Quick Recap: The Three States of Matter
Before we explore how matter changes state, let us quickly remind ourselves of the three states. All matter is made of tiny particles that are constantly moving. The way those particles are arranged and how much energy they have determines whether something is a solid, a liquid, or a gas.
In a solid, particles are tightly packed in a fixed, regular arrangement. They vibrate in place but cannot move freely, which is why solids hold their shape. In a liquid, particles are still close together but can slide past one another, which is why liquids flow. In a gas, particles are spread far apart and move rapidly in all directions, which is why gases spread out and fill any available space.
The key idea for this guide is simple: when you add or remove heat energy from matter, you can change it from one state to another. The particles do not change but the energy they have, and therefore the way they behave, changes completely.
What Is a Change of State?
A change of state is a physical change that occurs when matter moves from one state (solid, liquid, or gas) to another. It happens when energy, almost always in the form of heat, is either added to or taken away from a substance.
The crucial thing to understand about changes of state is that they are physical changes, not chemical changes. This means the substance itself does not change i.e. no new substance is created. Water that evaporates into steam is still water. Ice that melts into liquid water is still water. The molecules are identical before and after. Only the arrangement and energy of those molecules has changed.
There are five changes of state you need to know for. Each one has a specific name depending on which direction the change goes.
Melting
Solid → Liquid · Heat is added
Melting is the change of state from a solid to a liquid. It occurs when a solid is heated to its melting point and the particles gain enough energy to break free from their fixed positions.
Think about an ice cube sitting on a warm plate. At first it holds its shape perfectly. But as heat energy flows from the plate into the ice, the particles inside start to vibrate more and more vigorously. Eventually they gain enough energy to break free of the tight bonds holding them in place. The ice starts to flow. It has become liquid water.
This is melting: the point at which a solid gains enough energy for its particles to overcome the forces holding them in fixed positions and start moving freely past one another.
The temperature at which a substance melts is called its melting point. This is a fixed temperature that is specific to each substance. Ice always melts at 0°C. Chocolate melts at around 34°C, which is why it melts in your hand. Iron has a melting point of around 1538°C, which is why it stays solid in everyday conditions.
Freezing
Liquid → Solid · Heat is removed
Freezing is the change of state from a liquid to a solid. It occurs when a liquid loses enough heat energy for its particles to slow down and lock back into fixed positions.
Freezing is simply melting in reverse. Instead of adding heat, you are removing it. When liquid water is placed in a freezer, heat energy flows out of the water and into the cold surroundings. The particles slow down progressively. Eventually they lose so much energy that they can no longer slide past one another and instead lock back into the tight, regular arrangement of a solid.
The temperature at which a liquid freezes is called its freezing point. For water, the freezing point is 0°C — exactly the same as its melting point. This is not a coincidence. The melting point and freezing point of any substance are always the same temperature. At 0°C, water and ice can exist side by side. Add heat and the ice melts. Remove heat and the water freezes.
Evaporation and Boiling
Liquid → Gas · Heat is added
Evaporation is the change of state from liquid to gas that occurs at the surface of a liquid, at any temperature. Boiling is the change of state from liquid to gas that occurs throughout the entire liquid at the boiling point.
There are actually two ways a liquid can become a gas, and understanding the difference between them is important for your exams.
Evaporation
Evaporation happens at the surface of a liquid, at any temperature, not just at the boiling point. At any temperature, some particles near the surface of a liquid will have enough energy to escape into the air and become a gas. This is why a puddle dries up on a warm day without ever boiling, and why wet clothes dry on a washing line even in cool weather. The water evaporates slowly from the surface.
The warmer the temperature, the faster evaporation happens, because more particles have enough energy to escape. Wind also speeds up evaporation by carrying away the escaped particles before they can return to the liquid.
Boiling
Boiling happens when enough heat is added to a liquid that particles throughout the entire liquid, not just at the surface, gain enough energy to escape as a gas. This creates bubbles of gas forming throughout the liquid — which is the bubbling you see when water boils on a hob. Boiling happens at a specific temperature called the boiling point. For water, the boiling point is 100°C at sea level.
Key Difference: Evaporation vs Boiling
Evaporation occurs only at the surface, at any temperature, slowly. Boiling occurs throughout the entire liquid, only at the boiling point, rapidly with visible bubbles. Both result in a liquid becoming a gas, but they happen in very different ways.
Condensation
Gas → Liquid · Heat is removed
Condensation is the change of state from a gas to a liquid. It occurs when a gas loses heat energy and its particles slow down enough to come together and form a liquid.
Condensation is evaporation in reverse. Instead of a liquid gaining energy and becoming a gas, a gas loses energy and becomes a liquid. You see this happening all the time without realising it.
When you breathe on a cold mirror, water vapour in your warm breath hits the cold surface and loses heat rapidly. The water vapour particles slow down and come together to form tiny droplets of liquid water on the mirror. That misting of the mirror is condensation. When you take a cold can of drink from the fridge on a warm day, water vapour from the warm air around it condenses on the cold surface of the can, forming droplets. Again, condensation.
Condensation also plays a crucial role in the water cycle. Water vapour rises into the atmosphere where the temperature is much cooler. There, the vapour loses heat and condenses into tiny water droplets that form clouds. When enough droplets form, they fall as rain.
Sublimation
Solid → Gas directly · No liquid stage
Sublimation is the change of state from a solid directly to a gas, without passing through the liquid stage. It is a fascinating exception to the usual sequence of changes of state.
Most substances follow the familiar pattern: solid melts into liquid, liquid evaporates into gas. But some substances, under certain conditions, can skip the liquid stage entirely and go straight from solid to gas. This is called sublimation.
The most famous everyday example of sublimation is dry ice. Dry ice is frozen carbon dioxide. At normal atmospheric pressure, carbon dioxide cannot exist as a liquid. When dry ice warms up, it goes directly from a solid to a gas, producing that thick white fog you often see at concerts, parties, and in science demonstrations. The white fog is not the carbon dioxide itself — it is water vapour from the air that has been condensed by the cold carbon dioxide gas.
Another example you might have observed at home is ice cubes that slowly shrink and disappear in the freezer over weeks without ever melting into liquid. This is sublimation happening very slowly — the ice converts directly to water vapour without becoming liquid first.
Melting Points and Boiling Points
Every substance has its own specific melting point (the temperature at which it changes from solid to liquid) and boiling point (the temperature at which it changes from liquid to gas). These temperatures are fixed properties of the substance and are always the same under the same conditions.
Melting and boiling points are extremely useful in science. They can be used to identify unknown substances, to check the purity of a substance (impurities change the melting and boiling points), and to understand which state a substance will be in at any given temperature.
Why Do Different Substances Have Different Melting and Boiling Points?
It depends on the strength of the forces between the particles. Substances with stronger forces between particles require more energy to break those forces apart, so they have higher melting and boiling points. Iron has a very high melting point because the forces between iron atoms are extremely strong. Oxygen has a very low boiling point because the forces between oxygen molecules are very weak.
You can use the melting and boiling points of a substance to work out what state it will be in at any temperature. If the temperature is below the melting point, the substance is a solid. If it is between the melting and boiling points, it is a liquid. If it is above the boiling point, it is a gas.
Revision Summary (Key Points to Remember)
- Change of state is a physical change where matter moves between solid, liquid, and gas. The substance itself does not change but the arrangement and energy of its particles does.
- Melting is the change from solid to liquid when heat is added. It occurs at the melting point.
- Freezing is the change from liquid to solid when heat is removed. The freezing point and melting point of a substance are always the same temperature.
- Evaporation is the slow change from liquid to gas at the surface of a liquid, at any temperature.
- Boiling is the rapid change from liquid to gas throughout the entire liquid, at the boiling point.
- Condensation is the change from gas to liquid when heat is removed. It is the reverse of evaporation.
- Sublimation is the direct change from solid to gas with no liquid stage. Dry ice is the most well-known example.
- Every substance has a fixed melting point and boiling point. These depend on the strength of the forces between particles.
- If a temperature is below the melting point: solid. Between melting and boiling points: liquid. Above the boiling point: gas.
- Changes of state are reversible physical changes. No new substance is created.
