Section 4: Changes in State

• Objective:explain that thermal

energy flows from a warmer substance to a cooler substance

identify examples of changes in state and explain how thermal energy is involved in each example

I. Energy and Changes in State

A. Thermal energy always flows from a warmer substance to a cooler substance.

1 . A substance changes state when its thermal energy increases or decreases.

II. Changes Between Liquid and Solid

A. The change in state from a solid to a liquid is melting

B. Melting Point

1. The temperature when substance changes from solid phase liquid.

a. Each has own melting point.

b. physical property 1. used to identify the

substance.

C. Freezing

1. Freezing change liquid to solid. a. water freezes, the temperature stays at 0oC until freezing is complete.

D. Changes between Liquid and Gas

1. Vaporization-a. from liquid to gaseous b. Two types: 1. Evaporation

a. from surface 2. Boiling

b. at surface and inside liquid

Boiling Point and Air Pressure

Boiling point of a substanceDepends upon air pressure aboveThe liquid.

The lower the air pressure the Less energy the liquid moleculesNeed to escape into the air.

At higher elevations liquids boil at lower temperature because of less air pressure. Food will take longer to cook because of the lower temp.

E. Condensation

1. Gas particles lose thermal energy

a. become a liquid.

F. Changes between Solids and Gas

a. solid directly to gasa. Dry ice becomes gaseous carbon dioxide b. iodine crystals become gaseous iodine.

Dry ice Iodine crystals

1. Sublimation

FYI: Identifying Substance

Through Changes of State

Comparing melting points and boiling points can be important steps in identifying an unknown material.

Example: chloroform melts at -64 oC boils at 61oC ethanol melts at -117oC and boils at 79oC

water melts at 0oC and boils at 100oCAll are clear, colorless liquids.

G. Relationship between temperature, energy and particle

motion

As a sample of ice is warmed and the temperature is measuredevery 30 seconds, the particles of solid water absorb the appliedheat energy. What effect does this energy have on the motion ofthe particles?

As heat energy continues to be supplied, you may notice that the solid water now begins to appear as a liquid, but the temperature is not rising. To change a solid to a liquid, many forces must be overcome. For each kilogram of ice you want to melt, 334 kJ of energy must be supplied.

Once the work of melting is accomplished, the applied heat energyCauses the water particles making up the liquid to move faster.As you can see on the graph, the temperature begins to rise.

Once the boiling point has been reached, the liquid becomes a gas, andThe temperature again remains constant. At the heat of vaporization,The forces of attraction between particles are totally overcome.This requires 2260 kJ of energy.