Understanding States of matter: IGCSE Chemistry Explained

You are already an expert on states of matter

Here's something you probably haven't considered: you interact with all three states of matter every single morning. The ice in your drink? Solid. The water you pour over it? Liquid. The steam rising from the kettle? Gas. You instinctively know the difference between these states, but IGCSE Chemistry asks you to explain why they behave differently. And that is where the particle model comes in.

Distinguishing solids, liquids, and gases

Before we get into the particle-level explanation, you need to know the big-picture properties that separate the three states. Examiners love asking you to compare them, so a table like this is your revision power tool.

Notice that liquids and solids share a fixed volume, while liquids and gases both flow. That halfway position is exactly what makes liquids interesting: they've got enough energy to move around, but not enough to escape each other completely.

The particle model: what's actually happening inside

Everything you see in that table traces back to how particles are arranged and how they move. Picture this:

• In a solid, particles are packed tightly in a regular, ordered arrangement. They don't wander around. They vibrate on the spot, held in place by strong forces of attraction between them. That's why solids keep their shape.
• In a liquid, particles are still close together, but the arrangement is irregular and constantly shifting. They slide over one another. The forces between them are weaker than in a solid, which is why liquids flow and take the shape of whatever you pour them into.
• In a gas, particles are far apart with almost no regular arrangement at all. They move rapidly in random directions, colliding with each other and the walls of their container. The forces between them are negligible, which explains why gases expand to fill any space available and are easy to compress.

Changes of state: melting, boiling, and the rest

Substances don't stay in one state forever. Add enough energy and a solid becomes a liquid, then a gas. Remove energy and the process reverses. Here are the key terms you need:

• Melting: solid to liquid
• Boiling: liquid to gas (at a fixed temperature throughout the liquid)
• Evaporation: liquid to gas (at the surface only, at any temperature below the boiling point)
• Freezing: liquid to solid
• Condensing: gas to liquid

Evaporation is the one that catches people out. It is not the same as boiling. Boiling happens at a specific temperature (the boiling point) and occurs throughout the bulk of the liquid with visible bubbles. Evaporation happens at the surface at any temperature. A puddle drying on a warm day is evaporation, not boiling.

Kinetic particle theory and changes of state

So what actually happens to the particles during a change of state? This is supplement-level material, but it is tested regularly on Cambridge IGCSE papers, so it's worth getting comfortable with.

When you heat a solid, you're giving its particles more kinetic energy. They vibrate faster and faster. At the melting point, the particles have enough energy to break free from their fixed positions and start moving over each other. The solid becomes a liquid.

Keep heating, and the liquid particles move faster still. At the boiling point, they gain enough energy to overcome the remaining attractive forces entirely. They escape into the gas phase, moving rapidly in all directions.

The crucial detail? During a change of state, the temperature stays constant even though you keep supplying heat. That energy isn't raising the temperature. It's being used to overcome the forces of attraction between particles. This is why a pot of boiling water stays at 100 degrees C no matter how high you turn the flame.

Reading heating and cooling curves

Heating and cooling curves are a favourite on exam papers, and they test whether you truly understand what happens during state changes. Here's how to read them like a pro.

Heating curve (solid to liquid to gas)

Imagine you're heating a block of ice from well below 0 degrees C until it becomes steam above 100 degrees C. The graph of temperature against time looks like a staircase with two flat steps:

1. Rising section (solid heating): Temperature increases as the solid absorbs heat. Particles vibrate faster.
2. First flat section (melting): Temperature stays constant at 0 degrees C. Energy breaks the regular structure. Solid turns to liquid.
3. Rising section (liquid heating): Temperature increases again. Particles move faster, sliding past each other with more energy.
4. Second flat section (boiling): Temperature stays constant at 100 degrees C. Energy overcomes remaining forces. Liquid turns to gas.
5. Rising section (gas heating): Temperature increases once more. Gas particles move even faster.

Cooling curve (gas to liquid to solid)

A cooling curve is the reverse. Start with a hot gas, and the temperature drops as energy is lost. You'll see flat sections at the condensing point and the freezing point, where the temperature pauses while the state change completes. The shape mirrors the heating curve but slopes downward.

Worked example

A substance is heated steadily. Its temperature is recorded every minute:

Question: What is the boiling point of this substance?

Answer: The temperature stays constant at 80 degrees C from minute 3 to minute 5. During this flat section, the substance is changing from liquid to gas. The boiling point is therefore 80 degrees C.

How temperature and pressure affect gas volume

Gases are uniquely responsive to changes in temperature and pressure. Two relationships are essential here:

Temperature and volume: When you heat a gas (at constant pressure), its volume increases. The particles gain kinetic energy, move faster, and push further apart. Cool the gas, and the opposite happens: particles slow down and the volume shrinks. Think of a balloon left in a hot car: it expands. Put that same balloon in a freezer and it shrivels.

Pressure and volume: When you increase the pressure on a gas (at constant temperature), its volume decreases. You're squeezing the particles into a smaller space. The particles themselves don't get smaller; they're just forced closer together. Decrease the pressure, and the gas expands again. A bicycle pump demonstrates this perfectly: push the handle in with your thumb over the nozzle, and you feel the resistance as the trapped gas is compressed into a smaller volume.

Diffusion: particles on the move

Have you ever walked into a room and smelled perfume even though the person wearing it is on the other side? That's diffusion. It's the net movement of particles from a region of higher concentration to a region of lower concentration.

Diffusion happens because particles are in constant random motion. In a gas, where particles move rapidly and are far apart, diffusion is fast. In a liquid, it's slower because particles are closer together and collide more often. Diffusion in solids is so slow it's essentially negligible for Cambridge IGCSE purposes.

Demonstrating diffusion in gases

The classic experiment uses a glass tube with cotton wool soaked in hydrochloric acid at one end and cotton wool soaked in ammonia solution at the other. Both release gas molecules into the tube. After a few minutes, a white ring of ammonium chloride forms inside the tube, but it forms closer to the hydrochloric acid end. Why?

Ammonia molecules (NH₃, relative molecular mass 17) are lighter than hydrogen chloride molecules (HCl, relative molecular mass 36.5). Lighter molecules move faster at the same temperature, so the ammonia travels further along the tube in the same time. The white ring forms nearer to the heavier HCl end.

Factors affecting the rate of diffusion

• Temperature: Higher temperature means particles have more kinetic energy and move faster. Diffusion speeds up.
• Molecular mass: Lighter particles diffuse faster than heavier ones at the same temperature. This is the principle behind the ammonia-HCl experiment.
• State of matter: Diffusion is fastest in gases and slowest in solids (essentially zero for exam purposes).

Common mistakes that lose marks

After going through hundreds of student responses over the years, some errors come up again and again. Don't be the person who makes these on exam day.

1. Saying particles "expand" when heated. Particles do not get bigger. They move faster and further apart. The spaces between them increase, not the particles themselves. Examiners penalise this wording harshly.
2. Confusing boiling with evaporation. Boiling occurs at a fixed temperature throughout the liquid. Evaporation happens at any temperature and only at the surface. If a question asks about clothes drying on a line, that's evaporation, not boiling.
3. Forgetting that temperature is constant during a change of state. The energy input goes into breaking intermolecular forces, not into raising temperature. A flat section on a heating curve is not a mistake in the data; it's the change of state happening.
4. Mixing up diffusion direction. Particles move from high concentration to low concentration. Not the other way round. Think of it as particles spreading out to fill available space.
5. Writing that gas particles "don't move" in a solid. All particles move in all states. In a solid, they vibrate. In a liquid, they slide. In a gas, they move rapidly and randomly. Zero motion would mean absolute zero temperature.

Quick self-check questions

Test yourself on these. If you can answer them confidently without looking back, you're in great shape for your IGCSE exam.

1. Describe the arrangement and motion of particles in a liquid.
2. Explain why the temperature remains constant while a pure substance is melting, even though heat is still being supplied.
3. A gas at 25 degrees C occupies 500 cm³. If the temperature increases to 50 degrees C at constant pressure, will the volume exactly double? Explain your reasoning.
4. Bromine vapour is released at one end of a long glass tube. After two minutes, the brown colour has spread halfway along. Explain this observation using kinetic particle theory.
5. In the ammonia-HCl diffusion experiment, the white ring of ammonium chloride forms closer to the HCl end. Explain why, using the relative molecular masses of the two gases.
6. A student says, "When you heat water, the water particles expand and take up more space." Identify the error and write a corrected version of the statement.