Salts, acids, and bases are compounds that dissociate; that is, they ionize in water. For example, when a salt . such as sodium chloride (NaCI) is put into water, the -egative ends of the water molecules are attracted to the ium ions, and the positive ends of the water molecules are attracted to the chloride ions. This causes the sodium chloride to break apart, or dissociate, into individual ions:
Substances that break apart and release ions when put into water are called electrolytes (e-lek'tro-lits), since the ions can conduct an electrical current. The electrolyte balance in the blood and body tissues is important for good health because it affects the functioning of vital organs, such as the heart and the brain.
Substances like salts, acids, and bases that dissociate (that is, release ions when in water) are called electrolytes. The electrolyte balance in the blood and body tissues is important for good health.
Acids and Bases
When water ionizes, it releases an equal number of hydrogen ions and hydroxide ions. Only a few water molecules at a time are dissociated, and the actual number of ions is very small (0.0000001 of the water molecules in a pure sample have dissociated).
Lemon juice, vi negar, tomatoes, and coffee are all familiar acids. Acids (as'idz) are molecules that dissociate in water, releasi ng hydrogen ions. For example, hydrochloric acid (11 Cl) dissociates in the following manner:
HCI -----> H+ + Cl-
Because almost all hydrochloric acid molecules dissociate in water, hydrochloric acid is called a strong acid. If hydrochloric acid is added to a beaker of water, the number of hydrogen ions increases.
Milk of magnesia and ammonia are common bases.
Bases are molecules that either take up hydrogen ions or release hydroxide ions. For example, sodium hydroxide (NaOH) dissociates in the following manner:
NaOH ----> Na+ + OH-
Because almost all sodium hydroxide molecules dissociate in water, sodium hydroxide is called a strong base. If sodium hydroxide is added to a beaker of water, the number of hydroxide ions increases. These hydroxide ions can combine with hydrogen ions to form water. Thus, when a base is added to water, the number of hydrogen ions decreases.
pH
The pH scale ranges from 0-14. Any pH value below 7 is acidic, with the degree of acidity increasing toward the lower numbers. Any pH value above 7 is basic (or alkaline), with the degree of basicity increasing toward the higher numbers. A pH of exactly 7 is neutral. Water has an equal number of H+ and OH-, and therefore, one ion of each is formed when water dissociates. The fraction of water molecules that dissociate is 10-7 (0.0000001), which is the source of the pH value for neutral solutions. The pH scale was devised to simplify discussion of the hydrogen ion concentration [H+] without using cumbersome numbers. For example:
1 X 1O-6[H+] = pH 6 Each lower pH unit has 10
1 X 1O-7[H+] = pH 7 times the amount of H+ as
1 X lO-s[H+] = pH 8 the next higher unit.
Of the three values listed here, pH 6 has the greater number of H+ and is acidic, pH 7 has an equal number of H+ and OH- and is neutral, and pH 8 has the lower number of H+ and is basic.
All living things need to maintain the hydrogen ion concentration [1-r+], or pH, at a constant level. For example, the pH of the blood is held constant at about 7.4, or a person becomes ill. rfthis pH value drops below 7.35, the person is said to have acidosis (as'i-do'sis); if it rises above 7.45, the condition is called alkalosis (al"kah-Io'sis). Without medical intervention, a person usually cannot survive if the pH drops to 6.9 or rises to 7.8 for more than a few hours.
The presence of buffers helps keep the pH constant. A buffer is a chemical or a combination of chemicals that can take up excess H+ or excess OH-. When an acid is added to a buffered solution, the buffer takes up excess H+, and when a base is added to a buffered solution, the buffer takes up excess or r-. Therefore, the pH changes minimally whenever a solution is buffered.
Acids have a pH that is less than 7, and bases have a pH that is greater than 7. The presence of buffers helps keep the pH of body fluids constant at about neutral. or pH 7, because a buffer can absorb both hydrogen ions (H+) and hydroxide ions (OH-).
