Changes of state

 

Change of state

Solids, liquids and gases are different states of matter that many substances can be in.

The state of a substance can be changed by heating it up or cooling it down. There are four main ways that a substance can change its state:

 

• Melting: This is when a solid becomes a liquid. When a solid is heated, the particles get more energy and start to vibrate. This makes the forces between the particles weaker.
• Boiling: This is also called vaporisation, and it changes a liquid into a gas. When a liquid is heated, the particles start to vibrate and move around more. This breaks the bonds between the particles, and they become a gas.
• Condensation: This is when a gas becomes a liquid. To do this, the gas has to be cooled. This makes bonds form between the particles, and they turn into a liquid.
• Freezing: This is when a liquid becomes a solid. To do this, the liquid has to be cooled, which leads to bond formation between particles.

 

How water changes state
This diagram shows the process of water changing state.

 

 

There are four main changes of state: melting, freezing, evaporating and condensing

 

 

Sublimation

Some substances can change directly from solid to gas, or from gas to solid, without going through the liquid stage. This is called sublimation. Solid carbon dioxide ('dry ice') and iodine can sublime.

 

 

Conservation of mass

Mass does not change when a substance changes state. Only how the particles are arranged, how close they are, and how they move change. This is called the conservation of mass.

For example, 20 g of water boils to form 20 g of steam or freezes to form 20 g of ice. There is change of state, but no change of mass.

 

🔬 Knowledge Check: Changes of State

Test your knowledge of how substances transition between solids, liquids, and gases.

1. What is the name of the process when a liquid turns into a gas at a specific, fixed temperature?

• Evaporation
• Boiling
• Sublimation

2. Which change of state describes a gas cooling down to become a liquid?

• Melting
• Condensation
• Freezing

3. True or False: Evaporation only takes place at the surface of a liquid.

• True
• False

4. What happens to the temperature of a substance while it is melting?

• It increases steadily.
• It stays the same.
• It decreases.

5. Sublimation is a special change of state where:

• A liquid turns into a solid.
• A solid turns directly into a gas.
• A gas turns directly into a solid.

Click to Reveal Answers

1. Boiling (Boiling happens at a fixed temperature throughout the liquid).
2. Condensation (The process of gas turning back to liquid).
3. True (Unlike boiling, evaporation only occurs at the surface).
4. It stays the same (Temperature remains constant during any change of state).
5. A solid turns directly into a gas (Without passing through the liquid phase).

 

Summary:

 

• The state of matter of a substance can be solid, liquid or gas.
• The state of a substance can change. Energy has to be added to, or taken away from, the substance to change its state.
• The temperature at which a substance changes state can be found by using heating curves and cooling curves.