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Solubility, Henry's Law, and Raoult's Law

Henry's law relates the solubility of a gas in a liquid to its partial pressure, while Raoult's law relates the vapour pressure of a volatile component in an ideal solution to its mole fraction.

In one line: Henry handles gases under pressure; Raoult handles vapour pressure from mole fraction.

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Understand it

For gases dissolved in liquids, higher pressure generally increases solubility at a fixed temperature. Henry's law is commonly written as p = KH x, where x is mole fraction of gas in solution; some simplified contexts express solubility as directly proportional to pressure. Raoult's law states that the partial vapour pressure of a component equals its pure vapour pressure multiplied by its mole fraction in the solution.

Example

Carbonated drinks are bottled under high CO2 pressure, so more CO2 remains dissolved. When the bottle is opened, pressure decreases and CO2 escapes.

Rule to remember

Henry's law: p = KH x for a gas dissolved in a liquid.

Raoult's law for an ideal solution: pi = pi0 xi.

Total pressure for two volatile components: ptotal = pA0 xA + pB0 xB.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

  1. 1If pA0 = 80 kPa, pB0 = 50 kPa, xA = 0.30, and xB = 0.70 for an ideal solution, then ptotal = 80 × 0.30 + 50 × 0.70 = 24 + 35 = 59 kPa.
  2. 2Interpretation: the vapour pressure is the mole-fraction-weighted sum of partial pressures.

Method you can reuse

  1. 1Decide whether the question is about a gas dissolved in a liquid or a volatile liquid mixture.
  2. 2Use Henry's law for gas solubility under pressure.
  3. 3Use Raoult's law when vapour pressure and mole fraction of liquid components are involved.
  4. 4Substitute pressure units consistently and interpret whether solubility or vapour pressure increases or decreases.

Visual tip

A pressure-solubility graph can support Henry's law, but it is not mandatory for this concept. A straight-line relation at constant temperature is enough for most questions.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

In the form p = KH x, what happens to gas solubility when KH is large at the same pressure?

Think first, then show answer
The mole fraction x is smaller, so the gas is less soluble.

Common wrong answer

Using p = p0 x for gas solubility instead of Henry's law is a realistic formula-selection error.

Don't mix this up

A common confusion is treating Henry's constant as if a larger value always means higher solubility. In the form p = KH x, larger KH means lower solubility for the same pressure.

Exam tip

Write the exact form of the law before interpreting KH. For Raoult's law, check whether the solution is ideal and whether the component is volatile.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Questions often ask for gas solubility trends with pressure, applications such as soda bottles or deep-sea diving, calculation of vapour pressure using Raoult's law, or assertion-reason links between pressure and gas dissolution.
  1. 1

    Enough for 1 mark

    Henry's law relates the solubility of a gas in a liquid to its partial pressure, while Raoult's law relates the vapour pressure of a volatile component in an ideal solution to its mole fraction.

  2. 2

    Add for 2 marks

    Example: Carbonated drinks are bottled under high CO2 pressure, so more CO2 remains dissolved. When the bottle is opened, pressure decreases and CO2 escapes.

  3. 3

    Add for 3 marks

    For gases dissolved in liquids, higher pressure generally increases solubility at a fixed temperature.

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