The number preceding a chemical symbol or formula indicates the number of atoms or molecules reacting. The absence of a number indicates that only one atom or molecule is present. Acids turn blue litmus paper red and have a sour taste. The strength of an acid depends on the degree to which it ionizes in water, releasing hydrogen ions. Thus, HCl is a very strong acid because most of its molecules dissociate, producing hydrogen and chloride ions.
Bases turn red litmus paper blue.
Since the concentration of hydrogen or hydroxide ions is usually small, it is convenient to express the degree of acidity or alkalinity in a solution in terms of pH, formally defined as the negative logarithm of the hydrogen ion concentration. The pH scale is logarithmic, extending from 0, the pH of a very strong acid, to 14, the pH of a very strong base. A pH of 5 represents another tenfold increase. Therefore, a solution with a pH of 4 is 10 times 10 or 100 times more acidic than a solution with a pH of 6. The contents of most animal and plant cells are neither strongly acidic nor alkaline but are an essentially neutral mixture of acidic and basic substances.