Thermal Expansion of Water

(and other liquids)

This is an experiment that you can easily set up in a station format, or if you have access to heating plates, around your classroom.

Conceptually, this experiment can provide an easy access into explaining how thermometers work.

Concept: Thermal expansion of water

 

Set up and procedure

  1. Use a glass testtube
  2. Insert a 1-hole stopper in which you insert a glass tube (use a bit of glycerine around the bottom end which makes it slip easier into the stopper)
  3. Fill the testtube with colored water
  4. Place rubber stopper
  5. Make sure there are no air bubbles and that the column in the glass tube is low, as indicated
  6. Place testtube assembly into water which you will heat (hot plate)

Observations

You will observe that the water column in the glass tube moves up as the temperature of the water in the testtube increases. With a 20 mL testtube, and a temperature change from 20 degrees C to 80 degrees C, you should get something like 15-20 cm of change in height.

  1. What do YOU observe?
  2. How do YOU explain your observation?
  3. What is happening there?

Explanation

The water in the testtube expands as it is heated. There is only one direction to expand in, the glass tube. This glass tube is narrow so that a small change in volume will result in a considerable change in height; your signal is amplified.

This is the same phenomenon as in thermometers with a glass bulb on the lower end. Thermometers use a variety of liquids including alcohol (expands more than water) and mercury (is dangerous when spilled).

Extension

Bring thermometers. Have children look closely at thermometer. Put thermometer into water that you heat. Assist children in constructing the analogy with the experiment you did.

  1. What will happen with the expansion if we use salt water?
  2. What will happen with the expansion if we use different amounts of salt in the water?
  3. What will happen with the expansion if we use antifreeze?
  4. How can we find out if air expands as it is heated? What will our indicator be? (One solution is to put a water drop into the glass tube and have only air in the testtube. If you heat the testtube even with your hand, the water bubble will move up the glass tube.)