Charles’s Law states that the volume of a given mass of gas varies directly with the absolute temperature of the gas when pressure is kept constant. The absolute temperature is temperature measured with the Kelvin scale. The Kelvin scale must be used because zero on the Kelvin scale corresponds to a complete stoppage of molecular motion.
Examples 1: A helium balloon
If you take a helium balloon outside on an extremely cold day, the balloon would most likely crumble or deflate. Once it goes back into a warm area, it’ll go back to its original shape.
Example 2: A hot air balloon
A torch is used to make the air molecules move faster and disperse inside the balloon. The gas inside the balloon takes up more space, becoming less dense than the air surrounding it. As such, the hot air inside the balloon rises because of its decreased density and causes the balloon to float.
Example 3: Tire pressure
Measure the pressure of your car’s tires when they are cold. Driving heats up the tires and consequently causes the air within them to expand. If you measure the air when the tires are warm, the pressure will be higher. You can double-check that you haven’t overfilled your tires by checking them when they’ve cooled down.