States of Matter: Solids, Liquids, Gases, and Changes

In a solid, particles are packed closely and held in relatively fixed positions. They still vibrate, but they do not move freely past one another. That is why a solid usually keeps both its shape and its volume unless a force changes it.

In a liquid, particles remain close together but can move around one another. A liquid keeps roughly the same volume, yet it changes shape to fit its container. In a gas, particles are much farther apart and move freely in all directions. A gas spreads out to fill the available space and can be compressed much more easily than a liquid or solid.

The particle model makes the states easier to compare because the particles themselves do not need to change identity when the state changes. Water molecules in ice, liquid water, and water vapour are still water molecules. What changes is their energy, movement, spacing, and organisation.

When a solid is heated, its particles gain energy and vibrate more strongly. At the melting point, enough energy is available for the ordered structure to break down so particles can move around one another. The solid becomes a liquid. Cooling reverses the process at the freezing point.

Further heating can turn a liquid into a gas. Evaporation happens at the surface and can occur below the boiling point because some surface particles have enough energy to escape. Boiling happens throughout the liquid when vapour bubbles can form within it. Condensation is the reverse change, when gas particles lose energy and form a liquid.

Some substances can also move directly between solid and gas. Sublimation is the change from solid to gas without becoming liquid first, while deposition is the reverse. These changes broaden the simple picture and show that phase changes depend on conditions such as temperature and pressure.

Keep these ideas together when you revise:

• •Melting changes a solid into a liquid.
• •Freezing changes a liquid into a solid.
• •Evaporation and boiling change a liquid into a gas.
• •Condensation changes a gas into a liquid.
• •Sublimation changes a solid directly into a gas.

One detail that often surprises learners is that temperature can stay constant during a change of state even while heating continues. The added energy is being used to change the attractions and arrangement between particles rather than simply making their random motion faster. After the phase change is complete, further heating can raise the temperature again.

Pressure can shift when changes of state happen because pressure affects how closely particles are pushed together. This is especially important for gases. School diagrams often show one simple melting or boiling point, but real phase behaviour depends on both temperature and pressure. The particle model remains useful because it gives you a way to reason from movement and spacing instead of memorising a disconnected table.

Watch a kettle boiling and you will see a white cloud above the spout. Most people call this steam, but water vapour, the gas, is actually invisible. Look closely and there is often a small clear gap just above the spout before the cloud appears. The white mist forms when the hot vapour meets cooler air, loses energy and condenses into millions of tiny liquid droplets that scatter light. So the visible cloud is liquid water, not gas, and it is the same process that forms clouds in the sky.

A second misconception concerns the bubbles in boiling water. They are not air, and they are not hydrogen and oxygen produced by splitting water. They are pockets of water vapour that form inside the liquid once it reaches its boiling point. You may notice a different set of small bubbles earlier, while the water is still heating. Those are dissolved gases from the air coming out of solution, because warm water holds less dissolved gas than cold water. Telling the two kinds apart is a neat test of the particle model.

Heat a gas strongly enough, or pass a strong electric current through it, and electrons can be knocked off its atoms. The result is a plasma, a mixture of charged ions and free electrons that conducts electricity and responds to magnetic fields, so it behaves differently from an ordinary gas. Plasma is often called the fourth state of matter. It may sound exotic, yet you have seen it in lightning, the glow of neon signs and the aurora. Stars, including the Sun, are made of plasma, which means most of the ordinary matter you can see in the universe is in this state.

Glass is a puzzle of a different kind. Its particles are arranged in a disordered jumble, like a liquid, yet they are locked in place, so it behaves as a solid. Scientists call it an amorphous solid, and unlike ice it softens gradually when heated instead of melting at one sharp temperature. A popular myth says that old church windows are thicker at the bottom because the glass has slowly flowed downwards over the centuries. In fact, old methods produced uneven panes, and glaziers often set the thicker edge at the bottom. Estimates suggest room-temperature glass would take far longer than the age of the universe to sag visibly.

Numbers show how much energy changes of state really take. Start with 1 kilogram of ice at minus 10 degrees Celsius. Warming it to 0 degrees takes about 21 kilojoules. Melting it, with no change in temperature, takes about 334 kilojoules. Heating the resulting water from 0 to 100 degrees takes about 418 kilojoules. Then turning that boiling water into steam takes about 2,260 kilojoules, more than five times the energy needed to heat it all the way from freezing to boiling. In total the journey takes roughly 3,000 kilojoules, three quarters of it spent on boiling.

This energy is called latent heat, from a Latin word meaning hidden, because it goes in without showing up on a thermometer. The Scottish chemist Joseph Black studied it in the eighteenth century. The hidden energy comes back out when the change reverses, which explains some everyday effects. A scald from steam is usually worse than one from boiling water at the same temperature, because the steam releases its latent heat as it condenses on your skin. Sweating cools you for the opposite reason: every gram of water that evaporates carries a large amount of energy away from your body.

The states of matter become much simpler when you imagine particles instead of memorising labels. Solids have closely packed particles in fixed positions, liquids have close particles that can move, and gases have widely spaced particles moving freely. Add or remove energy, and the arrangement can change while the substance itself remains the same.