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TABLE 11.5

The approximate pH of some common substances
Substance pH (or pH Range)
Hydrochloric acid (4%) 0
Gastric (stomach) solution 1.6–1.8
Lemon juice 2.2–2.4
Vinegar 2.4–3.4
Carbonated soft drinks 2.0–4.0
Grapefruit 3.0–3.2
Oranges 3.2–3.6
Acid rain 4.0–5.5
Tomatoes 4.2–4.4
Potatoes 5.7–5.8
Natural rainwater 5.6–6.2
FIGURE 11.16 The pH increases as the acidic strength of
Milk 6.3–6.7
these substances decreases from left to right. Did you know that
Pure water 7.0 lemon juice is more acidic than vinegar? That a soft drink is more
Seawater 7.0–8.3 acidic than orange juice or grapefruit juice?
Blood 7.4
This is an ionic exchange reaction that forms molecular water,
Sodium bicarbonate solution 8.4 2+ –
leaving Ca and Cl in solution. As the water is evaporated,
Milk of magnesia 10.5
these ions begin forming ionic crystal structures as the solution
Ammonia cleaning solution 11.9
concentration increases. When the water is all evaporated, the
Sodium hydroxide solution 13.0
white crystalline salt of CaCl 2 remains.
If sodium hydroxide had been used as the base in stead of
calcium hydroxide, a different salt would have been produced:
A neutral solution has a pH of 7.0. Acidic solutions have
pH values below 7, and smaller numbers mean greater acidic prop- HCl(aq) + NaOH(aq) → NaCl(aq) + H 2 O(l)
– +
erties. Increasing the OH concentration decreases the H 3 O
Salts are also produced when elements combine directly,
concentration, so the strength of a base is indicated on the same
when an acid reacts with a metal, and by other reactions.
scale with values greater than 7. Note that the pH scale is loga-
Salts are essential in the diet both as electrolytes and as a
rithmic, so a pH of 2 is 10 times as acidic as a pH of 3. Likewise,
source of certain elements, usually called minerals in this con-
a pH of 10 is 100 times as basic as a pH of 8. Table 11.5 compares
text. Plants must have certain elements that are derived from
the pH values of some common substances (Figure 11.16).
water-soluble salts. Potassium, nitrates, and phosphate salts are
often used to supply the needed elements. There is no scientific
CONCEPTS Applied evidence that plants prefer to obtain these elements from natu-
ral sources, as compost, or from chemical fertilizers. After all, a
Acid or Base? nitrate ion is a nitrate ion, no matter what its source. Table 11.6
lists some common salts and their uses.
Pick some household product that probably has an acid
or base character (example: pH increaser for aquariums).
Write down the listed ingredients, and identify any you HARD AND SOFT WATER
believe would be distinctly acidic or basic in a water Salts vary in their solubility in water, and a solubility chart appears
solution. Tell whether you expect the product to be an acid in appendix B. Table 11.7 lists some generalizations concerning
or a base. Describe your findings from a litmus paper test. the various common salts. Some of the salts are dissolved by wa-
ter that will eventually be used for domestic supply. When the
salts are soluble calcium or magnesium compounds, the water
PROPERTIES OF SALTS will contain calcium or magnesium ions in solution. A solution
2+
2+
of Ca of Mg ions is said to be hard water because it is hard
Salt is produced by a neutralization reaction between an acid and
to make soap lather in the water. “Soft” water, on the other hand,
a base. A salt is defined as any ionic compound except those with
makes a soap lather easily. The difficulty occurs because soap is
hydroxide or oxide ions. Table salt, NaCl, is but one example of this
a sodium or potassium compound that is soluble in water. The
large group of ionic compounds. As an example of a salt produced
calcium or magnesium ions, when present, replace the sodium or
by a neutralization reaction, consider the reaction of HCl (an acid
potassium ions in the soap compound, forming an insoluble com-
in solution) with Ca(OH) 2 (a base in solution). The reaction is
pound. It is this insoluble compound that forms a “bathtub ring”
and also collects on clothes being washed, preventing cleansing.
2 HCl(aq) + Ca(OH) 2 (aq) → CaCl 2 (aq) + 2 H 2 O(l)
290 CHAPTER 11 Water and Solutions 11-16