Consider two aqueous solutions, one of sucrose (C12 H22 O11) and the other of nitric acid (HNO3)

step 1 In this example, we have two samples, sucrose and nitric acid. They both have in common a freezi

Consider two aqueous solutions, one of sucrose (C12 H22 O11)...

Consider two aqueous solutions, one of sucrose $\left(\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}\right)$ and the other of nitric acid $\left(\mathrm{HNO}_{3}\right)$. Both solutions freeze at $-1.5^{\circ} \mathrm{C}$. What other properties do these solutions have in common?

Key Concepts

Colligative Properties

Colligative properties are characteristic physical properties of solutions that depend solely on the number of solute particles present and not on their individual chemical identities. These properties include freezing point depression, boiling point elevation, vapor pressure lowering, and osmotic pressure. When two solutions have the same number of solute particles, they exhibit identical magnitudes for these effects.

Freezing Point Depression

Freezing point depression is a colligative property where the addition of solute particles to a solvent decreases its freezing point. The extent of the depression is proportional to the molal concentration of the solute particles and is independent of their chemical nature, meaning that different solutes can produce the same effect if they contribute an equal number of particles.

Boiling Point Elevation

Boiling point elevation is another colligative property, which reflects the increase in the boiling point of a solvent when a nonvolatile solute is dissolved in it. This effect arises because the solute particles lower the solvent’s vapor pressure, requiring a higher temperature for the vapor pressure to match the external pressure. As with freezing point depression, the magnitude of boiling point elevation depends only on the quantity of solute particles.

Vapor Pressure Lowering

Vapor pressure lowering is a colligative property that occurs when the addition of a solute reduces the vapor pressure of a solvent. This phenomenon happens because solute particles occupy space at the liquid surface, thereby decreasing the number of solvent molecules that can evaporate. The decrease in vapor pressure is directly related to the number of solute particles in the solution.

Osmotic Pressure

Osmotic pressure is the pressure required to stop the net flow of a solvent across a semipermeable membrane that separates solutions of different concentrations. As with other colligative properties, osmotic pressure depends solely on the concentration of solute particles. Two solutions with equivalent solute particle concentrations will have identical osmotic pressures.

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