Hot air balloons

Hot air balloons (sjlayne, iStockphoto)

Charles' Law and Gay-Lussac's Law

Let's Talk Science
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Summary

Learn about the theories of Charles’ Law and Gay-Lussac’s Law and explore examples of these laws in everyday life.

There are four laws, known as Gas Laws, which describe how gases behave. The four laws are Boyle’s Law, Charles’s Law, Gay-Lussac’s Law and Avogadro’s Law.

Charles’ Law

Jacques Charles, a French physicist, discovered in the 1780s that heating a gas will cause it to expand by a certain fraction. The image below shows how adding heat makes molecules move faster and hit the sides and lid with greater force, thus moving the lid up as the gas expands.

Charles’ Law in action
Charles’ Law in action (©2020 Let’s Talk Science).

Charles’ Law in Everyday Life

In order to make a hot air balloon rise, heat is added to the air inside the balloon. Adding heat causes the molecules to move further away from each other.

Burners heat up the air inside a hot air balloon
Burners heat up the air inside a hot air balloon (Source: Sunridin [CC BY] via Wikimedia Commons).

In everyday language, we would say that the air inside expands. When this happens, the total density (mass per unit of volume) of the balloon and the air inside it decreases. When the density of the balloon decreases to be less than the density of the outside air, the balloon rises. Conversely, the volume of a gas will shrink if its temperature decreases. 

Below you can see liquid nitrogen being poured over a green balloon. The cold liquid nitrogen cools the air inside the balloon. As a result the molecules of air slow down causing the volume of the balloon to decrease.

Liquid nitrogen poured over a balloon
Liquid nitrogen poured over a balloon (Source: © UW Madison Department of Chemistry. Used with Permission).

During the holidays, someone you know may have used a turkey thermometer. A turkey thermometer is stuck into the turkey while it cooks and then pops up when the meat is cooked enough. How does this wondrous piece of technology work? It has to do with Charles’s Law, of course! Inside the turkey thermometer is a small amount of air. As the temperature rises inside the turkey, the air inside the turkey thermometer expands. Once it reaches a certain volume, the top pops, telling the chef that the turkey is properly cooked. 

Turkey thermometer (white pin stuck in the turkey breast)
Turkey thermometer (white pin stuck in the turkey breast) (Source: Patrick Fitzgerald from Atlanta, GA, USA [CC BY] via Wikimedia Commons).

Gay-Lussac’s Law

Joseph Louis Gay-Lussac was a ­­French chemist and physicist who discovered in 1802 that if you keep the volume of a gas constant (such as in a closed container), and you apply heat, the pressure of the gas will increase. This is because the gases have more kinetic energy, causing them to hit the walls of the container with more force (resulting in greater pressure).

Gay-Lussac’s Law in action
Gay-Lussac’s Law in action (©2020 Let’s Talk Science).

Gay-Lussac’s Law in Everyday Life

Inside a pressure cooker the food that you want to cook sits in water. As the temperature of the liquid water is increased, water vapour (water in its gas state) is produced. This vapour cannot escape the pressure cooker – meaning the volume is not changing. The pressure of the water vapour keeps rising until the temperature of the water and the water vapour exceed the normal boiling point of water (100 °C). At this higher temperature food can be cooked much faster. Tough meat also comes out much more tender after being cooked in a pressure cooker.

Pressure cooker
Pressure cooker (LucaLuca [CC BY-SA] via Wikimedia Commons).

Did you know that the air pressure on the inside of car tires changes when the car is driven? After driving, the air pressure in a car’s tires goes up. This is because friction (a contact force) between the tires and road causes the air inside the tires to heat up. The air cannot expand because the tires are essentially a fixed-volume container, so the pressure increases – this is Gay-Lussac’s Law!

Measuring tire air pressure with a pressure gauge
Measuring tire air pressure with a pressure gauge (Source: (U.S. Air Force photo by Airman Frank Snider) [Public domain] via Wikimedia Commons).

Try this!

Measure your car’s tire pressure before and after driving somewhere to see Gay-Lussac’s Law in action!

 

 

Learn More

The Sci Guys: Science at Home - SE2 - EP10: Charles's Law of Ideal Gases (2014)

In this video, the Sci Guys use balloons to explore how gas molecules react to changes in temperature and volume, demonstrating Charles's law of ideal gases. 

The Sci Guys: Science at Home - SE2 - EP11: Gay-Lussac's Law of Ideal Gases (2014)

In this video, the Sci Guys use candles, water and a beaker to explore how gas molecules react to changes in temperature and pressure, illustrating Gay-Lussac's law of ideal gases. 

References