Molecular Matter — Cambridge IGCSE Co-ordinated Sciences 0654 - Physics Revision Notes & Topical Questions

Section 1 of 9The molecular (particle) model

The molecular (particle) model

All matter is made of tiny particles in constant motion.

Two ideas are central

• Two ideas are central the particles have kinetic energy, and there are forces between them.

The balance between kinetic energy and forces decides the state.

Solids

• Solids attractive forces dominate and hold particles in fixed positions, with small kinetic energy shown as vibration.

Liquids

• Liquids particles have enough energy to move past each other but not to escape completely.

Gases

• Gases kinetic energy is large compared with the forces, so the particles move freely.

Section 2 of 9Temperature and kinetic energy

Temperature

Temperature a measure of the average kinetic energy of the particles of a substance.

Heating makes particles move or vibrate faster, so the temperature rises.

At one temperature all particles have the same average kinetic energy, but individual speeds vary over a range.

Heavier particles move more slowly than lighter ones at the same temperature.

Section 3 of 9Internal energy

Internal energy

Internal energy the total of the random kinetic and potential energy of all the particles of a body.

Temperature measures the average energy while internal energy depends on the number of particles.

A cup of boiling water has a higher temperature than a bath of warm water, but the bath has more internal energy.

Section 4 of 9Evaporation explained

Liquid particles move at various speeds.

Particles near the surface with enough kinetic energy, moving the right way, overcome the attractive forces and escape as gas.

The fastest particles are lost, so the average kinetic energy of the remaining liquid falls and the liquid cools.

Evaporation is faster when:

• the temperature is higher, because more particles have enough energy,
• the surface area is larger,
• a draught removes escaped particles so they cannot return,
• the air is dry, so fewer vapour particles return to the liquid.

Boiling is different

• Boiling is different bubbles of vapour form throughout the liquid and it happens only at the boiling point,
  • where vapour pressure equals atmospheric pressure.

Section 5 of 9Explaining other properties

Density

• Density solids and liquids have closely packed particles and a high mass per volume, while gas particles are far apart and the density is low.

Compressibility

• Compressibility there is space between gas particles but almost none between those of solids and liquids.

Expansion

• Expansion on heating, particles vibrate or move more and push neighbours slightly farther apart.
• Gases expand most, then liquids, then solids.

Diffusion

• Diffusion particles spread through another substance by random motion.
• It is fast in gases, slower in liquids and negligible in solids at room temperature.

Section 6 of 9Using and limiting the model

Method

• Method decide what happens to the particles and link that to the observation.
  • (speed up, spread out, escape or collide more)

Wet washing dries faster on a warm, windy day,

• since warmth gives surface molecules more energy, wind carries vapour away and a spread-out garment has more surface.

The model does not claim that particles get hot individually or change in size.

Limits

• Limits it treats particles as hard spheres and says nothing about their internal structure.

Section 7 of 9More from the specification

1Kinetic Theory of Gas Pressure

• Gas pressure arises from continuous, random collisions of gas molecules with the interior walls of a container, exerting a net outward force per unit area.

2Absolute Temperature and Kelvin Scale

• The Kelvin temperature scale begins at absolute zero (0 K = -273°C);
  • the mean kinetic energy of gas particles is directly proportional to temperature in kelvin units.

3Gas Behavior in Elastic Containers

• When a gas-filled balloon is heated, particle kinetic energy increases,
• causing more frequent and energetic wall collisions that increase volume until internal and external pressures equalize.

4Brownian Motion

• The random, erratic zig-zag motion of microscopic smoke or pollen particles suspended in fluid demonstrates bombardment by invisible, fast-moving fluid molecules.

5Inversely proportional

• One quantity falls as the other rises in the same ratio.

6Kelvin

• SI unit of temperature on the absolute scale.

7Collision

• Short interaction with large internal forces.

8Brownian motion

• Random jiggling of small visible particles hit by fluid molecules.

9Absolute temperature

• Temperature measured in kelvin.

10p1V1

p1V1 p2V2

What the symbols mean: p₁, V₁ = pressure and volume before · p₂, V₂ = after; only for a fixed mass of gas at constant temperature

Section 8 of 9Key terms

Term	Meaning
Kinetic particle model	Matter is made of particles in constant motion with forces between them.
Temperature	Measure of the average kinetic energy of the particles.
Internal energy	Total random kinetic and potential energy of all particles.
Evaporation	Escape of fast surface particles from a liquid at any temperature.
Diffusion	Spreading of particles by random motion.
Expansion	Increase in size on heating as particles move further apart.

Section 9 of 9Exam technique

1

Saying particles themselves get hot or expand — their motion, spacing and arrangement change.

2

Mixing up temperature (average kinetic energy) with internal energy (total).

3

Saying evaporation happens only at the boiling point.

4

Forgetting that evaporation cools the liquid left behind.

5

Saying all particles at a given temperature move at the same speed — only the averages are equal.