What Happens When the Pressure and Temperature of a Fixed Sample of Gas Decreases?

By Rae Harris; Updated April 25, 2017
Gas laws explain the predictable behavior of gases.

One of the most interesting characteristics of gases is that regardless of their individual chemical properties, all gases basically follow the same set of gas laws. These laws describe the relationships between pressure, volume, temperature and the amount of a gas. According to these rules, gases will behave in a predictable way when one or more of these factors change. In order to understand how a decrease in both pressure and temperature will affect a fixed amount of a gas, we must first understand the laws that govern the behavior of gases.

Boyle's Law

Boyle's Law explains how the pressure and volume of a gas are related. This law states that when the temperature and mass of a gas sample are held constant, the pressure of the gas and its volume have an inverse relationship. So as the pressure increases, the volume of the gas decreases. As the pressure decreases, the volume increases.

Charles' Law

Charles' Law deals with the relationship between temperature and volume of a gas. This law says that when the pressure and mass of a gas are constant, the volume of a gas is directly proportional to the temperature as measured in kelvins. As the temperature increases, the volume increases. As the temperature decreases, the volume does too.

Amontons' Law

Amontons' Law explains how pressure and temperature are related. When a gas sample is held at a constant volume, the pressure and temperature have a direct proportional relationship. As the pressure of the gas increases, the temperature increases, and as the pressure decreases, the temperature decreases as well.

Avogadro's Hypothesis

Avogadro's hypothesis says that equal volumes of gases at the same temperature and pressure will have the same number of gas molecules, regardless of the type of gas. The relationship between the number of gas molecules and the volume of the gas is directly proportional. As the number of molecules increase, the volume of the gas increases.

Ideal Gas Law

All of these gas laws are combined to make the Ideal Gas Law. This law gives the relationship between pressure, volume, temperature and amount of a gas. The Ideal Gas Law is represented as PV=nRT, where P is the pressure, V is the volume, n is the number of moles of the gas, R is a constant known as the universal gas constant and T is the temperature in kelvins.

Pressure and Temperature Decreases

While a decrease in pressure will cause an increase in the volume of a gas, a decrease in temperature will cause a decrease in volume. The net effect on volume can be determined using the ideal gas law. The formula PV=nRT can be solved for volume. The resulting formula is V=nRT/P. The effect of pressure and temperature decreases is found by inserting these values, along with the number of moles of the gas, into this equation. This will give the final volume of the gas.

About the Author

Rae Harris is an educator and writer with an academic background in health and fitness. She graduated from Brigham Young University with a Bachelor of Science degree in exercise science. She began writing professionally in 2004. Harris' work has been published in various magazines, including "Schooled Magazine" and "YM."