What Is Henry's Law?

In the science of chemistry, Henry’s law is a gas law stating that the mass of a dissolving gas within a certain amount of liquid is equally proportional to the pressure exerted on the gas. Simply put, the more pressure there is, the more gas can dissolve and blend into the liquid. Henry’s gas law also states that the gas’s solubility is inversely proportional to the temperature. If there is an increase in temperature, the gas’s solubility decreases.

The establishment of Henry’s law is attributed to William Henry, who in the 1800s, experimented with gases contained in water, using different temperatures and pressures. In formulaic terms, the gas law can be summed up this way: p = khc, where “p” represents the gas’s partial pressure, and the c is the solute’s concentration. The kh is the constant variable, depending on which gas is analyzed. The formula can also be expressed in other inverse forms, such as kh,pc = p/c or kh,cp = c/p.

The principle of Henry’s law is more easily understood when applied in everyday objects, such as in soft drinks, when the element of pressure is concerned. One can notice that a carbonated drink fizzes and forms bubbles when the bottle cap is removed and pressure is released, proof that the carbon gas is being released as well. If the bottle cap stays on, the pressure inside somehow forces and presses on to the carbon to dissolve in the sugared liquid, proving that a higher amount of pressure results in the gas being dissolved. This is why soft drinks do not taste as delicious when they are exposed in the air for too long, as much carbon has already been released.

Henry’s law can also be experienced by divers who usually feel heavier the more they descend into deeper waters. This is because the nitrogen is absorbed more by the bodily tissues because of the increased pressure underwater. When divers swim up, they experience a lighter feeling because of the release of gases, a very similar occurrence of the carbons fizzing out of the soda. Divers are advised, however, against ascending to the surface too quickly, as this can lead to decompression sickness, in which the gases are released all over the body and can cause severe pain, inflammation, and even seizures.

The element of temperature in Henry’s law can also be seen in soda drinks, or any gas-containing liquid, for the matter. As said before, the gas’s solubility decreases when the temperature increases, as observed when water is boiled and gas bubbles form on the surface. This also explains why experienced divers do not soak in a hot bath right after diving, because the hot water makes for a less soluble nitrogen that breaks away from the body, causing decompression sickness.