The three states of matter are solid, liquid and gas. Everything around you, from the screen you’re reading this on to the air filling your lungs right now, is built from matter, and matter only ever behaves in one of three familiar ways. Scientists call these the states of matter: solid, liquid and gas. Understanding why ice, water and steam are really the same substance behaving differently sits at the heart of the UK National Curriculum for primary and lower secondary science, and it’s one of the first places children encounter the idea that everything physical is made of tiny moving particles.
This guide goes beyond a simple list of the three states. It explains the particle model that makes sense of why solids hold their shape, why liquids pour, but gases don’t, and why a handful of unusual substances, like the plasma inside a star or a bowl of cornflour slime, don’t fit neatly into any of the three categories at all. Along the way, you’ll find curriculum milestones for Year 4 and Key Stage 3, practical classroom activities, and kitchen experiments parents can try with children at home.
Whether you’re a teacher planning a science unit, a parent helping with homework, or a curious learner working through the topic for the first time, this article is built to answer the questions that come up most: what the three states of matter actually are, why gases compress and liquids don’t, and what the mysterious “fourth state” of matter turns out to be. LearningMole’s science resources for KS2 cover this topic in more depth, with curriculum-aligned videos and activities for Years 3 to 6.
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Matter is anything that has mass and takes up space, which covers everything physical in the universe, essentially: the chair you’re sitting on, the water in your glass, and the air around you. What determines whether a particular sample of matter is solid, liquid, or gas isn’t what it’s made of, but how its particles are arranged and how much energy they have.
All matter is built from atoms and molecules, and these particles are constantly moving. The kinetic theory of matter explains that a substance’s state depends on two things: the spacing between its particles and the energy those particles have to move. In a solid, particles are packed tightly and mostly just vibrate in place. In a liquid, particles have enough energy to slide past each other while staying close together.
In a gas, particles have so much energy that they fly apart and move independently in every direction. This is the “big idea” that Key Stage 3 science builds on directly: the particle model isn’t just a way of describing solids, liquids and gases, it’s the explanation for almost everything about how materials behave.
Adding or removing heat energy changes how fast particles move, and that’s what pushes a substance from one state into another. Heat a solid enough and its particles vibrate hard enough to break free of their fixed positions, so it melts into a liquid. Heat that liquid further, and its particles gain enough energy to escape each other’s pull entirely, turning it into a gas.
Cool a gas down, and the reverse happens. Nothing about the atoms themselves changes during this process, only their energy and spacing. Water is the clearest UK classroom example: the same two hydrogen atoms and one oxygen atom exist whether the water is ice, liquid or vapour.
A solid holds its own shape and doesn’t need a container to keep its form, because its particles are locked into a regular, tightly packed arrangement.
Inside a solid, particles sit in fixed positions relative to one another and vibrate without moving far from their spot. This rigid arrangement is why solids maintain a fixed shape and volume, and why they’re generally difficult to compress. There’s simply very little space between the particles for them to be squeezed into. Some solids are soft, like a lump of clay, and some are extremely hard, like granite, but in every case, the particles maintain their relative positions.
Children can spot solids everywhere without needing a laboratory: a wooden ruler, a slice of bread, a radiator, a school bag, and ice cubes from the freezer. A useful classroom or kitchen-table test is to ask whether an object keeps its shape when it’s moved to a different container.
A brick keeps its shape in a box, on a shelf, or on the floor, which confirms it’s solid. This kind of hands-on sorting activity, comparing solids, liquids and gases around the house, works particularly well for Years 1 and 2, building the foundation that Key Stage 2 then extends with the particle explanation.
A liquid flows and takes the shape of its container, but unlike a gas, it keeps a fixed volume rather than spreading to fill all available space.
Liquid particles stay close together, similar to a solid, but they aren’t locked into fixed positions. They can slide and roll past one another, which is what allows a liquid to flow and reshape itself. Pour milk from a carton into a bowl, and the milk takes the shape of the bowl, but the amount of milk stays the same. This is also why some liquids flow faster than others.
Thicker liquids like honey have stronger forces between their particles, resisting the sliding motion, a property scientists call viscosity. Thinner liquids, like water, have weaker forces between particles, so they flow much more readily.
Because liquid particles aren’t held in a rigid pattern, nothing stops them from moving to fill the lowest available space in a container, which is why liquids always settle with a flat, level surface. Compressing a liquid into a smaller space is difficult, though, because even though the particles can move around each other, they’re still packed closely together with very little gap between them.
A gas has no fixed shape or volume. Its particles are spread far apart and move quickly and randomly in all directions, allowing a gas to expand to fill any container completely.
This is one of the questions children ask most often, and the answer comes straight from the particle model. Gas particles are separated by much more empty space than in solids or liquids, roughly 10 times further apart than in a liquid. Squeezing a gas pushes those particles closer together, closing that empty space, which is exactly why a bicycle pump works and why a fizzy drink can be carbonated under pressure. Solids and liquids barely compress at all because their particles are already touching or nearly touching, so there’s almost no gap left to close.
The air around us is mostly nitrogen and oxygen, with smaller amounts of carbon dioxide and other gases. Steam is water in its gas state, and helium is a gas light enough to lift a balloon. It’s worth clearing up a common mix-up here: strictly speaking, “gas” describes a substance that is naturally gaseous at room temperature, while “vapour” describes the gas form of something that’s usually a liquid or solid, such as water vapour. This is a small distinction, but it’s one that separates a solid understanding of the topic from a surface-level one.
Solids, liquids and gases don’t cover every kind of matter in the universe, and two unusual examples make excellent extension material for curious learners.
Plasma forms when a gas is given so much energy that its particles lose their electrons entirely, creating a charged, glowing state of matter that behaves differently from an ordinary gas. Stars, including the Sun, are made almost entirely of plasma, which makes it the most common state of matter in the universe, even though it’s rarely seen in everyday life on Earth. Lightning and the glow inside a neon sign are both plasma, too.
Mix cornflour with water in the right ratio, and you get a substance nicknamed oobleck, which behaves like a liquid when left alone but turns firm when squeezed or hit. This is a non-Newtonian fluid, a substance that doesn’t follow the usual rules for how liquids respond to force. Punch it quickly, and your fist meets something almost solid; scoop it slowly, and it drips through your fingers like a liquid. Custard and quicksand behave the same way.
This is a brilliant classroom challenge because it forces children to question the assumption that every substance fits neatly into one category, which is exactly the kind of scientific thinking Key Stage 2 and Key Stage 3 both aim to build.
Most changes between states of matter are physical changes, meaning they can be reversed simply by adding or removing heat, without creating a new substance.
Melting turns a solid into a liquid, and freezing reverses it back into a solid; water melts and freezes at 0°C. Evaporation turns a liquid into a gas at its surface, as in a puddle drying on warm pavement, while boiling is the same liquid-to-gas change happening throughout the whole liquid at once, which is why a kettle needs to reach 100°C before it properly boils.
Condensation is the reverse of evaporation, turning a gas back into a liquid, as when warm breath meets a cold window and leaves a misty film. These four processes make up the water cycle that children study across Key Stage 2, and LearningMole’s guide to the water cycle for kids extends this topic with the stages of evaporation, condensation and precipitation in full.
A small number of solids skip the liquid stage entirely and turn straight into a gas when heated, a process called sublimation. Dry ice, which is solid carbon dioxide, is the classic classroom example: it produces a dramatic fog effect as it sublimates at room temperature without ever forming a puddle. The reverse process, gas turning directly into a solid, is called deposition, and it’s what forms frost on a cold window overnight.
Not every change of state can be undone quite so easily. When a chemical reaction happens rather than a purely physical one, for example, baking a runny cake mixture into a solid sponge, the change becomes irreversible because the ingredients have been permanently transformed into something new.
“Children often think of a gas as just invisible air, but every state of matter has real particles with real mass,” says Michelle Connolly, Founder of LearningMole and a former primary teacher with over 15 years of classroom experience. “When I taught Year 5, the moment a balloon visibly got heavier after being pumped full of air was the moment gases stopped being ‘nothing’ to my class and started being matter they could actually reason about.”
Bringing the states of matter to life in the classroom or at home works best when children can see and touch the changes happening, not just read about them.
The Human Particle Model is a simple and effective way to physically demonstrate all three states. Ask a group of children to stand shoulder to shoulder and barely move to represent a solid, then let them walk freely around a defined space to represent a liquid, then have them move quickly and randomly across the whole room to represent a gas. It costs nothing, needs no equipment, and gives children a physical memory of the particle model that a diagram alone can’t provide.
Parents don’t need a laboratory to explore this topic at home. Watching ice melt in a warm room, timing how quickly a puddle dries on a sunny versus a shady patio, or making a simple batch of oobleck with cornflour and water are all low-cost ways to reinforce what children are learning at school.
LearningMole is a UK educational platform that provides curriculum-aligned science videos and teaching resources for primary schools, and its materials and properties give parents a structured way to extend classroom learning into everyday kitchen science.
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Fixed shape? | Yes | No, takes the shape of a container | No |
| Fixed volume? | Yes | Yes | No, fills container |
| Can it flow? | No | Yes | Yes |
| Can it be compressed? | Very little | Very little | Yes, easily |
| Particle arrangement | Tightly packed, fixed positions | Close together, able to slide | Widely spread, moving freely |
| Process | Change | Everyday Example |
|---|---|---|
| Melting | Solid to liquid | Ice melting into water |
| Freezing | Liquid to solid | Water freezing in a freezer |
| Evaporation | Liquid to gas (at the surface) | A puddle drying on a warm day |
| Boiling | Liquid to gas (throughout) | A kettle reaching 100°C |
| Condensation | Gas to liquid | Breath misting on a cold window |
| Sublimation | Solid to gas (no liquid stage) | Dry ice fogging at room temperature |
| Deposition | Gas to solid (no liquid stage) | Frost forming overnight |
The three states of matter are solid, liquid and gas. Solids hold a fixed shape and volume; liquids take the shape of their container while maintaining a fixed volume; and gases expand to fill whatever space they’re given. Every state is made of the same basic particles; only the arrangement and energy of those particles differ. This is one of the first ideas taught in the UK National Curriculum for primary science.
Fire isn’t really any of the three states on its own. The flame you see is largely made up of hot gases and glowing particles, and at very high temperatures, a flame can contain plasma, the same charged state of matter found in stars. This question often catches children out, which makes it a useful discussion starter for introducing plasma as an extra state beyond the standard three.
Plasma is widely considered the fourth state of matter. It forms when a gas is heated or energised so strongly that its particles lose their electrons, creating a charged, glowing substance with different properties from an ordinary gas. Stars, lightning and neon signs are all everyday, or not so everyday, examples of plasma in action.
Some substances behave unusually and don’t fit neatly into a single category. Non-Newtonian fluids like cornflour and water mixtures, sometimes called oobleck, act like liquids when left still but behave almost like solids when hit or squeezed quickly. Sand behaves partly like a liquid when poured but supports weight like a solid when packed down, which is another example worth exploring with children.
Both processes turn a liquid into a gas, but evaporation happens only at the surface of a liquid and can occur at any temperature, which is why a puddle dries up even on a cool day. Boiling happens throughout the entire liquid at once and only occurs at a specific temperature, 100°C for water at sea level. This distinction comes up often in KS2 science assessments.
Generally, gas particles have more energy than liquid particles, which in turn have more energy than solid particles, because each state up the scale involves faster, more independent particle movement. Plasma particles have even more energy than an ordinary gas, since the energy involved is enough to strip electrons away entirely.
LearningMole provides curriculum-aligned videos and teaching resources covering states of matter, materials and their properties, and related KS2 science topics. Practical activities like the Human Particle Model, alongside simple kitchen experiments such as observing ice melt or mixing oobleck, give children a hands-on route into a topic that can otherwise feel abstract.
Solids, liquids and gases might seem like a simple starting point in science, but the particle model underneath them explains an enormous amount about how the physical world behaves, from why a bicycle pump works to why a puddle disappears on a warm day. Once children grasp that every state is built from the same particles, just arranged and energised differently, the rest of the topic tends to click into place far more easily.
The extensions beyond the standard three states, plasma and non-Newtonian fluids in particular, are what turn a straightforward topic into a genuinely engaging one. They give children permission to question tidy categories and notice that real science is often messier and more interesting than a textbook diagram suggests, which is exactly the habit of mind that Key Stage 2 and Key Stage 3 science both aim to build.
LearningMole’s curriculum-aligned videos and classroom resources are designed to support exactly this kind of teaching, providing teachers with ready-made activities and parents with a straightforward way to back up what their children are learning at school.
Explore LearningMole’s science resources for more videos and activities covering states of matter and the wider primary science curriculum, or browse the KS2 science experiments library for hands-on lesson ideas.
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