A base is a proton acceptor. As was discussed in this previous post on acids, a proton is an H⁺ ion.
Many bases are oxides or hydroxides of metals. However, bases are any substance that accept protons, which includes substances like calcium carbonate.
If you look at the following reactions bewteen acids and bases, you can see how the base accepts the proton from the acid:
Base + acid → salt + water
MgO(s) +2HCl → MgCl₂(aq) + H₂O(l)
NaOH(aq) +HNO₃(aq) → NaNO₃(aq) + H₂O(l)
This becomes more obvious when you see the ionic equations for these reactions:
O²⁻(s) + 2H⁺(aq) → H₂O(l) (first equation)
OH⁻(aq) + H⁺(aq) → H₂O(l) (second equation)
Hence, bases accept protons.
In the second equation, sodium hydroxide is used, which is a base that is soluble in water. Bases that are soluble in water are known as alkalis.
Like acids, alkalis can also be classified as weak or strong based on their completeness of ionization in aqueous solution.
Strong alkalis ionize completely in aqueous solution (all the charged particles are completely separated and dissociated). For example, sodium hydroxide is a strong alkali.
Weak alkalis do not ionize completely in aqueous solution. For example, ammonia is a weak alkali.
Characteristics of bases
They give characteristic colours with certain indicators, e.g. they turn litmus blue and phenolpthalein from colourless to pink.
They conduct electricity when in aqueous solution
They neutralize acids to form a salt and water
They react with ammonium salts to produce ammonia gas, a salt and water, for example: (NH₄)₂SO₄(s) + CaOH₂(s) → 2NH₃(g) + 2H₂O(l) + CaSO₄(s)
Alkalis are slippery to the touch because the alkali converts oils on the skin to soap. (Strong alkalis can damage skin)