When you dissolve a substance into a pure liquid, the liquid becomes harder to freeze. The dissolved particles get in the way and block the liquid from forming solid crystals. This effect is known in chemistry as freezing point depression.
The cryoscopic constant, written as Kf, helps predict exactly how much the temperature drops. This constant is also known as the molal freezing point depression constant. The constant depends entirely on the identity of the pure liquid solvent.
It relies on properties like the solvent’s molar mass and heat of fusion. It does not care what type of solid you dissolve into the liquid. Students often mistakenly think the dissolved solid changes the constant value itself.
The dissolved solid only changes the total temperature drop, not the fixed constant. Chemists use this constant in an older lab technique called cryoscopy. In this technique, they measure the new freezing point of a liquid mixture.
The temperature drop is equal to the constant times the molality and van’t Hoff factor. They can rearrange this math to figure out the molar mass of an unknown solid.