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# 2.26: Charles's Law

Difficulty Level: At Grade Created by: CK-12
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Practice Charles' Law

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In the 1700s and 1800s, hot air balloons were very popular. Why wouldn’t they be? Riding in a hot air balloon allowed people to travel through the air and get a bird’s eye view of the landscape at a time when airplanes did not yet exist. Do you know why hot air can inflate a balloon? Read on to find out.

### Up, Up, and Away

The popularity of hot air balloons got scientists thinking about gases and what happens to them when they heat up. In the early 1800s, two French scientists—Jacques Charles and Joseph Gay-Lussac—decided to investigate how changes in the temperature of a gas affect the amount of space it takes up, or its volume. They heated air and measured how its volume changed. The two scientists already knew that the pressure of a gas affects it volume. This had been demonstrated back in the 1660s by the English scientist Robert Boyle. So Charles and Gay-Lussac controlled the effects of pressure by keeping it constant in their experiments.

Based on the results of the research, Charles developed a scientific law about gases. It is one of three well-known gas laws, the others being Boyle’s law and Amontons’ law. According to Charles’s law, when the pressure of a gas is held constant, increasing its temperature increases its volume. The opposite is also true: decreasing the temperature of a gas decreases it volume.

### Summary

• According to Charles’ law, when the pressure of a gas is held constant, increasing its temperature increases its volume.
• Heating a gas gives its particles more energy so they move faster. If the speedy particles have room to spread out, the volume of the gas will increase.

### Review

1. State how increasing the temperature of a gas changes its volume, assuming pressure is held constant.
2. With the help of his dad, a boy in the Figure below blew up a bunch of balloons outside on a hot, sunny day. The outdoor temperature was 26°C. Then the boy took the balloons inside his air-conditioned house, where the temperature was 20°C. What do you think happened to the balloons after they had been inside the cool house for a while? Explain.

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