### GCSE Physics

Energy
Forces
Forces and Motion
Waves in Matter
Electricity
Magnetism and Electromagnetism
Particle Model of Matter
Atomic Structure
Space Physics

# Changes of State

Depending on the temperature, a substance can be in the form of a solid, a liquid or a gas.

For example, at room temperature, water is a liquid. However, when heated, it turns into water vapour, which is a gas. When water is put in a freezer, the temperature decreases, so it turns into ice, which is a solid.

Physical changes do not change the nature of the substance. The particles are the same in all three states, but their arrangement and movement are different.

## Different Changes of state

All matter can change between states. There are 5 different changes of state, which you can see below.

### Melting

Melting is the process that changes a solid to a liquid. When a solid is heated, the particles begin to vibrate as they gain more energy. This causes the forces between the particles to become weak.

The point at which a solid melts is known as the melting point. Each pure solid has a unique melting point.

### Evaporation and boiling

#### Boiling

Boiling is a process that changes a liquid to a gas. Applying heat to liquid will cause the particles to vibrate and move around even more. This also causes the bonds between the particles to break, forming a gas.

The point at which a liquid boils is known as the boiling point. Each pure substance has a unique boiling point.

#### Evaporation

Evaporation is a process that changes a liquid changes to a gas. However, evaporation is slower than boiling and it only occurs from the surface of the liquid. So, evaporation does not produce bubbles and it leads to cooling.

Evaporation takes place over a range of temperatures, but heating the liquid will speed up the evaporation. As the particles need high energy to escape from the surface.

### Condensation

Condensation is the process that changes a gas to a liquid. To change a gas to a liquid, the gas needs to be cooled. This causes bonds to form between the particles, which leads to liquid formation.

This process does not require energy, it can take place over a range of temperatures.

An example is when water condenses on a window.

Warm air holds more moisture than cold air. So, when warm air comes into contact with a cooler window, the excess moisture in the air will condense on the window.

### Freezing

Freezing is the process that changes a liquid to a solid. To freeze a liquid, it will need to be cooled, which causes bond formation.

As freezing is the opposite of melting, the boiling point and freezing point are the same. Each pure substance has its own freezing point.

### Sublimation

Under certain conditions, a solid can change to a gas, which is known as sublimation. An example of this is the sublimation of iodine.

When you heat iodine crystals, they form a purple iodine gas. This is an example of sublimation because iodine changes directly from a solid to a gas.

## Conservation of mass

When a substance changes state, the mass does not change. Only the arrangement, closeness and motion of the particles change. This is known as the conservation of mass.

• For example, 10g of water will evaporate to form 10g of water vapour.
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