What is Dalton's Law?

Dalton's law is a principle used in chemistry to predict the concentration of mixed gases in terms of pressure. Also known as Dalton's law of partial pressure, it states that the sum of exerted pressure of the whole mixture of gases is equal to the sum of all pressures in the mixture. Introduced in the early 1800s by John Dalton, an English chemist and physicist, Dalton’s law applies to ideal gases as an absolute empirical law and not real gases. The reason for this is due to the elastic properties of the molecules involved in the former, as well as low particle volume. However, the margin of error when applied to real gases is generally considered minimal in most cases.

Mathematically, Dalton’s law can be expressed as P(1) + P(2) + …P(n), where P = Pressure. As the law observes, the combined pressure of each gas component in the mixture is equal to the total pressure of each gas in the entire mixture. The measurement to represent pressure is expressed in kilopascal units and written as kPa.

Nearly everyone has observed this phenomenon first-hand at some point, whether formally familiar with Dalton’s law or not. One might even recall the classic science experiment from their school years that involved water being displaced from a glass bottle as it was held under a water-filled trough. The lesson learned was that, although the bottle was emptied of water, it was not actually left empty at all. Instead, it filled up with invisible gas as the water was being displaced. This same effect can be observed when doing something as mundane as washing a glass in a kitchen sink full of water, or watching a young child play with a plastic cup in the bathtub.

In the above scenario, it is possible to determine the amount of pressure exerted by the invisible gas captured in the bottle, namely hydrogen. This is done by referring to a table showing the pressure of water vapor at varying temperatures, since a certain amount of water vapor must be accounted for in the equation. The calculation would be the total amount of pressure less the pressure of water vapor. The result would equal the pressure of the hydrogen gas.

While Dalton’s law primarily serves a purpose in the laboratory, it also has real life applications. For instance, scuba divers are concerned with knowing how air and nitrogen are affected at different pressures at varying depths of water. It is also used to determine the concentration of specific gases in the atmosphere.