Henry's Law

Henry's Law

Henry's Law is one of the Gas Laws formulated by a British Chemist William Henry in 1803.

It states that " At a constant Temperature the solubility of a Gas in a Liquid (unless it is highly soluble) is DIRECTLY proportional to the Partial Pressure applied to the Gas ".

Mathematically Henry's Law can be stated as :-

p = kH x c

Where,

p is the partial pressure of the solute in the gas above the solution,

c is the concentration of the solute,

kH is a constant known as the Henry's Law constant with the dimensions of pressure divided by concentration.

The Henry's constant depends on the solute, solvent and the temperature.

The Henry's law constant "kH" is different for every gas, temperature and solvent .

A very good example of Henry's Law is Carbonated soft drinks. We know that in such drinks carbondioxide gas is at the top and the drink below it. Also some amount of carbondioxide is dissolved in the drink. When the bottle is sealed the pressure at the cap of the bottle is greater than the Atmospheric Pressure,But as soon as the bottle cap is opened the carbondioxide gas escapes which is indicated by the "phiss" sound heard when the cap is opened. Due to this,the Pressure at the cap of the bottle drops down the Atmospheric Pressure and some carbondioxide comes out of the drink in the form of bubbles.If the drink is left open for sometime the carbon dioxide in the drink will come into equilibrium with the carbon dioxide in the air, and the drink will go "flat". One key thing to remember in this example is that the pressure acting above the drink in a sealed bottle is due to the partial pressure of carbondioxide.Thus from this example and according to Henry's Law we can say that more the partial pressure of carbondioxide more will be it's solubility in the drink.