Chemistry

Bases
Bases are substances that are soapy to touch and bitter in taste.
A base is a substance, usually the oxide or the hydroxide of a metal, which can react with an acid to produce salt and water.
For example, sodium oxide (Na₂O) , calcium oxide (CaO), cupric oxide (CuO), iron oxides (FeO, Fe₂O₃), sodium hydroxide (NaOH) and calcium hydroxide(Ca(OH))₂ are all bases.
Certain substances are also called bases, though they do not fit into the above definition. For example, ammonia (NH₃). It forms salt with an acid without giving water. So, it should not be treated as a base. But ammonium hydroxide (NH₄OH), the aqueous solution of NH₃, is a base as it reacts with an acid to give salt and water
NH₄OH + HCl → NH₄ Cl + H₂O
Alkalis :
Bases that are soluble in water are called alkalis. For example, sodium hydroxide, potassium hydroxide, calcium hydroxide are soluble in water. Therefore, they are alkalis. But bases like copper hydroxide Cu(OH)₂ ferric hydroxide (Fe(OH)₃) aluminium hydroxide (Al(OH)₃)do not dissolve in water. They are, therefore, not alkalis.
Hence, all alkalis are bases, but all bases are not alkalis. Some of the bases are listed here in Table.

Lime water, baking soda and washing soda are all bases