# Gases, Pressure, & Laws

It is common in chemistry to have to deal with gases. Naturally, scientists have uncovered various laws that describe how gases act. This post will look at concepts such as pressure and the development of various laws related to gases and pressure.

Pressure and Units

Pressure is defined as (force / area). To make this practical, scientists have found that our bodies are constantly exposed to 14.7 pounds of pressure per square inch by the air around us. Our bodies are so used to this constant external pressure that without it breathing would be difficult, if not impossible.

There are various units of measurement of pressure. The Pascal, named after Blaise Pascal, is newtons per meter square. However, Pascals are rarely used by scientists. Another common unit is standard atmospheric pressure or atm for short, which is the average amount of pressure exerted by air at sea level. As a fact, one atm is the equivalent of 101,325 pascals.

One more unit for pressure is the torr, which is 1 /760th of an atm. In terms of measuring pressure, it is common to use a barometer, and a barometer measures pressure using millimeters of mercury or mmHg. The units on a barometer are almost the same as for the torr.

Laws Related to Gases

There are several laws related to gases. For example, Boyle’s law states an inverse relationship between pressure and volume with the assumption that temperature is constant. In other words, when the pressure goes up, the volume will go down and vice versa. Boyle’s law was developed by Robert Boyle, an Irish scientist from the 17th century.

Breathing is based on Boyle’s law. When we breathe, inhaling causes the volume of our lungs to grow, which leads to a drop in pressure. The pressure drop is what allows air to flow into the lungs. The opposite takes place when we exhale. Our lungs become smaller, raising the pressure and forcing the air out of our bodies.

Charles’s laws are somewhat of a variation on Boyle’s law. This law was developed by Jacques Charles, a French scientist of the 18th century. Charles law states that if pressure is constant, then temperature and volume are proportional. In other words, when the temperature goes up or down, then the volume will go up or down.

An interesting by-product of Charles’ law is the idea behind absolute zero. Essentially, as we lower the temperature, the volume of a gas will shrink. However, gas is made of matter, and it can’t go to zero. This implies that there is a lower limit to temperature, and this lower limit is called absolute zero and is -273.15 C.

As shown below, the combined gas law combines Boyle and Charles’ law into one equation.

(p * v) / T

Pressure times volume captures a value to describe a gas in a particular context. However, we use the equation to solve for unknown values, so it is more appropriate to show it as follows.

(p1 * v1) / T1 = (p2 * v2) / T2

Conclusion

People generally dislike pressure, but the pressure is literally needed for life, at least when it comes to gases. Thanks to the work of many excellent scientists, we have a better understanding of how gases behave in the world around us.