Base (chemistry)

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In chemistry, a base is a compound that is the opposite of an acid in the sense that it will neutralize an acid. Common bases include baking soda, sodium carbonate and ammonia. Many metal oxides and hydroxides will also form basic solutions.

Table of contents

1 Bases and pH
2 Neutralization of acids
3 Alkalinity of non-hydroxides
4 See also

Bases and pH

The pH of (impure) water is a measure of its acidity. In pure water, about one in ten million molecules dissociate into hydronium ionss (H⁺) and hydroxyl ionss (OH⁻), according to the equation

The concentration (in mole per liter) of the ions is indicated as [H⁺] and [OH⁻]; their product is the dissociation constant of water with and has the value 10⁻¹⁴ mol²/l². The pH is defined as −log [H⁺]; thus, pure water has a pH of 7. (These numbers are correct at 23 °C and slightly different at other temperatures.)

A base accepts (removes) hydronium ionss (H⁺) from the solution, or donates hydroxyl ionss (OH⁻) to the solution. Both actions will lower the hydronium concentration, and thus raise pH. By contrast, an acid donates H⁺ ions to the solution or accepts OH⁻, thus lowering pH.

The pH of a solution can be calculated. For example, if 1 mol of sodium hydroxide (40 g) is dissolved in 1 liter of water, the concentration of hydroxyl ions becomes [OH⁻] = 1 mol/l. Therefore [H⁺] = 10⁻¹⁴ mol/l, and pH = −log 10⁻¹⁴ = 14.

Neutralization of acids

When dissolved in water, sodium hydroxide decomposes into hydroxyl and sodium ions:

and similarly, hydrochloric acid forms hydronium and chloride ions:

When the two solutions are mixed, the H⁺ and OH⁻ ions combine to form water molecules:

If equal amounts of NaOH and HCl (measured in moles, not grams) are dissolved, the base and the acid exactly neutralize, leaving only NaCl (table salt) in solution.

Alkalinity of non-hydroxides

Both sodium carbonate and ammonia are bases, although neither of these substances contains OH⁻ groups. That is because both compounds accept H⁺ when dissolved in water: