Changing Matter - Causes of Phase Change

Another important physical property of matter is phase. The three most common phases of matter are solids, liquids, and gases. Water can exist in the solid, liquid, or gas phase. Most substances can exist in different phases. Changes in phase are also physical changes. For example, the physical properties of ice and steam are quite different but they are both water. There is no change in the chemical nature of the two substances. Solid gold and liquid gold are exactly the same chemically even though the phases (solid and liquid) are different.

Ice - H2O as a solid

Image courtesy of Dot Zangari - her Web site has several glacier photos!

Water - H2O as a liquid

Gas - H2O is usually found in air as a gas

Examples of phase changes include melting, freezing, condensation, evaporation, and sublimation. Melting occurs when a solid changes to a liquid. Freezing occurs when a liquid becomes a solid. Condensation involves a gas becoming a liquid. Evaporation involves a liquid becoming a gas and sublimation is the change of a solid directly to a gas. Phase changes require either the addition of heat energy (melting, evaporation, and sublimation) or subtraction of heat energy (condensation and freezing).

Drag your mouse over the link to open a new browser window - then watch a demonstration of the differences between the different phases of water. Close the window to return to this page.

Changing the amount of heat energy usually causes a temperature change. However, DURING the phase change, the temperature stays the same even though the heat energy changes. This energy is directed into changing the phase and not into raising the temperature. That's why the temperature of water doesn't increase as it is boiling. The temperature remains constant until the phase change is complete.

All atoms have motion. The higher the temperature of an atom, the faster the motion. A solid will merely vibrate, although your eyes cannot detect this. A liquid will allow atoms to roll around each other but not bounce out of their container. A gas is of such a high temperature that the molecules are literally bouncing off the container and taking up as much space as they are allowed. Remember--the higher the energy level or temperature, the faster and farther apart the atoms move.

Temperature is usually measured in one of three scales; degrees Fahrenheit, degrees Celsius, & Kelvin. When converting the temperature of 0o C to the Kelvin scale, you would say 0o C is equal to 273 Kelvins.

The formula for temperature conversion are as follows:

F = (9/5 C) + 32 for Fahrenheit temperature

C = 5/9 (F-32) for Celsius temperature

K = C + 273 for Kelvin


  1. Which factor(s) cause a phase change?
  2. Which type of energy is being transferred during a phase change?
  3. Which phase changes release heat?
  4. Which phase changes require that energy is added to make it occur?
  5. Which phase has the greatest molecular motion?
  6. What do you call the process of a solid changing to a gas?
  7. Why does the temperature not change while a liquid boils even though heat is being added to the liquid?
  8. What do you call the process of a liquid changing to a solid?
  9. What do you call the process of a liquid changing to a gas?