A base is a compound which, when dissolved in water, dissociates to produce hydroxide ions, $\text{OH}^{-}$
- Alternative definition: A base is a metal oxide or hydroxide which reacts with an acid to form water and a metal salt only.
Several examples of bases are as follows:
- Sodium hydroxide: $\text{NaOH} \rightarrow \text{Na}^{+} + \text{OH}^{-}$
- Potassium hydroxide: $\text{KOH} \rightarrow \text{K}^{+} + \text{OH}^{-}$
- Calcium hydroxide: $\text{Ca}(\text{OH})_{2} \rightarrow \text{Ca}^{2+} + 2 \text{OH}^{-}$
The hydroxide ions, $\text{OH}^{-}$ are responsible for all the alkaline properties.
Bases react with acids to produce a salt and water. This reaction is called neutralization. (Note: Neutralization will be dealt with in the next sub-topic) Two examples of the reaction are as follows:
- $\text{HCl} (\text{aq}) + \text{NaOH} (\text{aq}) \rightarrow \text{NaCl} (\text{aq}) + \text{H}_{2}\text{O} (\text{l})$
- $\text{H}_{2}\text{SO}_{4} (\text{aq}) + \text{ZnO} (\text{s}) \rightarrow \text{ZnSO}_{4} (\text{aq}) + \text{H}_{2}\text{O} (\text{l})$
Alkalis
Bases consist of metal oxides and metal hydroxides. Most bases are insoluble in water. Bases that dissolve in water are called alkalis.
Some examples of bases and alkalis are given below:
| Insoluble Bases | Soluble Bases (Alkalis) |
|---|---|
| Copper (II) oxide $$\text{CuO}$$ | Sodium hydroxide $$\text{NaOH}$$ |
| Magnesium oxide $$\text{MgO}$$ | Potassium hydroxide $$\text{KOH}$$ |
| Lead(II) oxide $$\text{PbO}$$ | Aqueous ammonia $$\text{NH}_{3} (\text{aq})$$ |
A solution containing hydroxide ions ($\text{OH}^{-}$) is produced when alkalis dissolve in water. The properties of alkalis are due to these hydroxide ions, $\text{OH}^{-}$.
Strong and Weak Alkalis
A strong alkali dissociates/ionises completely into ions. All the alkali molecules become ions in water.
- E.g. Aqueous Sodium Hydroxide is a strong alkali. It only contains $\text{Na}^{+} (\text{aq})$ and $\text{OH}^{-}(\text{aq})$ ions, and no $\text{NaOH}$ molecules are present.
- $\text{NaOH} (\text{aq}) \rightarrow \text{Na}^{+} (\text{aq}) + \text{OH}^{-} (\text{aq})$
- If 4 molecules of $\text{NaOH}$ are dissolved in water, all 4 molecules will dissociate into ions, producing $4 \text{ Na}^{+}$ ions and $4 \text{ OH}^{-}$ ions.
- Sodium hydroxide, Potassium hydroxide and Barium hydroxide are examples of strong alkalis.
Weak alkalis are only slightly/partially ionised. Not all the alkali molecules become ions in water.
- E.g. Aqueous ammonia, $\text{NH}_{3}$ is a weak alkali. Most of the alkali molecules remain unchanged in water as very few molecules are ionised to produce hydroxide ions.
- $\text{NH}_{3} (\text{aq}) + \text{H}_{2}\text{O} \left( \text{l} \right) \rightleftharpoons \text{NH}_{4}^{+} (\text{aq}) + \text{OH}^{-} (\text{aq})$
- The solution of ammonia will contain a large number of $\text{NH}_{3}$ molecules and a small number of $\text{NH}_{4}^{+}$ and $\text{OH}^{-}$ ions.
- Ammonia and magnesium hydroxide are examples of weak alkalis.
Basicity
The basicity of an alkali refers to the number of moles of hydroxide ions that can be produced by one mole of alkali.
Sodium hydroxide is a monobasic alkali:
$$\text{NaOH} (\text{aq}) \rightarrow \text{Na}^{+} (\text{aq}) + \text{OH}^{-}(\text{aq})$$
1 mole of sodium hydroxide produces 1 mole of hydroxide ions.
Barium hydroxide is a dibasic alkali:
$$\text{Ba}(\text{OH})_{2} (\text{aq}) \rightarrow \text{Ba}^{2+} (\text{aq}) + 2 \text{OH}^{-} (\text{aq})$$
1 mole of barium hydroxide produces 2 moles of hydroxide ions.
Uses of Alkalis
Alkalis have two main uses:
- Neutralise acids – Toothpaste usually contains magnesium hydroxide which neutralizes acids in the mouth, which are produced by bacteria.
- Dissolve dirt and grease – Soaps and detergents are mild alkalis (soapy feeling)
