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Thermal expansion: Experiments

Thermal expansion: Experiments

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True or false? The weight of air trapped in a balloon remains constant irrespective of changing temperatures.

Thermal expansion: Experiments

Sophia’s been to a birthday party and got a balloon. It's a cold day, and as she and Maria walk home, the balloon starts changing. Oh no! My balloon is shrinking! Is the air escaping?

Don't worry Sophia! Because it's cold, the gas in the balloon takes up less space now. Let’s go home and I’ll show you how it works! At home, Maria gets out a glass bottle with a narrow neck, two bowls, kitchen scales, and a new balloon from a packet. She fills one bowl with hot water and the other bowl with cold water and ice.

Maria stretches the opening of the balloon around the open end of the bottle and secures it tightly. This seals the bottle and traps the air inside. She weighs the bottle on the scales – it weighs 200 grams. This weight is the combined mass of the bottle, the balloon and air trapped inside. Then, Maria carefully places the bottle into the hot water.

The balloon starts growing! Maria weighs the bottle again. It weighs exactly the same as before! This means the amount of air has not changed. But somehow the air filled up the balloon.

How? When Maria places the bottle in hot water, the heat from the water transfers to the air inside the bottle. The gas particles in the bottle start moving faster and spread out. This means the volume of the gas inside increases. This is why the balloon is inflating!

The air inside the bottle is expanding because of increasing temperature. This phenomenon is known as thermal expansion. Next, Maria takes the same bottle and places it in the ice cold water. The balloon deflates immediately, as if all the air has escaped. When the girls weigh the bottle, the mass has not changed this time either.

This means that the same air must all still be inside. When Maria places the bottle in cold water, both bottle and the air inside cool down. The particles slow down, and are pulled closer together. The air takes less space, and the balloon shrinks. Decreasing temperature causes the air to contract.

The same thing can happen to liquids and solids too! Let’s see! Take a thermometer. It has a bulb filled with coloured liquid, and a very narrow tube connected to it — a capillary. Note the level of liquid in the capillary.

Next, stick the thermometer first into a cup of warm water, then into cold water. Do you notice any change in the level of the liquid inside the capillary? Just like the air in the balloon, the liquid inside the thermometer’s capillary expands when the temperature rises, and contracts when its temperature decreases. So that's how the thermometer works! To observe how solids expand, have a look at this experiment.

It involves two objects: a metal ball and a metal ring. At room temperature, the diameter of the ball and the ring are almost the same, but the ball can just pass through the ring. Now, heat up the ball for a couple of minutes. Then try to pass it through the ring again. It won’t fit!

The heated metal has increased in volume — it expanded. But let the ball cool in cold water, and it will go through the ring easily once more! Sophia, look! Since we came inside to do these experiments your balloon has gone back to normal! Hooray!