Key concepts
Physics
Gases
Temperature
Expansion and contraction

Introduction
You may know many objects expand (or get bigger) when they get hot and shrink when they cool down. For example, this is true for metals, wood and concrete. But did you know that gases do the same? It is just difficult to see when it happens. This activity will let you “see” air contract using water!

Background
Matter, such as a solid, liquid or gas, is defined as “a substance that has mass and takes up space by having volume.” If matter is heated, it has the tendency to change its shape, area and volume. This property is called thermal expansion. The reason this change happens is due to the fact that all matter is made up of atoms and molecules, which behave differently depending on whether they are hot or cold. When heated, molecules begin to vibrate and move around very fast. This leads to their greater separation, which results in the expansion of the material. When cooled, however, molecules tend to move much less, which makes them take up much less space.

Liquid thermometers are a great application of thermal expansion. The liquid, which is trapped in a little glass tube, changes its volume due to an increase or decrease in temperature. If it gets hotter, the liquid expands and rises in the glass tube, indicating higher temperature. If it gets colder, the liquid contracts and its level in the tube drops, indicating a lower temperature. The exact amount of volume change can then be correlated to the temperature change. This allows us to read the temperature from the liquid level inside the thermometer.

In this activity you will also make liquid rise inside a glass—not due to thermal expansion of the liquid but due to thermal contraction of the gas above the liquid. Confused? Then do this activity and find out how it works!

Materials

• Two identical flat plates
• Ice cubes
• Two identical, transparent drinking glasses
• A large bowl (It should be deep enough so it one of the glasses can be submerged inside it.)
• Tap water (hot and cold)
• Food coloring (optional)

Preparation

• Carefully fill the bowl with hot tap water (the hottest setting that you have).
• Submerge one of the glasses in the water so that it is completely covered and filled by water.
• Keep the other glass at room temperature.
• Place the ice cubes on one of the flat plates and add cold water, so that the whole plate is covered with ice water.
• Pour some room temperature water on the second plate so that it is also covered with water.
• Optionally, you can add one or two drops of food coloring to the water on both of the plates to make the phenomenon more visible.

Procedure

• Feel the temperature of the water on both plates with your finger. Are the temperatures very different?
• Carefully take the glass out of the bowl with hot water. How does the glass feel? Is it very hot?
• Push the ice cubes to the sides on the plate and place the glass upside down on the flat plate with the ice water. What happens to the glass when you put it on the plate? What happens to the liquid on the plate?
• Observe the glass for five to 10 minutes. Does anything change with the liquid or the glass over that time period? If yes, what happens? Can you explain your observations?
• After 10 minutes remove the glass from the plate. How does the glass feel this time? Did its temperature change during the test?
• Next take the second glass that was not submerged in the bowl and place it upside down on the second plate with room temperature water. What do you think will happen to the water on the plate and the glass this time?
• Again, observe the glass for five to 10 minutes. Do you see any changes happening? Are the results the same as before? Whether you answer yes or no, can you explain why?
• Extra: Can you find out exactly how much water your glass can suck up? Add different amounts of water to the plate and test it! You might need to use a bigger plate if you want to add more water.
• Extra: How much of a temperature difference between the glass and the water on the plate is required to suck the water up? Fill the bowl with warm water (instead of hot water) and repeat the test. You can also vary the temperature of the water on the plate. Does it need to be ice water? How big does the temperature difference need to be?
• Extra: Does changing your glass container to something bigger or smaller change your results? Does the shape of the glass matter? Design tests to find out!

Observations and results
When you put the warm glass onto the plate with cold water, you should have seen the water rise up inside the glass. The glass is initially heated when submerged in hot water. Once you take it out of the bowl the water inside the glass is replaced by air, which then is warmed, too. When you put the glass upside down onto the layer of ice water on the plate, the ice water cools down the glass and the warm air inside. This causes the air inside the glass to contract (its molecules do not bounce around as much).

The contracting air inside the glass exerts a lower pressure (the “push” air molecules exert on surfaces they are in contact with) on the water in the glass than the room-temperature air outside the glass. This causes water to be pushed up into the glass, compressing the air inside. Eventually the pressure difference balances out, and the rising water stops. In other words the shrinking volume of the air inside the glass is replaced with water. This does not happen if the glass and the water on the plate are at the same temperature. The air inside the glass has no reason to cool down, so it remains at the same volume and pressure, and there is nothing to push water up into the glass.

More to explore
Getting Hotter = Getting Bigger, from Physics 4Kids.com
Balloon Morphing: How Gases Contract and Expand, from Science Buddies
Steamy Science: Demonstrating Condensation, from Scientific American
Stretch It! How Does Temperature Affect a Rubber Band?, from Scientific American
Science Activity for All Ages!, from Science Buddies

This activity brought to you in partnership with Science Buddies