1. Introduction to Chemistry Chapter 2 Lesson 1
Changes of State
Introduction to Chemistry
Chapter 2
Lesson 1

2. Objectives
1. Explain what happens to a substance during changes between solid and liquid.
2. Explain what happens to a substance during changes between liquid and gas.
3. Explain what happens to a substance during changes between solid and gas.
We are covering NGSS
DCI-MS-PS1.A.6
The changes of state that occur with variations in temperature or pressure can be described and predicted using these models of matter.
PE-MS-PS1-4
Develop a model that predicts and describes changes in particle motion, temperature, and state of a pure substance when thermal energy is added or removed.

3. On the Boil Planet Diary
You might have noticed that as an uncovered pot of water boils, the water level slowly decreases. The water level changes because the liquid is changing to a gas.
As you heat the water, the thermal energy of its molecules increases. The longer you leave the pot on the hot stove, the more energy is absorbed by the water molecules.
When the water molecules gain enough energy, they change state from a liquid to a gas.
The graph shows the temperature of a small pot of water on a stove set to high heat. The starting temperature of the water is 20°C.
How long does it take for the water to start boiling? At what temperature does the water boil?
Does it take more energy to heat the water to 100°C or to boil it?

4. What Happens to the Particles of a Solid as It Melts?
The change in state from a solid to a liquid is called melting.
Most pure substances have a specific temperature at which this happens, called the melting point.
At its melting point, the particles of a solid vibrate so fast that they break free from their fixed positions.
The reverse of melting is the change of state from a liquid to a solid. This is called freezing.
At its freezing point, the particles of a liquid are moving so slowly that they begin to take on fixed positions.
The freezing point of water, 0°C, is the same as its melting point.

5. What Happens to the Particles of a Liquid as It Vaporizes?
The change from a liquid to a gas is called vaporization.
Vaporization occurs when the particles of a liquid gain enough energy to move independently.
Vaporization that takes place only on the surface of a liquid is called evaporation.

6. What Happens to the Particles of a Liquid as It Vaporizes?
Vaporization that takes place both below and at the surface of a liquid is called boiling.
The temperature at which a liquid boils is called its boiling point. 
The reverse of vaporization is called condensation; it is the change in state from a gas to a liquid.
Condensation occurs when particles in a gas lose enough thermal energy to form a liquid.

7. What Happens to the Particles of a Solid as It Sublimes?
Sublimation occurs when the 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.