To pass from liquid
a substance needs heat
added to it to change the temperature
, plus an additional amount called the latent heat
Commonly abbreviated B.P. or b.p.
The boiling point of a solution is in general higher than that of the pure solvent. The extra temperature is called the boiling point elevation. Each solvent has a constant called the boiling point elevation constant, symbol kb. The constant for water is 0.52 K kg mol-1.
The increase in boiling point is not affected by what solute is dissolved in the solvent, only by what amount (in moles) of solute is present. (This is called a colligative property.) The formula is
ΔTb = kb · m
where m is the molality (concentration in molals, or moles per kilogram). These are not the same as the elevation constants.
Note that one mole of NaCl placed in solution is two moles, one each of Na+ and Cl-. It is the number of ions that counts.
Similarly the presence of ions in solution takes a freezing point depression from a pure freezing point.
Here are some boiling points in °C and their elevation constants for common solutes:
water 100.0 0.51
acetic acid 118.5 2.5
benzene 80.15 2.58
chloroform 60.19 3.66
ethanol 78.26 1.22
ether 34.60 2.11
nitrobenzene 210.9 5.01
Note that the boiling point of water is no longer exactly 100°C, since the Celsius
and absolute scales have been redefined in terms of the triple point