Changes of State Textbook pages. 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: Melting Freezing Evaporation Condensation 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 is an endothermic change.

Evaporation-Boiling Boiling occurs throughout a liquid Boiling point: the point at which a liquid boils. Water boils at 100ºC Boiling is and endothermic change.

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 STATE Change of State Direction Endothermic or exothermic Example 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 Liquid water vaporizes into steam at 100°C Condensation Gas to liquid Steam condenses into liquid at 100°C Sublimation Solid to gas Dry ice sublimes into a gas