Base (chemistry)

The common (Arrhenius) definition of a base, also known as an alkaline compound, is a chemical compound that either donates hydroxide ions or absorbs hydrogen ions when dissolved in water. Bases and acids are seen as opposites because the effect of an acid is to increase the hydronium ion (H3O) concentration in water, whereas bases reduce this concentration. Arrhenius bases are water-soluble and these solutions always have a pH greater than 7.

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 hydrogen ions (H+) and hydroxide ions (OH−), according to the equation

Related Topics:
PH - Acidity - Hydrogen ion - Hydroxide ions

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:mbox{H}_2mbox{O}leftrightarrowmbox{H}^++mbox{OH}^-.

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The concentration (in mole/liter) of the ions is indicated as and ; their product is the dissociation constant of water with and has the value 10−14 mole2/l2. The pH is defined as −log ; thus, pure water has a pH of 7. (These numbers are correct at 23 °C and slightly different at other temperatures.)

Related Topics:
Concentration - Mole - Liter - Dissociation constant

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

Related Topics:
Hydrogen ion - Hydroxide ions

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

Related Topics:
Sodium hydroxide - G

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