colligative properties

Some properties of a solution care only about how many particles are dissolved, not about what those particles are. These are the colligative properties. Whether you dissolve sugar, salt or a drug, what shifts these properties is the head-count of dissolved particles per amount of solvent, not their chemical identity.

There are four classic colligative properties: lowering of the solvent's vapour pressure, raising of its boiling point, lowering of its freezing point, and the appearance of osmotic pressure. Salts count for more than their formula suggests because they split into several ions one mole of sodium chloride dissolving contributes close to two moles of particles, so it depresses freezing point about twice as much as one mole of sugar.

Pharmacy leans on these properties to make injections and eye drops safe and comfortable. Because a solution that matches body fluid in particle number is isotonic, the freezing-point depression method lets formulators measure exactly how much sodium chloride or other agent to add to reach the right tonicity, turning an abstract physical principle into a routine compounding calculation.