Cracking the SAT Chemistry Subject Test

m   =   molality    =

i    =   whole   number  equaling    the     number  of  particles   a   substance

dissolves   into

Again, why is this? Boiling point elevation and freezing point depression are
directly proportional to the number of particles dissolved in a solution, but
independent of the type of particle (i.e., sucrose and KCl equally affect melting
point and freezing point at the same molality). Sucrose doesn’t dissociate and
KCl does. When 1 mole of sucrose is dissolved in water, it yields 1 mole of
dissolved particles. When 1 mole of KCl is dissolved in water, it yields 2 moles
of dissolved particles. If the sucrose elevated the water’s boiling point by 0.5°C,
the KCl would raise it by 1°C. If the sucrose depressed the water’s freezing point
by 2°C, the KCl would depress it by 4°C. Remember the relationship of
proportionality we’ve just described.

Boiling Point

Elevation   and Freezing

Point   Depression

These   points  depend

only    on  the type    of

solvent and the number

of  solute  particles.

There are many practical applications of boiling point elevation and freezing
point depression. For example, spreading calcium chloride onto roadways during
snowstorms makes it harder for ice to form on them (since the freezing point of
water is made lower).