SAT Subject Test Chemistry,10 edition

Solutions   that    obey    Raoult’s    law are called  ideal   solutions.

FREEZING-POINT DEPRESSION

Pure water (H 2 O) freezes at 0°C at 1 atm; however, for every mole of solute particles dissolved in 1 L
of water, the freezing point is lowered by 1.86°C. This is because the solute particles interfere with
the process of crystal formation that occurs during freezing; the solute particles lower the
temperature at which the molecules can align themselves into a crystalline structure.

A CLOSER LOOK

Freezing-point  depression  is  the principle   behind  spreading   salt    on  ice:    The freezing    point   of

water   is  lowered by  the presence    of  the salt,   and so  the ice melts.  Antifreeze  (mostly ethylene

glycol) also    operates    by  the same    principle.

The formula for calculating this freezing-point depression is:

∆Tf =   Kf m

where ∆Tf is the freezing-point depression (the number of degrees or Kelvin the freezing point is
lowered by), Kf is a proportionality constant characteristic of a particular solvent, and m is the
molality of the solution (mol solute/kg solvent). Each solvent has its own characteristic Kf . The
larger the value, the more sensitive its freezing point is to the presence of solutes.

Note that the molality in question is the total molality of all particles present. A 1 m aqueous
solution of NaCl, for example, would correspond to 2 m in solute particles since it dissociates to give
1 m of Na+ ions and 1 m of Cl− ions. It would lead to a freezing-point depression that is twice the
magnitude of that of a 1 m aqueous solution of sugar.

Freezing-point depression is the principle behind spreading salt on ice: The freezing point of water is
lowered by the presence of the salt, and so the ice melts. Antifreeze (mostly ethylene glycol) also