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H + H +
O – O –
H + H + Dissolved
Cl – negative salt
Water molecules H + H + ion
around salt crystal O – O –
H + H + H + H +
O – H + H + O –
H + O – H +
Cl – Na + Cl – H + O – H + O – H + H +
H + H + O –
+
+
+
Na Cl – Na Cl – Na Cl – H + H +
O – Na + O –
H + H + H +
–
+
Cl – Na Cl – Na + Cl Na + O – –
H + H + O H +
+
Na + Cl – Na Cl – Na + Cl – H +
O – Dissolved
positive salt
Cl – Na + Cl – Na + Cl – Na + H + ion
Salt crystal
FIGURE 11.6 An ionic solid dissolves in water because the number of water molecules around the surface is greater than the number of
other ions of the solid. The attraction between polar water molecules and a charged ion enables the water molecules to pull ions away from
the crystal, so the salt crystals dissolve in the water.

Concentration ratios that describe small concentrations The concentration term of percent by volume is defined as
of solute are sometimes reported as a ratio of parts per million the volume of solute in 100 volumes of solution. This concentra-
(ppm) or parts per billion (ppb). This ratio could mean ppm by tion term is just like any other percentage ratio, that is, “part”
volume or ppm by weight, depending on whether the solution divided by the “whole” times 100 percent. The distinction is that
is a gas or a liquid. For example, a drinking water sample with the part and the whole are concerned with a volume of solute
+
1 ppm Na by weight has 1 weight measure of solute, sodium and a volume of solution. Knowing the meaning of percent by
ions, in every 1,000,000 weight measures of the total solution. volume can be useful in consumer decisions. Rubbing alcohol,
By way of analogy, 1 ppm expressed in money means 1 cent in for example, can be purchased at a wide range of prices. The vari-
every $10,000 (which is 1 million cents). A concentration of ous brands range from a concentration, according to the labels,
1 ppb means 1 cent in $10,000,000. Thus, the concentrations of of “12% by volume” to “70% by volume.” If the volume unit is
very dilute solutions, such as certain salts in seawater, minerals mL, a “12% by volume” concentration contains 12 mL of pure
in drinking water, and pollutants in water or in the atmosphere, isopropyl (rubbing) alcohol in every 100 mL of solution. The
are often reported in ppm or ppb. “70% by volume” contains 70 mL of isopropyl alcohol in every
100 mL of solution. The relationship for percent by volume is

volume solute
CONCEPTS Applied __

× 100% solution = % solute
volume solution
ppm or ppb to Percent or
Sometimes it is useful to know the conversion factors _

between ppm or ppb and the more familiar percent V solute × 100% solution = % solute
concentration by weight. These factors are ppm ÷ (1 × V solution
10 ) = percent concentration and ppb ÷ (1 × 10 ) = equation 11.1
7
4
percent concentration. For example, very hard water (water
2+ 2+
containing Ca or Mg ions), by definition, contains more The concentration term of percent by weight is defined as
than 300 ppm of the ions. This is a percent concentration the weight of solute in 100 weight units of solution. This concen-
4
of 300 ÷ 1 × 10 , or 0.03 percent. To be suitable for tration term is just like any other percentage composition, the
agricultural purposes, irrigation water must not contain difference being that it is concerned with the weight of solute
more than 700 ppm of total dissolved salts, which means (the part) in a weight of solution (the whole). Hydrogen per-
a concentration no greater than 0.07 percent salts. oxide, for example, is usually sold in a concentration of 3% by
weight. This means that 3 oz (or other weight units) of pure
11-7 CHAPTER 11 Water and Solutions 281