Chemistry students at any level are confronted with a variety of laws that serve to explain the reactions of gases to various different conditions. In broad terms, these laws are the study of thermodynamics, and are vital to much modern engineering and science. Boyle’s Law is a law that describes the relationship between pressure and volume of a gas when the temperature of a closed system is held constant.
Boyle’s Law was first described in the 17th Century by a man named Robert Boyle (shocking, isn’t it?). The law can be written in text, or as an equation. In words the Law states the following; if the amount of a gas and the temperature are held constant, the pressure and volume of that gas sample are inversely proportional. In other words, if you don’t change the amount of gas molecules, and the temperature isn’t allowed to change, the volume of the gas will go up if the pressure goes down (or the volume will go down if the pressure increases).
As an equation, Boyle’s Law can be written very simply. It is pV = k. p is the pressure of the system. V is the volume. k is a constant that is calculated via a rather complicated set of standards. The equation will sometimes be written as p1V1 = p2V2. In this case it is possible to compare the pressure or volume before and after one of the variables changes.
Let’s use an example for that to make more sense. Let’s say you have a volume of gas that is 1 liter. It has a pressure of 1 atmosphere. p1 = 1 liter. V1 = 1 atmosphere. Following?
Now, if you were you increase the pressure of the system to 3 atmospheres, what is the new volume? In this case p2 = 3 atmospheres.
Then we plug in the three known values in to the equation: p1V1 = p2V2. You get: (1 atmosphere)(1 liter) = (3 atmospheres)(V2).
Solving the equation for V2, which is the volume of the gas after the pressure is increased, you get a volume of .333 liters (a third of a liter). Does that make sense?
The same calculation could be done if any of the four variables where missing. As you can see, Boyle’s Gas Law can be a very simple yet powerful tool for calculating the pressure and volumeof a gas sample.
In practical chemistry, Boyle’s Law is rarely used alone. It is commonly combined with other Laws to make the Ideal Gas Law. These other law include Charles’s Law, Avogadro’s Law, and Gay-Lussac’s Law. The ideal gas law may be a little more complicated, but it takes many more factors in to account and can be used in a wider range of circumstances. Despite it’s limitations however, Boyle’s Law is important for understanding the thermodynamics of a gas.