Changes of State

Textbook pp. 74-79

A change of state is the conversion of a substance from one physical form to another. All changes of state are

physical changes.

Changes Of State

There are five changes of state. They are:

1. Melting

2. Freezing

3. Evaporation

4. Condensation

5. Sublimation

During a change of state, the energy of a substance changes.

If energy is added to a substance, its particles move faster.

If energy is removed from a substance, its particles move slower.

TEMPERATURE

The temperature of a substance is a measure of the speed of its particles and is therefore a measure of its energy.

As substances heat up, the particles move faster

As particles cool down the particles move slower

MELTING

The change of state from a solid to a liquid. The melting point of a substance is the

temperature at which it changes from a solid to a liquid

Melting is an endothermic change because energy is absorbed by the substance as it changes state.

FREEZING

The change of state from a liquid to a solid. The freezing point of a substance is the temperature

at which a liquid changes into a solid. Freezing is the opposite process of melting. Freezing and melting occur at the same

temperature. Freezing is an exothermic change because energy

is taken out of (exits) the substance as it changes.

EVAPORATION

The change in state from a liquid to a gas.

Evaporation-Boiling

Boiling occurs throughout a liquid

Boiling point: the point at which a liquid boils

Water boils at 100ºC

EVAPORATION – LIQUID TO GAS

Evaporation – occurs at the surface of a liquid at a temperature below its boiling point.

Perspiration is the cooling off of the body through evaporation.

BOILING POINT

Pressure affects the boiling point of a substance.

Water boils at a lower temperature on the top of a mountain than it does at sea level.

Vacuum Pump Demo.

TEMPERATURE CHANGES VS. CHANGE OF STATE When a substance changes state its

temperature does not change until the change of state is complete.

CONDENSATION

The change of state from a gas to a liquid. Condensation is an exothermic change.

(energy exits)

SUBLIMATION

Change of state directly from a solid to a gas. Dry ice sublimes; iodine sublimes; moth balls

sublime. Sublimation is an endothermic change.

SUMMARIZING THE CHANGES OF STATEChange of State Direction Endothermic or

exothermicExample

Melting Solid to liquid Endothermic ice melts into liquid water at 0°C

Freezing Liquid to solid Exothermic Liquid water freezes into ice at 0°C

Evaporation Liquid to gas Endothermic Liquid water vaporizes into steam at 100°C

Condensation Gas to liquid Exothermic Steam condenses into liquid at 100°C

Sublimation Solid to gas Endothermic Dry ice sublimes into a gas