Shrinking and Expanding Balloons : Charles’s Law

This experiment shows how balloons (gasses) change size (volume) when their temperature is changed.

Be careful of heating devices and hot water. Burns can occur.

Required

1. balloon
2. heating device (stovetop or hotplate)
3. 2 containers (pots or beakers)
4. ice (or dry ice)
5. water

1. string

Optional

1. thermometer

1. ruler

Note – The experiment is written with two balloons – you could use one balloon, and either heat it first andthen cool it or vice versa – two is quicker because you do both at once.
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1. Blow up 2 balloons (ideally different colors so you don’t get confused) to about the same size. Do not fully inflate them, or they will pop later.

2. Measure the circumference of each balloon (how far around) with a piece of string, and then hold the string up to a ruler to record the number. (or just mark the string with a marker).

3. Heat a pot/beaker of water. You can use a thermometer or not. Hotter water will give a bigger result, but is more dangerous. Water above 60 C or 140 F is ppainful to touch, but if you are careful you can use boiling water.

4. Fill a separate container with ice. A beaker or pot works – if you have a lid that helps keep it cold (a small picnic cooler, or pot and lid are great).

5. Put one balloon on top of the hot water, put another balloon on top of/ice the ice. Remember the time.

6. After 10 minutes carefully remove the two balloons and measure each circumference. Compare to the original ones. You can either put them back and wait longer to see if anything else happens, or you can let them go back to room temperature and see if they go back to their original sizes, or you can stop here.

What did you observe?
(heated balloon grew, cooled balloon shrunk)

How can you quantify this?
(Measure circumference, or water displacement (see below) and use a thermometer on the hot water/ice – although you can’t easily get a good reading of the temperature of the balloon…)

Do other things grow and shrink when we heat and cool them?
(In general, yes, but gasses do it most, then liquids, and solids least. Also, sometimes things don’t follow this rule – water grows as it freezes, changing from a liquid to a solid – think about how a soda can left in the freeer can explode!)

Basic Explanation
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The amount of air in the balloon is roughly constant (it will slowly leak out through the balloon, which is why balloons eventually deflate, but this takes days). When you heat up a gas or mixture of gasses, such as air, it expands. This is because hotter gas molecules move more, and spread out.

Some Math
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where V and T are volume and temperature (in Kelvin) and 1 and 2 are two different conditions (condition 1, and condition 2)

This means if you double the temperature of a gas, its volume should double. Because the elastic of the balloon resists stretching you will probably not see as dramatic a volume change as the formula indicates, but withut the balloon to help visualize, it is hard to see a gas expanding.

If you pretend the balloon is a perfect sphere:
You can find the volume of the gas by 4/3 * pi * radius^2
radius is circumference divided by 2 * pi.

For a more accurate measure of the volume of the balloon you can see how much water it displaces. To do this, submerge the balloon in a full container of water (push it in with a ruler or something so your hand doesn’t displace water) that is inside a bigger, empty container. An equal volume of water to the size of the balloon will spill out. Measure how much water spilled out into your spill container using a measuring cup.

To find a Kelvin temperature add 273.15 to the Celcius tmperature. (Room temperature is about 300 Kelvin)

For a realistic example, the temperature will increase by 375/300 if you heave boiling water, so the volume should only increase ~ 30%.

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