Chemistry

# Charles Gas Law

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Gas Laws are one of the most difficult modules for the first year chemistry student to master. Gas Laws require students to apply skills learned in Algebra class. Yet, the Gas Laws are the most relevant science subject studied in High School and at University.

Jacques-Alexandre-Cesar Charles experimented with balloons in 1783. Charles's experiments showed that the volume of a gas is directly proportional to the temperature when the pressure and weight or moles of the gas remain the same. This relationship between the temperature and volume of a gas is now known as Charles' Law. Go to YouTube for a demonstration of Charles' Law.

Charles' Law explains how a hot air balloon works. A gas expands when it is heated to take up more volume. Hot air is not as dense as cold air. Once the air in a balloon gets hot enough, the weight of the balloon is less than the weight of cold air, so the hot air balloon rises. The hot air balloon floats in air just like a boat floats in water on the lake. When the gas in the balloon cools, the balloon returns to the ground. Charles' Law also explains why in the fall when outdoor temperatures start to approach freezing at night that our automobile tires seem to be under-inflated. We have to add air because the volume of air in the tires decreases with the decrease in temperature.

Warm yeast and sugar produce carbon dioxide gas. Bread rises as the carbon dioxide expands as it warms. Charles' Law also explains how bread and cakes rise. Eventually the heat of the oven kills the yeast so carbon dioxide is no longer formed and the rising of the bread stops.

The mathematical representation showing that the volume of a gas is directly proportional to the temperature when the pressure and amount of the gas remain the same is the ratio

V1/V2 = T1/T2.

Proportional relationships are demonstrated by a straight line on a graph. However, you will not see a straight line if you graph temperatures in Fahrenheit vs. volume or if you graph temperatures in Celsius vs. volume. The straight line is only seen when the temperature is on the Kelvin scale vs. volume. The temperature in Kelvin can be calculated by adding 273 to the Celsius temperature.

Charles's plots of volume vs. temperature show that the volume of a gas is almost zero when the temperature reaches 273 degrees Kelvin. Charles's plots are used to define the Kelvin temperature scale.

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