Kinetic Particle Model of Matter

Everything around you — your chair, the air, the water in a glass — is made of tiny particles too small to see, even with a light microscope. These particles are in constant motion. Their arrangement, spacing and speed determine whether the substance is a solid, liquid or gas.

Solids. Particles are packed tightly in regular, fixed positions. They cannot move past each other, but they vibrate about their fixed points. This is why solids have a definite shape and volume.

Liquids. Particles are still close together — they touch — but the arrangement is no longer fixed. They can slide and roll past each other. This is why liquids take the shape of their container but keep a definite volume.

Gases. Particles are very far apart (about 10× the separation in a liquid) and move randomly at high speed in straight lines until they collide. A gas has no fixed shape and no fixed volume — it fills any container it is put in, and it can be compressed (the empty space between particles can be squeezed out).

The forces of attraction between particles are strongest in solids, weaker in liquids, and almost negligible in gases. Adding energy (by heating) makes the particles move faster and can overcome these attractions, changing the state.

Temperature is a measure of the average kinetic energy of the particles in a substance. When you heat a substance, you transfer energy to its particles — they move (or vibrate) faster, so the kinetic energy goes up, and the temperature goes up.

The reverse is also true. Cooling a substance removes energy and slows the particles down. At a temperature called absolute zero (−273 °C, or 0 kelvin), all particle motion stops. This is the theoretical lowest temperature possible.

Brownian motion. In 1827 the botanist Robert Brown noticed that tiny pollen grains in water jiggled randomly under a microscope. We now know the jiggling is caused by invisible water molecules bombarding the pollen grain from all directions. This is direct evidence that the particles in a fluid (gas or liquid) are in random motion.

You can see Brownian motion in a school lab using smoke in a glass cell lit from the side and viewed through a microscope. The bright specks of smoke (large particles, visible) move erratically because they are being hit at random by even smaller, faster air molecules.

A few useful temperature reference points (memorise these):

Substance	Melting point (°C)	Boiling point (°C)
Water (pure)	0	100
Oxygen	−219	−183
Iron	1538	2862

Notice that "boiling" and "melting" temperatures are properties of the substance — they don't depend on how much of it you have.

When a substance is heated steadily, its temperature does not rise smoothly all the way up. Watch what happens to ice as you heat it:

The temperature rises until it reaches 0 °C, then stays flat while the ice melts. During melting, the energy added is being used to break the bonds holding the solid lattice together, not to raise the temperature. Once all the ice has melted, the temperature rises again until it reaches 100 °C, where it stays flat while the water boils.

The main changes of state:

Process	Direction	Energy
Melting	solid → liquid	absorbed
Freezing	liquid → solid	released
Boiling / evaporating	liquid → gas	absorbed
Condensing	gas → liquid	released
Subliming	solid → gas directly	absorbed

Evaporation vs boiling. Evaporation happens only at the surface of a liquid at any temperature — the fastest-moving particles at the surface escape into the air. Boiling happens throughout the liquid when the temperature reaches the boiling point and bubbles of vapour form inside.

Three factors increase the rate of evaporation:

1. Higher temperature (particles already faster on average).
2. Larger surface area (more particles exposed at the surface).
3. Stronger draught / wind (faster-moving particles carried away, so they don't return).

This is why hanging wet washing on a windy, warm day dries it fastest.

When you blow up a balloon, the gas inside pushes outwards on the rubber. That outward push is gas pressure. The kinetic particle model explains it neatly:

• Gas particles are in random, high-speed motion.
• They constantly collide with the walls of the container.
• Each collision exerts a tiny outward force.
• The total force per unit area = pressure (measured in pascals, Pa).

Three changes affect gas pressure:

Increasing temperature at constant volume → particles move faster, hit the walls harder and more often → pressure increases.

Decreasing volume at constant temperature → particles hit the walls more often per second → pressure increases. This is why squeezing a syringe with the nozzle blocked is hard.

Adding more gas (more particles in the same volume) → more collisions per second → pressure increases. This is what happens when you pump up a bicycle tyre.

In the IB MYP these are usually described qualitatively (in words) rather than via the ideal gas equation, but the underlying picture is the same.

A typical MYP criterion-B inquiry: How does temperature affect the rate of evaporation of water?

A good plan identifies:

• Independent variable: water temperature (e.g. 10, 25, 40, 55, 70 °C).
• Dependent variable: mass of water lost per minute (g/min).
• Controlled variables: same beaker (same surface area), same volume of water at the start, same room conditions (no draught), same total time of measurement.
• Safety: care with hot water; eye protection if heating; insulating gloves for hot beaker.

A sensible method:

1. Measure 100 mL of water into a beaker and weigh it on a balance.
2. Set it on a heating mat at the target temperature; record the mass every 2 minutes for 10 minutes.
3. Repeat each temperature 3 times and calculate the mean evaporation rate (g/min) for each.
4. Plot a graph of evaporation rate (y) against temperature (x).

For criterion C (Processing and Evaluating) you'd then comment on the trend (rate rises as temperature rises), the gradient of the graph, anomalous points, and the source of any random error (slight draughts, timing, balance precision). For criterion D you'd discuss real-world relevance: drying clothes, sweat cooling, the global water cycle, or industrial freeze-drying.