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phasechangeimage
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.
 

glacierimage Ice - H2O as a solid

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

waterfillingglassimage


Water - H2O as a liquid
balloonimage

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.)
 
Watch a QuickTime video demonstration of the differences between the different phases of water. Be sure to return to this page when you finish watching the video!
 

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 going into changing the phase and not into raising the temperature. That's why water doesn't get hotter while it boils. 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; Fahrenheit, Celsius, & Kelvin.

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 temperature

 

Analysis:

  1. What factor(s) cause a phase change?
  2. What type of energy is being transferred during a phase change?
  3. Which phase changes release heat?
  4. Which phase changes require added energy to make them occur?
  5. Which phase has the greatest molecule motion?
  6. What term describes the change from a solid to a gas?
  7. Why does the temperature remain unchanged while a liquid boils even though heat is being added to the liquid?
  8. What do you call the process of changing from a liquid to a solid?
  9. What do you call the process of changing a liquid to a gas?
 
Visit Pioneer Library and search e-Media for the video "Changes in the Properties of Matter (Physical and Chemical)."

Review Science safetey rules here.

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Teachers should view the Teacher Site Map to relate Sci-ber text and the USOE 8th grade science core.


Updated October 24, 2008 by: Glen Westbroek

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