High School Physics + more

Bases and alkalis – fundamentals

In this post, we will study the fundamentals of Bases and Alkalis (fundamentals, definition, differences, examples, reactions of alkalis with water, strong and weak alkalis, etc.)


Bases are substances that neutralize acids. They are the chemical opposites of acids. The largest group of bases are metal oxides, hydroxides, and carbonates, such as sodium oxide, copper oxide, sodium hydroxide, and copper carbonate.


A special sub-set of bases is called alkalis.

Alkalis are bases that are soluble in water. Their solutions have a pH above 7. They turn litmus blue and give a green, blue or purple color with a universal indicator.

The most common alkalis are sodium hydroxide (NaOH), calcium hydroxide (Ca(OH)2), and ammonia (NH3).

Calcium hydroxide is much less soluble than sodium hydroxide. A solution of calcium hydroxide in water is often called ‘limewater’.

reaction of alkalis with water

Sodium oxide (Na2O), potassium oxide (K2O), and calcium oxide (CaO) react with water to form their hydroxides.

So, the reactions of these three metal oxides with water produces alkalis. For example:
Na2O(s) + H2O(l) → 2NaOH (aq)
CaO(s) + H2O(l) → Ca(OH)2 (aq)
Most other metal oxides and hydroxides are insoluble in water. These insoluble metal oxides and hydroxides are bases but not alkalis.

Like acids, alkalis can be classified by the extent of their ionization in water.

strong alkalis & weak alkalis

Strong alkalis, such as sodium hydroxide and calcium hydroxide, are completely ionized in water, for example:
NaOH(aq) → Na+(aq) + OH(aq)

Weak alkalis, such as ammonia, are only partly ionized in water:
NH3(aq) + H2O(l) = NH4+(aq) + OH(aq)

All alkalis dissolve in water to produce hydroxide ions, OH. This has led to alkalis being defined as sources of hydroxide ions.

See also  Alkanes & The first four alkanes - revision notes
Scroll to top