Changes of State

All changes of state are physical changes, so the identity of the substance doesn’t change.

Energy and changes of states.

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

• The energy is related to the motion of the substance.

• Therefore, which state has the most energy?

Plasma.

• If energy is added, the particles move faster.

• If energy is removed, the particles move slower.

Temperature

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

Temperature

• The transfer of energy, called heat, causes the temperature of the substance to change.

Melting

• Solid Liquid

Ice water

The melting point is the temperature at which a solid becomes a liquid. Many times the melting point can be used to identify a substance.

This would make it a characteristic property!

Endothermic

• To melt, particles in a solid must overcome their attraction to each other. THIS WOULD REQUIRE AN INCREASE IN ENERGY TO INCREASE THE MOTION = ENDOTHERMIC

Freezing

• Liquid Solid

• Freezing is the reverse of melting, therefore energy must be removed. THIS IS CALLED EXOTHERMIC.

To change the freezing point of water, just add salt. People rely on this phenomenon when making homemade ice cream. To achieve temperatures cold enough to freeze ice cream, rock salt is added to the ice, thus lowering the freezing point of the ice-salt mixture!

During which changes are substances gaining energy?

Vaporization

• Liquid Gas

Steam ironing

Vaporization• Is vaporization endothermic or exothermic?

Particles must move faster = energy needed = Endothermic!

So, why do you sweat?Energy from your body helps sweat evaporate = you get cooled off!!

Condensation

• Gas Liquid• It is the same temperature as the boiling point!• For particles to be attracted together again,

energy must be lost to slow them down.

Therefore, condensation is?

Yes, exothermic!

Sublimation

• Solid Gas• See figure 18

For a solid to change to a liquid, the atoms go from being tightly packed to spread apart. Their attraction has to be overcome. What type of change is this?

Endothermic