It’s important to realize the denominator is not the mass of the solvent only; it
represents the mass of the total solution. If you dissolve 15 g of sugar in 100 mL
of water (density of water: 1.0 g/mL), the percent by mass would be calculated
by dividing 15 g by 115 g, NOT dividing 15 g by 100 g.
Solubility and Saturation
Suppose you take a glass of water and add table salt to it. The table salt
dissolves. Suppose you keep adding table salt to it. After a while, the table salt
doesn’t dissolve, it just sits at the bottom of the glass. At that point the water is
saturated with table salt. Another way to describe this is to say that the table salt
has reached the limit of its solubility in water.
The temperature of the solvent affects solubility. Generally, a solid solute is more
soluble in a liquid solvent at higher temperatures and less soluble at lower
temperatures. If we took the glass of water and heated it, some of the table salt
that hadn’t dissolved would dissolve. The increased temperature increases the
solubility of table salt in water.
Substances that are held together by ionic bonds (such as table salt, NaCl) are
generally soluble in water.
Other solutes are completely insoluble. For instance, if you place a pat of butter
in a glass of water, it won’t dissolve, ever, even if you heat it. This illustrates an
important general principle; polar solutes such as NaCl and HCl dissolve in polar
solvents such as water, and nonpolar solutes dissolve in nonpolar solvents.
Butter, which is a fat, is nonpolar. Just remember: “Like dissolves like.”
The solubility of gases in water is quite different from that of solids. Think about
a bottle of soda; its carbonation is the result of dissolved carbon dioxide gas.
Once the bottle has been opened, should you store it where it’s warm or cold to
prevent it from going flat? You should store it where it’s cold, of course. The
CO 2 gas is more soluble in water at lower temperatures, and flat soda is simply
soda after its CO 2 has diffused out into the air. This is typical of the solubility of
gases in water. One more thing about the solubility of gases in water: the higher
the pressure, the more soluble the gas. Again consider soda, bottled under
pressure. Once you open the bottle, the pressure over the soda decreases, and
CO 2 starts to come out of solution.