Solids, liquids and gases

Water is the only common substance that is naturally found as a solid, liquid or gas. Solids, liquids and gases are known as states of matter. Before we look at why things are called solids, liquids or gases, we need to know more about matter.

Matter is everything around us

Matter can be a confusing word because it has several meanings. We often hear phrases like “What is the matter?” or “It doesn’t matter”. Scientists have a different meaning for matter – matter is anything that occupies space and has mass.

Matter is made up of tiny particles. These can be atoms or groups of atoms called molecules. Atoms are like individual LEGO blocks. They are the smallest unit that anything can be broken down into without doing something extreme (like hitting a LEGO block with a hammer or smashing atoms in the Large Hadron Collider.) If atoms are like LEGO blocks, molecules are the structures you build with them. The physical characteristics of atoms and molecules decide the form or state the matter is in.

Water in its three states of matter

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Water behaves differently to most other substances because, in its solid state (ice), its particles are less densely packed than in its liquid state. This is why ice floats.

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Solid

Right now, you are probably sitting on a chair, using a mouse or a keyboard that is resting on a desk – all these things are solids. Something is usually described as a solid if it can hold its own shape and is hard to compress (squash). The particles in most solids are closely packed together. Even though the particles are locked into place and cannot move or slide past each other, they still vibrate a tiny bit.

Ice is water in its solid form or state. Ice keeps its shape when frozen, even if it is removed from its container. However, ice is different from most solids: its molecules are less densely packed than in liquid water. This is why ice floats.

Liquid

The simplest way to determine if something is a liquid is to ask this question: If I try and move it from one container to another (i.e. by pouring), will it conform to (take on the shape of) the new container?

If you have a glass of water and pour it into another glass, it clearly conforms – it takes on the shape of the glass. If you spill the water, it will go everywhere. Because it isn’t in a container, it conforms to the shape of the floor, making a big puddle!

In most liquids, the particles are less densely packed, giving them the ability to move around and slide past each other. While a liquid is easier to compress than a solid, it is still quite difficult – imagine trying to compress water in a confined container!

Water is an example of a liquid, and so are milk, juice and lemonade.

Find out more about water by looking at our wide range of resources under the water topic.

Gas

The atoms and molecules in gases are much more spread out than in solids or liquids. They vibrate and move freely at high speeds. A gas will fill any container, but if the container is not sealed, the gas will escape. Gas can be compressed much more easily than a liquid or solid. (Think about a diving tank – 600 L of gas is compressed into a 3 L cylinder.) Right now, you are breathing in air – a mixture of gases containing many elements such as oxygen and nitrogen.

Water vapour is the gaseous form or state of water. Unlike ice or water, water vapour is invisible. We exhale water vapour whenever we breathe out. We cannot see the water vapour as we exhale, but if we hold our eyeglasses or smartphone to our mouths, we can see the water vapour condensing (becoming liquid) on these objects.

Other states of matter

We’ve known about solids, liquids and gases for hundreds of years, but scientists have discovered other states. One state is plasma, which naturally occurs in lightning, and we create it in fluorescent light bulbs and plasma TVs. Another state of matter is Bose-Einstein condensate, but this state only occurs with super-low temperatures.