The Henry's law states that at a constant temperature
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas present over the surface of the liquid.
Henry’s law is about a gas dissolving in a liquid, and its usual form is:
\( \displaystyle p=K_H\,x \)
where p is the partial pressure of the gas and x its mole fraction in the solution; a larger KH means a lower solubility.
Why the other options are incorrect
Option (1) the solubility of a solid in a liquid is directly proportional to the pressure of gas over the liquid. — Incorrect. Henry’s law is about the solubility of a gas, not a solid.
Option (2) the solubility of a solid in a gas is directly proportional to the pressure of gas over the liquid. — Incorrect. A solid does not dissolve in a gas; the law is about a gas dissolving in a liquid.
Option (4) the solubility of a liquid in a liquid is directly proportional to the pressure of the gas over the surface of the liquid. — Incorrect. Henry’s law is about a gas in a liquid, not a liquid in a liquid.
NCERT (2026-27 edition):
Solubility of a Gas in a Liquid – “… Henry was the first to give a quantitative relation between pressure and solubility of a gas in a solvent which is known as Henry’s law. The law states that at a constant temperature, the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas present above the surface of liquid or solution.”