Phase changes are physical changes that occur when a substance changes from one state of matter to another. These changes happen because energy is either added to or removed from a substance, causing its particles to move differently. As particles gain energy, they move faster and spread farther apart. As they lose energy, they slow down and move closer together.

One common phase change occurs when heat is added to a liquid. As the temperature increases, the particles gain more kinetic energy and begin moving fast enough to escape into the air as a gas. This process is called vaporization, the change from a liquid to a gaseous state. The temperature at which a liquid begins to boil and rapidly vaporize is called its boiling point. During boiling, bubbles of water vapor form inside the liquid and grow larger as more heat is added.

Vaporization includes both boiling and evaporation. While boiling occurs throughout the liquid at its boiling point, evaporation happens only at the surface of a liquid and can occur at temperatures below the boiling point. For example, if a cup of water is left on a counter, the water will slowly evaporate into the air over time.

Another important phase change is melting, which occurs when a solid gains enough energy to change into a liquid. The temperature at which this happens is called the melting point. The opposite process is freezing, which occurs when a liquid loses energy and changes into a solid. The temperature at which a liquid freezes is called the freezing point.

Matter can also change from a gas back into a liquid through a process called condensation. Condensation occurs when gas particles lose energy, slow down, and move closer together. The temperature at which a gas changes into a liquid is called the condensation point. Water droplets forming on the outside of a cold glass are a common example of condensation.

Review:

1. What is boiling point?
2. When does condensation occur?
3. Explain evaporation.