Suck it Up – with Cooling Air!
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!
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.
- Two identical flat plates
- Ice cubes
- Two identical, transparent drinking glasses
- A large bowl that can fit a glass inside it
- Tap water (hot and cold)
- Optional: food coloring
- 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 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.
- Feel the temperature of the water on both plates with your finger. Is the temperature 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 ten 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 experiment?
- 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 ten minutes. Do you see any changes happening? Are the results the same as before? If yes or no, can you explain why?
Extra: Can you find out exactly how much water you 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 experiment. 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 experiments 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 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.
Ask an Expert
- Dispose of the ice cubes and water in the sink. Rinse the glasses and your hands with warm water and soap. Clean your workspace with a wet cloth.
- 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