# Section 2: Changes of State

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Section 2: Changes of State
What happens to a substance during changes between solid and liquid? What happens to a substance during changes between liquid and gas? What happens to a substance during changes between solid and gas?

Thermal Energy Gases have the most thermal energy, then liquids, then solids. So changing from a solid to a liquid takes an increase in thermal energy, whereas going from liquid to solid is a decrease in thermal energy. A change in thermal energy means a change in molecular motion. Remember that particles of gases are the most free, then liquids, then solids.

Changes Between Solid and Liquid
The change in state from a solid to a liquid is called melting.

Melting Solid  Liquid Occurs at a characteristic temperature called a melting point When a substance melts, the particles in the solid vibrate so fast that they break free from their fixed positions. Increased thermal energy causes the particles to vibrate faster and temperature to increase. Once the melting point temperature is reached, energy causes particles to break free and collide with one another, thus becoming a liquid. Physical change, not chemical change

Changes Between Solid and Liquid
The change from a liquid to a solid is called freezing.

Freezing Liquid  Solid
Occurs at a characteristic temperature called a freezing point (should be same as melting point) When a substance freezes, the particles in the liquid move so slowly that they begin to take on fixed positions. Decreased thermal energy causes the particles to lose energy and move more slowly. Eventually, the liquid becomes a solid. Not always at cool temperatures, Example: Candle wax, butter, chocolate Physical change, not chemical change

Changes Between Liquid and Gas
The change from a liquid to a gas is called vaporization.

Vaporization Liquid  Gas
Takes place when the particles in a liquid gain enough energy to move independently, forming a gas. When a substance freezes, the particles in the liquid move so slowly that they begin to take on fixed positions. Freedom of motion increases Physical process, not chemical 2 main types of vaporization: Evaporation and Boiling

Vaporization EVAPORATION: BOILING:
Vaporization that takes place only on the surface of a liquid. Example: a shrinking puddle Vaporization that takes place both below the surface and at the surface of a liquid. Bubbles form below surface and rise and break the surface. Temperature a liquid boils at is called its boiling point. Boiling point of a substance depends on the pressure of the air above it. Example: at sea level water boils at 100°C, but in mountains where air pressure is lower, it boils at 95°C

Let’s look at these pictures again:

Changes Between Liquid and Gas
The change from a gas to a liquid is called condensation.

Condensation Gas  Liquid
During condensation, the particles in a gas lose enough thermal energy to forma liquid. Gas particles cannot overcome attractive forces between them, freedom of motion decreases, and they condense into a liquid. Example: clouds forming, breath on mirror Physical process, not chemical

Changes Between Solid and Gas
The change from a solid to a gas is called sublimation. Does anyone know what this a picture of?

Sublimation Solid  Gas
Occurs when surface particles of a solid gain enough energy that they form a gas During sublimation, particles of a solid do not pass through the liquid state as they form a gas Freedom of motion of particles increases Physical process, not chemical Example: Dry ice, disappearing of snow in temperatures below freezing

Let’s make a prediction!!
If you allowed the dry ice to stand at room temperature for several hours, what would be left in the glass dish? Why? Nothing would be left, because all of the carbon dioxide would sublimate and mix with the air.

Let’s Review!! True or False:
All of the processes we talked about today were physical processes. What physical state is skipped during the sublimation of a substance? True! All phase changes involve only physical processes, no chemical. The liquid state.

Let’s Review!! True or False: The reverse of vaporization is melting.
Evaporation is going from what state to what state? False! Vaporization is Liquid  Gas, so the reverse must be Gas Liquid, or condensation. (Melting is Solid  Liquid) Liquid  Gas