Build Your Own Thermometer

In this experiment you will build your own thermometer.

Don’t drink rubbing alcohol, or drink out of a container that used to hold rubbing alcohol.

• Tap water
• Rubbing alcohol
• Clear, narrow-necked plastic bottle
• Food coloring
• Clear plastic straw

• Tape or clay

1. Pour equal volumes of rubbing alcohol and water into the bottle. You want the bottle to be at most 1/4 full.
2. Add a couple drops food coloring.
3. Put the straw in the bottle such that the bottom is under the liquid, but not touching the bottom of the bottle.
4. Fix the straw in place using tape or clay. Be sure to seal the bottle so that air can not get in or out of the bottle around the straw.
5. Heat the bottle and watch what happens. The easier way is to hold it in your hands for a few minutes.

1. You can cool the bottle too by putting it in the fridge.

Optional

If you want, you could try to mark on the bottle how high the liquid inside gets at different temperatures. One safe, although slow, method of doing this is to check the temperature of the air with “normal” thermometer and mark the bottle every few hours or days, depending on the temperature change.

Q: What happened when the bottle was heated? Cooled?

A: The liquid went up the straw. Down the straw (hopefully not all the way or the thermometer might not work anymore).

Q: Why did the level of the liquid change with temperature?

A: Because the air (and to a lesser degree, the liquids) in the bottle change volume with temperature. As air is heated it either expands or exerts more pressure. In trying to expand and in exerting pressure, it fights gravity and pushes some liquid up the straw.

Q: Do you think this is how a “real” thermometer works?

A: Most day to day thermometers work with exactly these principles.

Most matter expands as it is heated. Predicting the expansion of liquids with temperature can be complicated. The expansion of a gas with temperature is easier to predict.

The properties of a gas can be explained by the Ideal Gas Law, which states:

PV = nRT.

P is pressure, V is volume, n is moles (amount of a material), R is a constant (like pi), and T is temperature.

This means if a gas has its temperature doubled, the product of the volume it takes up and the pressure it exerts will double.

Several other experiments on this site discuss pressure and gasses:
Crushing a soda can with air
Air takes up space
Shrinking and Expanding Ballons

You can pour the liquids down the sink, and recycle the plastic bottle.

EnergyQuest Link

None.