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The presence of the hydronium ion gives the solution new chemi- been observed to add a pinch of salt to a pot of water before
cal properties; the solution is no longer hydrogen chloride but is boiling. Is this to increase the boiling point and therefore
hydrochloric acid. Hydrochloric acid, and other acids, will be dis- cook the food more quickly? How much does a pinch of salt
cussed shortly. increase the boiling temperature? The answers are found in
the relationship between the concentration of a solute and the
boiling point of the solution.
Myths, Mistakes, & Misunderstandings It is the number of solute particles (ions or molecules) at
the surface of a solution that increases the boiling point. Recall
Teardrops Keep Falling? that a mole is a measure that can be defined as a number of
particles called Avogadro’s number. Since the number of par-
It is a mistake to represent raindrops or drops of falling water
ticles at the surface is proportional to the ratio of particles in
with teardrop shapes. Small raindrops are pulled into a spheri-
the solution, the concentration of the solute will directly influ-
cal shape by surface tension. Larger raindrops are also pulled
into a spherical shape, but the pressure of air on the bottom of ence the increase in the boiling point. In other words, the boil-
the falling drop somewhat flattens the bottom. If the raindrop is ing point of any dilute solution is increased proportionally to
too large, the pressure of air on the falling drop forms a concave the concentration of the solute. For water, the boiling point is
depression on the bottom, which grows deeper and deeper until increased 0.521°C for every mole of solute dissolved in 1,000 g
the drop breaks up into smaller spherical drops. of water. Thus, any water solution will boil at a higher tempera-
ture than pure water. Since it boils at a higher temperature, it
also takes a longer time to reach the boiling point.
It makes no difference what substance is dissolved in the
BOILING POINT water; 1 mole of solute in 1,000 g of water will elevate the boiling
point by 0.521°C. A mole contains Avogadro’s number of particles,
Boiling occurs when the pressure of the vapor escaping from a
so a mole of any solute will lower the vapor pressure by the same
liquid is equal to the atmospheric pressure on the liquid. The
amount. Sucrose, or table sugar, for example, is C 12 H 22 O 11 and has
normal boiling point is defined as the temperature at which
a gram-formula weight of 342 g. Thus, 342 g of sugar in 1,000 g
the vapor pressure is equal to the average atmospheric pres-
of water (about a liter) will increase the boiling point by 0.521°C.
sure at sea level. For pure water, this temperature is 100°C
Therefore, if you measure the boiling point of a sugar solution,
(212°F). It is important to remember that boiling is a purely
you can determine the concentration of sugar in the solution. For
physical process. No bonds within water molecules are broken
example, pancake syrup that boils at 100.261°C (sea-level pres-
during boiling.
sure) must contain 171 g of sugar dissolved in 1,000 g of water.
The vapor pressure over a solution is less than the vapor
You know this because the increase of 0.261°C over 100°C is one-
pressure over the pure solvent at the same temperature. Mol-
half of 0.521°C. If the boiling point were increased by 0.521°C over
ecules of a liquid can escape into the air only at the surface of
100°C, the syrup would have the full gram-formula weight (342 g)
the liquid, and the presence of molecules of a solute means that
dissolved in a kg of water.
fewer solvent molecules can be at the surface to escape. Thus,
Since it is the number of particles of solute in a specific sam-
the vapor pressure over a solution is less than the vapor pressure
ple of water that elevates the boiling point, different effects are
over a pure solvent (Figure 11.12).
observed in dissolved covalent and dissolved ionic compounds
Because the vapor pressure over a solution is less than that
(Figure 11.13). Sugar is a covalent compound, and the solute
over the pure solvent, the solution boils at a higher tempera-
is molecules of sugar moving between the water molecules.
ture. A higher temperature is required to increase the vapor
Sodium chloride, on the other hand, is an ionic compound and
pressure to that of the atmospheric pressure. Some cooks have
dissolves by the separation of ions, or
+ – + –
Na Cl (s) → Na (aq) + Cl (aq)
This equation tells you that one mole of NaCl separates into one
mole of sodium ions and one mole of chlorine ions for a total
of two moles of solute. The boiling point elevation of a solution
made from one mole of NaCl (58.5 g) is therefore multiplied
by 2, or 2 × 0.521°C = 1.04°C. The boiling point of a solution
made by adding 58.5 g of NaCl to 1,000 g of water is therefore
101.04°C at normal sea-level pressure.
Now back to the question of how much a pinch of salt
increases the boiling point of a pot of water. Assuming the pot
contains about a liter of water (about a quart), and assuming that
a pinch of salt has a mass of about 0.2 g, the boiling point will be
increased by 0.0037°C. Thus, there must be some reason other
FIGURE 11.12 The rate of evaporation, and thus the vapor
pressure, is less for a solution than for a solvent in the pure state. than increasing the boiling point that a cook adds a pinch of salt
The greater the solute concentration, the less the vapor pressure. to a pot of boiling water. Perhaps the salt is for seasoning?
11-11 CHAPTER 11 Water and Solutions 285