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Pure water has a pH of 7 at 25°C, meaning it is neutral. When an acid is dissolved in water, the pH will be less than 7, while a base, or alkali, will have a pH greater than 7. A strong acid, such as hydrochloric acid, at concentration 1 mol dm −3 has a pH of 0, while a strong alkali like sodium hydroxide, at the same concentration, has a pH ...
Water is amphoteric: it has the ability to act as either an acid or a base in chemical reactions. [87] According to the Brønsted-Lowry definition, an acid is a proton (H +) donor and a base is a proton acceptor. [88] When reacting with a stronger acid, water acts as a base; when reacting with a stronger base, it acts as an acid. [88]
Water is also central to acid-base neutrality and enzyme function. An acid, a hydrogen ion (H +, that is, a proton) donor, can be neutralized by a base, a proton acceptor such as a hydroxide ion (OH −) to form water. Water is considered to be neutral, with a pH (the negative log of the hydrogen ion concentration) of 7 in an ideal state.
In pure water, there is an equal number of hydroxide and H + ions, so it is a neutral solution. At 25 °C (77 °F), pure water has a pH of 7 and a pOH of 7 (this varies when the temperature changes: see self-ionization of water). A pH value less than 7 indicates an acidic solution, and a pH value more than 7 indicates a basic solution. [7]
In and of themselves, pH indicators are usually weak acids or weak bases. The general reaction scheme of acidic pH indicators in aqueous solutions can be formulated as: HInd (aq) + H 2 O (l) ⇌ H 3 O + (aq) + Ind − (aq) where, "HInd" is the acidic form and "Ind −" is the conjugate base of the indicator. Vice versa for basic pH indicators ...
A strong base is a basic chemical compound that can remove a proton (H +) from (or deprotonate) a molecule of even a very weak acid (such as water) in an acid–base reaction. Common examples of strong bases include hydroxides of alkali metals and alkaline earth metals, like NaOH and Ca(OH)
Self-ionizable compounds like water are also amphiprotic. Ampholytes are amphoteric molecules that contain both acidic and basic functional groups. For example, an amino acid H 2 N−RCH−CO 2 H has both a basic group −NH 2 and an acidic group −COOH, and exists as several structures in chemical equilibrium:
Bases are proton acceptors; a base will receive a hydrogen ion from water, H 2 O, and the remaining H + concentration in the solution determines pH. A weak base will have a higher H + concentration than a stronger base because it is less completely protonated than a stronger base and, therefore, more hydrogen ions remain in its solution.