The dilute solutions of non-volatile solutes exhibit certain characteristic properties which do not depend upon the nature of the solute but depend only on the number of particles (molecules or ions) of the solute i.e., on the molar concentration of the solute. These are called colligative properties (colligative, from Latin: co means together, ligare means to bind). Thus, the properties of the solutions which depend only on the number of solute particles but not on the nature of the solute are called colligative properties.

The four important colligative properties are:

1. Relative lowering in vapour pressure
2. Elevation in boiling point
3. Depression in freezing point
4. Osmotic pressure

A pair of solution bears the same osmotic pressure. What is this pair of solutions called?

Isotonic

The answer is 3. Isotonic.

Isotonic solutions have the same osmotic pressure. This means that the solute concentration is the same in both solutions. When two solutions are isotonic, there is no net movement of water between the two solutions.

Hypertonic solutions have a higher osmotic pressure than hypotonic solutions. This means that the solute concentration is higher in the hypertonic solution than in the hypotonic solution. When two solutions are hypertonic and hypotonic, there is a net movement of water from the hypotonic solution to the hypertonic solution.

Osmolarity is a measure of the total solute concentration in a solution. It is not the same as osmotic pressure, but it is related to osmotic pressure. The higher the osmolarity of a solution, the higher the osmotic pressure of the solution.

Therefore, the pair of solutions that bears the same osmotic pressure is called isotonic.