Suck it Up – with Cooling Air!

Introduction

You might know that 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 such as air can 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, which 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 it gets hot, molecules begin to vibrate and move around very fast. This leads to a greater separation of the molecules, which results in the expansion of the material. However, when it is cold, 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 a change in temperature. If it gets hot, the liquid expands and rises in the glass tube, indicating higher temperature. If it gets cold, 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

Preparation

Procedure

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 up when submerged in hot water. Once you take it out of the bowl, the water inside the glass is replaced by air, which also gets warm. 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 (the 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 have 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.

Cleanup

More to Explore

• Getting Hotter = Getting Bigger, from Physics 4Kids.com
• Thermal Expansion - Real-life applications, from Science Clarified
• Temperature and Thermometers, from the Physics Classroom
• Balloon Morphing: How Gases Contract and Expand, from Science Buddies
• Science Activity for All Ages!, from Science Buddies

Credits