7.1 States of Matter & the Particle Model

Everything around you - the air in the room, the water in your bottle and the bottle itself - is matter, which simply means anything that has mass and takes up space. Questions about matter appear on ICAS Science papers from the earliest years right through to the senior papers, and from Paper F onwards you are expected to explain matter using the particle model: the idea that all matter is made of tiny particles far too small to see, and that these particles are always moving.

The Three States of Matter

A state of matter is one of the forms matter can take. You meet three of them every day, and each one is explained by how its particles are arranged and how they move.

The key insight of the particle model is that the particles never stop moving. In a solid they can only vibrate about fixed positions, which is why a block of wood or an ice cube holds its shape. In a liquid, the particles have enough energy to slip past one another while staying close together, which is why water flows and can be poured but keeps the same volume. In a gas, the particles move fast and in straight lines until they bump into each other or into the walls of their container. Those constant bumps on the walls are exactly what we feel as air pressure inside a tyre or a balloon.

Because gas particles are so far apart and moving freely in every direction, a gas always spreads out to fill any container it is placed in. That is why the smell of toast soon reaches every corner of the kitchen, and why there is no such thing as a balloon that is half full of air - the air always fills the whole space available to it.

One substance, three states

The same substance can exist as a solid, a liquid or a gas, depending on its temperature. Water is the classic example: ice is solid water, and steam is water as a gas. Here is the crucial point - the particles themselves do not change. A water particle is a water particle in all three states; only its arrangement, its spacing and its speed change. ICAS questions often test exactly this idea, asking what happens to the particles when ice melts or steam condenses, and the answer is never that new particles are made.

Changing State

When a substance moves from one state to another we call it a change of state. Each change has its own name, and you should know all five:

• Melting: solid to liquid (an ice cube softening on a hot day)
• Freezing: liquid to solid (water turning to ice in the freezer)
• Evaporation: liquid to gas (a puddle slowly disappearing)
• Condensation: gas to liquid (droplets forming on a cold bathroom mirror)
• Sublimation: solid directly to gas, skipping the liquid state (dry ice producing stage fog)

Each change is simply particles gaining or losing energy. Heating gives particles more energy, so they move faster and can overcome the forces holding them together. Cooling removes energy, so particles slow down and lock into place. Nothing is created and nothing is destroyed - the same particles are just rearranged.

Heating and cooling curves

If you heated a beaker of crushed ice and recorded the temperature every minute, your graph would look like a staircase: rising slopes separated by flat sections called plateaus. During a plateau the temperature stays constant even though you are still heating. This happens at the melting point (0 degrees Celsius for ice) and again at the boiling point (100 degrees Celsius for water). During a plateau, the energy you add is not making the particles move faster - it is doing the work of breaking particles free from their fixed arrangement. Once every particle is free, the temperature rises again. A cooling curve shows the same staircase in reverse, with plateaus where the substance condenses and freezes. ICAS loves to show an unfamiliar heating curve and ask what is happening during a flat section - the answer is always a change of state, with the temperature staying constant.

Evaporation versus boiling

Both processes turn a liquid into a gas, but they are not the same thing, and mixing them up is a classic exam trap:

• Evaporation happens at any temperature, but only at the liquid's surface, where the fastest-moving particles escape into the air.
• Boiling happens at one exact temperature - the boiling point - throughout the whole liquid, with bubbles of gas forming inside it.

A puddle evaporates on a 25 degree day without ever boiling; a kettle boils at 100 degrees even though it started evaporating gently long before that.

Worked example, ICAS style

A student hangs a wet towel on the line in the morning. By the afternoon it is dry. The next week she boils water near the bathroom sink and notices the mirror becomes misty. Explaining both events with the particle model: warmth from the sun and moving air give water particles in the towel enough energy to escape from the surface - that is evaporation. In the bathroom, fast-moving steam particles hit the cool mirror, lose energy, and clump together as tiny liquid droplets - that is condensation. Notice that in both cases nothing was created or destroyed; the same water particles simply changed how fast they were moving and how they were arranged. If you can tell that story for any stimulus the exam shows you, you have mastered this section.