Changes of State

Chapter 3 Section 3

Energy & Change of State• The change of a substance from one physical

form to another is a change of state or phase.• Remember, in a physical change the identity

of the substance does not change, only it’s physical form.

• Ice, liquid water, and steam are all made of the same molecules.

• These substances are different only in the arrangement and motion of the molecules.

Movement of Particles

• In different states, or phases, the molecules move differently because they have different amounts of energy.

• To change a substance from one state of matter to another you must add or remove energy.

• The five phase changes are melting, freezing, condensation, vaporization, and sublimation.

Melting• When heat is added to a substance to cause

it to go from a solid to a liquid, melting has occurred.

• Adding energy increases the temperature of the solid, causing the molecules to move faster.

• Melting will occur when a certain temperature has been reached, the melting point.

Melting• Look at figure 2 page 75. Why wouldn’t

gallium, a metal element, be useful as jewelry?

• Gallium melts at 30°C. Normal human body temperature is 37°C, so gallium will melt in your hand.

• Melting occurs when molecules move fast enough to overcome their attractions to each other.

• Melting is endothermic because energy is added.

Freezing

• When heat is removed from a substance and it changes from a liquid to a solid, freezing has occurred.

• The temperature at which a liquid changes to a solid is the freezing point.

• Freezing and melting will occur at the same temperature.

Freezing

• Look at figure 3 page 75. At a certain temperature, what will determine whether water will melt or freeze?

• The addition or removal of energy. • For a liquid to freeze the attractions between the

molecules most overcome their motion. • Removing energy will cause molecules to lock

into place. • This is exothermic because energy is removed.

Vaporization

• A term not used in your textbook is vaporization.

• This is the phase change from liquid to gas.• There are two types of vaporization:

– Evaporation– Boiling

• Vaporization requires the addition of heat, so it is endothermic.

Evaporation

• When liquid changes to a gas at temperatures below the boiling point, evaporation has occurred.

• Evaporation occurs at the surface of the liquid.

• Sweating helps cool your body because the water evaporates from your skin, taking the heat with it.

Boiling

• When liquid changes to a gas throughout the liquid, not just at the surface, boiling is taking place.

• The temperature at which a substance boils is the boiling point.

• Pressure will effect the boiling point of a liquid. When pressure is less, so is the b.p.

Condensation

• The change of state from a gas to a liquid is condensation.

• The condensation point is the temperature at which the gas becomes a liquid and is the same as the boiling point of the substance (at a given pressure).

• Energy must be removed for condensation to occur, so it is exothermic.

Sublimation

• The change of state from a gas to a solid or a solid to a gas (where the liquid phase is bypassed) is sublimation.

• Sublimation can be endothermic or exothermic.

• Dry ice, iodine crystals, and ice can all sublime.

Change of Temp vs. Change of State

• When most substances lose or gain energy, one of two things will happen to the substance (not both):

– The substance will change temperature

OR– The substance will change state

Change of Temp vs. Change of State

• When temperature changes, the speed of the molecules changes.

• Temperature cannot change until the change of state is complete.

• The temperature of boiling water stays at 100°C until all of the water has vaporized, no matter how much more heat you add.

Figure 7 Page 79

• Look at Figure 7 on page 79. This is a graph of the “Heat Curve” of water.

• At what temperature does water melt?

• At what temperature does water boil?

• Does the temperature change when the water is changing states?

• Why? Why not?

Additional Questions

• Why is antifreeze added to automobile engines during the summer as well as the winter?

• What is the purpose of adding salt to boiling water when making pasta, and putting salt on icy roadways in the winter?

• Answer these questions by referring to melting and boiling points.