States of matter and changes of state

In a nutshell

Matter can exist in three different 'states': solid, liquid or gas. These differ in the behaviour and arrangement of the particles. Melting, evaporation and sublimation are three reversible changes of state.

Solids

Particles in a solid are held very close together by strong forces of attraction. The particles are held in a fixed, regular arrangement and are able to vibrate, but the strong forces of attraction between them prevents them from being able to move freely. 

The mass of a solid object occupies a small volume because the particles are packed tightly together, meaning solids tend to be the most dense state of matter. 

Liquids

Particles in a liquid tend to be close together and touching, but not as close together as in a solid. This is because the forces of attraction between particles are weaker in a liquid, meaning particles in a liquid are able to move past each other in random directions and at slow speeds.

Particles in a liquid tend to occupy a larger volume than in a solid, their mass is more spread out, meaning liquids tend to be less dense than solids. Particles in a liquid form irregular arrangements due to their random motion. Because they are able to move past each other, particles in a liquid have more kinetic energy than particles in a solid. 

Gases

In a gas there are almost no forces of attraction holding the particles together, so they are able to move freely in random directions and at high speeds. This means gas particles have a lot of kinetic energy. The mass of a gas is spread out over a very large volume, so gases are less dense than liquids. 

The key differences between solids, liquids and gases are summarised in the table below:

Curiosity: Although solids tend to be more dense than liquids, this is not true for water: liquid water is more dense than ice. This is why ice floats in water!

1. Solid 2. Liquid 3. Gas

Melting

Melting is a reversible physical process in which a solid transforms into a liquid. Melting a solid requires breaking the bonds between the particles, which requires energy. The energy required to break the bonds between the particles depends on the strength of the forces between the particles, which depends on the particles involved. Every substance has its own melting point. 

Evaporation and boiling

Evaporation and boiling are reversible physical processes in which a liquid transforms into a gas. Evaporating or boiling a liquid requires overcoming the remaining forces between particles, which requires energy. 

Evaporation occurs on the surface of a liquid, whereas boiling occurs throughout the entire liquid. 

Boiling is faster, produces lots of bubbles, and does not result in cooling. Boiling occurs at the substance's boiling point.

Evaporation is much slower, does not produce bubbles, and causes the liquid to cool down. Evaporation can occur below the substance's boiling point.

Evaporation and boiling are reversible. The reverse process is condensing - where a gas turns to a liquid - during which the forces of attraction between particles get stronger.

Note: Evaporation is the process where some of the liquid turns into a gas, whereas boiling is the process whereby all the particles have enough energy to turn into a gas. 

Freezing

Freezing is the reversible physical process in which a liquid turns into a solid. A liquid freezes when enough energy has been removed from the particles that bonds start to form between the particles. The amount of bonds formed between the particles depends on the substance being frozen.

Note: The freezing point is the same as the melting point.

Condensation

Condensation is the reversible process which turns a gas into a liquid. Similar to freezing, if enough energy is removed from gas particles, they will start to condense and form weak bonds in between the particles. 

Note: The temperature for condensation to occur is the same as the boiling point. 

Sublimation

Sublimation is a reversible physical process in which a solid transforms (directly) into a gas, without passing through the liquid phase. Most substances can undergo sublimation if they are at a low enough pressure.

Sublimation is a reversible process. The reverse process is called deposition, in which a gas transforms directly into a solid without passing through the liquid phase. 

Example

Solid carbon dioxide (also known as 'dry ice') sublimates directly from a solid to a gas at atmospheric pressure.  

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