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True or false? One way to cause a substance to change its state is to decrease or increase its temperature.

You have probably noticed water changing its state — ice cubes melt in your drink, water turns into steam when you boil it, vapour turns to ice in the freezer. Changes of state: melting, freezing, vaporisation, condensation, sublimation, and deposition, are known as phase transitions. It’s not just water that undergoes phase transitions. In the right conditions, all matter can change states! Think of butter melting on hot toast, lava turning into solid rock, or soot depositing inside a chimney.

Many plastic, metal and glass objects are made through phase transitions too. To make these objects, we need to know when a phase transition will happen, and be able to control it. How? The state of a substance depends on the amount of energy in it. If you want a substance to change its state, you need to add or remove energy.

One way to do this is by changing the temperature of the substance. Let’s take a candle, for example. When the candle is burning, its temperature increases. Energy in the form of heat is transferred from the burning wick to the nearby wax. This heat is transformed into energy of motion in the particles in the wax.

The particles gain kinetic energy. The particles start to move faster and faster, the bonds between them break, and the particles move further apart. The wax melts. When you blow out the candle, the flame extinguishes and the temperature decreases. The heat is transferred out of the wax.

The particles lose their kinetic energy and slow down. This makes it easier for the particles to stay close together and form fixed structures. The wax turns back into a solid. As we add or remove heat, the temperature increases or decreases until it reaches the point at which phase transition occurs. During the phase transition, the energy is used to change the state of the substance, rather than to change the temperature.

The temperature remains the same, and the substance is in two different states at the same time until the phase transition is completed. There are two distinct points at which phase transitions happen. One is when a substance exists both as solid and as liquid, which happens during melting or freezing. This is the melting point. The other is when a substance transitions between liquid and gas — during boiling or condensation.

This is the boiling point. Each substance needs a different amount of energy to change from one state to another, so different substances have different melting points, and different boiling points. Another way to change the amount of energy in a substance is by changing pressure. In high pressure, particles are pressed together and forced to stay close to each other. It requires more energy to break the structure or move the particles apart.

For most substances, the higher the pressure, the higher the melting point and boiling point will be. The only exception is water — in high pressure, ice melts at lower rather than higher temperatures. In low pressure, particles can easily move away from each other. This means that less energy is needed for a substance to melt or evaporate, so the melting and boiling points are lower. So, by adjusting temperature and pressure, we can change the energy of a substance, and control phase transitions — in different substances, not just water!