Change of state
Substances can change state, usually when they are heated or cooled. For example, liquid water turns into steam when it is heated enough, and it turns into ice when it is cooled enough.
The closeness, arrangement and motion of the particles in a substance change when it changes state. Simple diagrams of particles in a solid, liquid and a gas are shown like this:
Gaining energy
The table summarises what happens to the particles in a substance when it gains energy, and it melts or boils (ie changes state):
| Melting | Evaporating or boiling | |
| Description | Solid to liquid | Liquid to gas |
| Closeness of particles | Stay close together | Become much further apart |
| Arrangement of particles | Regular to random | Stay random |
| Motion of particles | Start to move around each other | Become able to move quickly in all directions |
| Description | |
|---|---|
| Melting | Solid to liquid |
| Evaporating or boiling | Liquid to gas |
| Closeness of particles | |
|---|---|
| Melting | Stay close together |
| Evaporating or boiling | Become much further apart |
| Arrangement of particles | |
|---|---|
| Melting | Regular to random |
| Evaporating or boiling | Stay random |
| Motion of particles | |
|---|---|
| Melting | Start to move around each other |
| Evaporating or boiling | Become able to move quickly in all directions |
Evaporation happens below the boiling point of a liquid. When the liquid reaches its boiling point, evaporation happens very quickly and the liquid boils.
How substances change state
Losing energy
The table summarises what happens to the particles in a substance when it loses energy, and it freezes or condenses (ie changes state):
| Condensing | Freezing | |
| Description | Gas to liquid | Liquid to solid |
| Closeness of particles | Become much closer together | Stay close together |
| Arrangement of particles | Stay random | Random to regular |
| Motion of particles | Stop moving quickly in all directions, and can only move around each other | Stop moving around each other, and only vibrate on the spot |
| Description | |
|---|---|
| Condensing | Gas to liquid |
| Freezing | Liquid to solid |
| Closeness of particles | |
|---|---|
| Condensing | Become much closer together |
| Freezing | Stay close together |
| Arrangement of particles | |
|---|---|
| Condensing | Stay random |
| Freezing | Random to regular |
| Motion of particles | |
|---|---|
| Condensing | Stop moving quickly in all directions, and can only move around each other |
| Freezing | Stop moving around each other, and only vibrate on the spot |
Conservation of mass
The particles in a substance stay the same when it changes state - only their closeness, arrangement or motion change. This means that the mass of the substance stays the same. For example, 10 g of water boils to form 10 g of steam, or freezes to form 10 g of ice. This is called The total mass of reactants before a reaction is equal to the total mass of products after a reaction, because no atoms can be created or destroyed so the mass must remain unchanged..