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Charles’ Law
Jean Brainard, Ph.D.
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Printed: January 13, 2015
AUTHOR
Jean Brainard, Ph.D.
www.ck12.org
C HAPTER
Chapter 1. Charles’ Law
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Charles’ Law
• Describe how the volume of a gas is related to its temperature.
• Explain what happens to gas particles when they are heated.
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’ 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.
You Try It!
At the URL below, you can simulate Charles’ and Gay-Lussac’s work. The simulation lets you choose different
temperatures for a gas and see how it affects the volume of the gas. After you have recorded several sets of values in
the data table, use the graphing feature to plot the points.
http://group.chem.iastate.edu/Greenbowe/sections/projectfolder/flashfiles/gaslaw/charles_law.html
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Q: What does the graph of the data look like?
A: The graph is a straight line that rises from left to right. This type of graph shows that both variables increase
together at a constant rate.
Q: Could you use the graph to find the volume of a gas at a temperature you didn’t choose for your data table?
A: Yes, you could. You could find the temperature on the temperature scale, trace straight up from this point to the
graph line, and then trace straight across to the volume scale to find the volume for that temperature.
Amazing Motion
Can you guess why the volume of a gas increases when it is heated? Heating a gas gives its particles more energy.
With more energy, the particles move faster. You can see this in the simulation at the URL below. Drag the bar to
increase the temperature, and you’ll be amazed at how fast the particles zigzag around inside the box! If the speedy
particles had more room, they would spread out and the volume of the gas would increase. http://www.colorado.
edu/UCB/AcademicAffairs/ArtsSciences/physics/PhysicsInitiative/Physics2000/bec/temperature.html
Q: Now that you know how a gas behaves when it is heated, can you explain how hot air causes a balloon to inflate?
A: As the air heats, its particles spread out and occupy more space, causing the balloon to inflate.
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.
Explore More
Do the activity on Charles’ law at the following URL. Be sure to answer the questions at the end of the activity.
http://www.wisc-online.com/objects/ViewObject.aspx?ID=HYP4107
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 1.1 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.
References
1. Image copyright Warren Goldswain, 2014. http://www.shutterstock.com . Used under license from Shutterstock.com
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Chapter 1. Charles’ Law
FIGURE 1.1
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