depth in a liquid with pressure decreasing with height in the atmosphere.
• Using E = mcΔθ during a change of state.
• Forgetting to convert g/cm³ to kg/m³ (× 1000), or cm³ to m³, when units are mixed.

Self-test

1. How would you find the density of an irregular solid?
2. Why is melting a physical rather than chemical change?
3. Why does temperature stay constant while a substance boils?
4. Explain why heating a gas in a sealed container raises its pressure — give both effects.
5. Why does a bicycle pump get warm?
6. Convert a density of 2.7 g/cm³ into kg/m³.
7. Calculate the total energy to turn 0.50 kg of ice at 0 °C into water at 20 °C (L of fusion = 334,000 J/kg, c of water = 4200 J/kg°C).

Answers: 1. Measure its mass on a balance, then find its volume by the water displaced in a displacement can, and divide mass by volume. 2. No new substance is formed and the change is reversible — the particles are unchanged, only their arrangement and energy differ. 3. The energy supplied is used to overcome the forces between particles, increasing potential energy rather than kinetic energy, and temperature depends on kinetic energy. 4. The particles move faster, so they hit the walls more frequently and each collision exerts a greater force. 5. Work is done on the air as it is compressed, which increases its internal energy and therefore its temperature. 6. 2.7 × 1000 = 2700 kg/m³. 7. Melting: E = mL = 0.50 × 334,000 = 167,000 J; heating: E = mcΔθ = 0.50 × 4200 × 20 = 42,000 J; total = 209,000 J.