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### Theory:

We have studied that in an aqueous solution, an acid can produce hydrogen ions ($$H^+$$), while a base can produce hydroxyl ions ($$OH^-$$). When an acid and a base react, a neutral product is formed, which is called salt.
According to Arrhenius theory, bases are substances that ionise in water to form hydroxyl ions ($$OH^-$$).
$\begin{array}{l}\mathit{NaOH}\phantom{\rule{0.147em}{0ex}}\stackrel{}{⟶}\phantom{\rule{0.147em}{0ex}}{\mathit{Na}}^{+}+{\mathit{OH}}^{-}\\ \mathit{Base}\phantom{\rule{3.381em}{0ex}}\mathit{Hydroxyl}\phantom{\rule{0.147em}{0ex}}\mathit{ion}\end{array}$

Some metal oxides give salt and water on reacting with acids. These are also called bases.

$\begin{array}{l}{\mathit{ZnO}}_{\left(s\right)}+{2\mathit{HCl}}_{\left(l\right)}\phantom{\rule{0.147em}{0ex}}\stackrel{}{⟶}\phantom{\rule{0.147em}{0ex}}{\mathit{ZnCl}}_{2\left(\mathit{aq}\right)}+{{H}_{2}O}_{\left(l\right)}\\ \mathit{Zinc}\phantom{\rule{0.147em}{0ex}}\mathit{oxide}\end{array}$

Bases that are highly soluble in water are called alkalis.

Example for alkalis: Sodium hydroxide, potassium hydroxide, calcium hydroxide and ammonium
hydroxide.

Example for bases: Sodium carbonate, sodium bicarbonate, calcium carbonate.

When a base reacts with water, salt and water are produced.

$\mathit{Base}\phantom{\rule{0.147em}{0ex}}+\phantom{\rule{0.147em}{0ex}}\mathit{Acid}\phantom{\rule{0.147em}{0ex}}\stackrel{}{⟶}\phantom{\rule{0.147em}{0ex}}\mathit{Salt}\phantom{\rule{0.147em}{0ex}}+\phantom{\rule{0.147em}{0ex}}\mathit{Water}$

Bases contain one or more replaceable oxide or hydroxyl ions in solution.

Important!
All alkalis are bases, but not all bases are alkalis. For example, $$NaOH$$ and $$KOH$$ are alkalis, whereas $$Al(OH)_3$$ and $$Zn(OH)_2$$ are bases.
Bases can be categorised in different ways based on the following factors:
1. Acidity
2. Ionisation
3. Concentration
Classification of base