Changes of State

1. Changes of State Pages 38 - 43

2. Change of State • The conversion of a substance from one physical form to another. • The physical identity of the substance does not change. • H2O is the same substance as a solid (ice), liquid (water) or gas (steam).

3. Energy and State Changes • Energy of a substance changes with a change in state. • Energy is related to the motion of the particles of a substance. • If energy is added to a substance  particles move faster. • If energy is removed from a substance  particles move slower.

4. Temperature • A measure of the speed of the particles of the substance and therefore a measure of energy. • A transfer of energy, like heat, causes the temperature of a substance to change. • This can also cause a change in state.

5. Melting • The change of state from a solid to a liquid. • Ex) When an ice cube melts into water • Each substance has its own melting point • Everything does NOT melt at 32 degrees Fahrenheit, 0 degrees Celsius. • Ex) Gallium melts at 30 degrees Celsius

6. For a substance to melt its particles must overcome their attraction to each other. • When melting, a substance’s particles are absorbing energy and increasing their motion. • Since particles are increasing their motion, we call melting an endothermic change.

7. Freezing • The change of state from a liquid to a solid. • The temperature at which a substance freezes is the freezing point. • For a substance to freeze, its particles must slow their motion to the point where they cannot overcome their attraction to each other.

8. At the freezing point of a substance, the particles are releasing energy and slowing down their motion. • Freezing is an exothermic process. • Freezing and Melting occur at the same temperature. • Melting and freezing point are the same.

9. Vaporization • The change of state from a liquid to a gas. • Boiling is vaporization that occurs throughout a liquid. • Energy is being added to particles so vaporization is endothermic. • The temperature at which a liquid boils is boiling point. • Boiling point of water is 100 degrees Celsius or 212 degrees Fahrenheit.

10. Evaporation • Vaporization that occurs at the surface of a liquid below its boiling point. • Ex) sweating – your body cools off because water is absorbing energy from your skin as it evaporates

11. Pressure and boiling point • Water boils at 100 degrees Celsius at sea level. • Atmospheric pressure (pressure of air) depends on your altitude. • Pressure decreases with increasing altitude. • Higher altitudes allow substances to boil at lower temperatures because of less atmospheric pressure.

12. Condensation • The change of state from a gas to a liquid. • Condensation point is the temperature at which a substance changes to a liquid. • It is the same temperature as boiling point. • For a gas to become a liquid, particles will clump together. • Energy is being removed from the particles so condensation is exothermic.

13. Sublimation • The change of state from a solid to a gas. • Ex) dry ice • For a solid to change directly into a gas, the particles must move from being very tightly packed to being very spread apart. • A large amount of energy is absorbed by the particle in a short amount of time. • Sublimation is endothermic.

14. Temperature and Change of State • When particles of a substance gains or loses energy, it will change its temperature or its state. • One or the other will happen, not both at the same time.

15. State of Matter Diagram