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Changes of State

This page is a recap of prior knowledge from KS3, it covers what happens during a change of state, the names we give to these changes, heating curves and how we show this in a chemical reaction.

By the end of this page you should be able to:

  • Define melting, freezing, condensation, boiling and sublimation
  • Explain why substances change state
  • Identify state symbols

Five Changes You Need To Know

You need to know the names for five changes of state:

  • Melting - A solid changing into a liquid
  • Freezing - A liquid changing into a solid
  • Boiling - A liquid changing into a gas
  • Condensing - A gas changing into a liquid
  • Sublimation - A solid changing into a liquid

This information is shown in the diagram to the right. Sublimation is the rarest of the changes, only a few chemicals are know to do it, Iodine and carbon dioxide are the best known examples. If you have never seen it before you may find this video interesting Sublimation of Iodine.

Diagram showing changes of state

N.B. Knowledge of deposition is not required at GCSE it is just included for completeness.

Why Do Substances Change State?

All substances have internal energy, this causes their particles to vibrate and/or move. These particles are held together by forces. In a solid the particles do not have enough energy to overcome the forces holding them in place so they are unable to move and hence can only vibrate around a fixed position. This is the solid state of a substance.

If you heat a subtance the particles gain more internal energy. Eventually they have enough internal energy to overcome the forces holding them in place and move around each other (i.e. they are still in contact with each other). This is the liquid state of a substance.

If you continue to heat the substance then eventually the particles will gain enough energy to totally overcome the forces holding them in place and move freely. This is the gasesous state of a substance.

This means that the higher the melting and boiling point of a substance the stronger the forces between the particles in the substance.

Diagram of changes of state

Melting and Boiling Points

The melting point of a substance is the temperature at which a substance can change state from a solid to a liquid and vice versa. This is also the temperature at which it can be in either state. For example water at 0°C can be either liquid or solid, below this temperature it's a solid and above it it's a liquid.

Boiling points are similar - they're the temperature at which liquids can turn into gases and vice versa. Again a substance at its boiling point can be in either state. For example water at 100°C can be either a gas or a liquid.

You can show this using heating curves, the heating curve to the right illustrates what happens to the temperature of an ice cube when it melts.

Heating curve to show the temperature change when ice melts

1 - The ice is heated and its temperature rises
2 - Temperature stops rising - the ice is changing state to water.
3 - The temperature rises above 0°C once every ice molecule has changed state.

State Symbols

In a chemical reaction it's sometimes useful to know the state of the substances in a chemical reaction. We use the state symbols on the right to show this. Let's take a look at the example equation below:

CaCO3(s) + 2HCl(aq) → CaCl2(aq) + CO2(g) + H2O(l)

Notice how the state symbols are used. In this equation they show that a solid and a liquid are reacting to make a gas, a liquid and a dissolved substance.

State Symbols You Need To Know

s = solid
l = liquid
g = gas
aq = dissolved

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