# Science projects: how hot & cold water changes a balloon

Written by kay tang
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Science projects on how hot and cold water changes a balloon allow students to explore the concepts of the density of matter, air pressure and surface tension. When a balloon is exposed to heat or cold, the gas inside the rubber will either expand or contract. The change in the balloon's size becomes a visual gauge of the change in air pressure. Adding hot and cold water to an experiment also enables students to explore the properties of liquids.

## Air Pressure

Investigate air pressure in a simple experiment with one balloon, a bottle and a pan of hot water. Inflate the balloon, but don't knot it. Stretch it over the mouth of an empty bottle. Set the bottle in a pan filled with hot water. Leave this experiment alone for a few minutes, and return to observe the change in the size of the balloon. Note that the hot air inside the bottle has moved into the balloon, causing the gas inside the balloon to grow warmer. Observe how the heated gas molecules repel each other, forcing the balloon to expand.

## Air Expansion and Contraction

Measure the changes in a balloon's size when exposed to hot and cold environments. Gather three identical balloons, a thermometer and a tape measure. Inflate the balloons. Measure the room temperature, and then measure the circumference of the balloons. Consider that water can turn into gas or a solid depending on the temperature. Use a shower in a small bathroom to create an environment filled with steam. Measure the temperature of the bathroom and place the three balloons in hot air. Wait about 10 minutes or until you notice that the balloons expand. Measure their circumferences. Return the balloons to room temperature, which should take about 20 minutes. Measure the temperature of a large ice box. Put the balloons in frosty air. Wait 10 minutes, remove the balloons and then measure their circumferences. Consider how hot air takes up more space than cold air, accounting for changes in a balloon's size when placed in environments with varying temperatures.

## Mixing Hot and Cold Water

Investigate what happens to a balloon when hot and cold water are mixed. Gather two narrow jars and a plastic card that can be slipped between the jars to prevent water leaking from one jar into the other. Pour hot water into one jar and cold water into the other. Place the card over the mouth of the hot jar and invert the jar over the cold one, then quickly slip the card out. Stick a balloon over the mouth of the cold jar. Measure the circumference of the balloon when it reaches its maximum diameter. Repeat the experiment; however, invert the jar of cold water over the hot one. Observe the difference in the size of the two balloons even though you use the same amounts of hot and cold water. Explain how a change in procedure causes the temperature of the water's surface to vary, which affects the balloon's size.

## Hot and Cold Fronts

Use balloons and hot and cold water to explore weather events, such as when a warm front meets a cold front. Gather two jars and food colouring. Pour hot water into one jar and cold water into the other. Use food colouring to label the jars, and then combine the contents of the jars into a bowl. Observe and record how the same liquid at two different temperatures mixes. Gather two balloons. Fill one with cold water and the other one with hot water. Draw a hot bath. Place the water balloons in the tub and observe how they move. Observe how the balloons repel each other like oil and water. Consider how the cold balloon sinks due to its greater density.

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