Bases and Alkalis Are Not the Same

What students typically write

“A base is the same as an alkali.” Or: “Alkalis and bases both neutralise acids, so they are the same thing.” Or: “Copper oxide is an alkali.”

Students learn that both bases and alkalis react with acids and conclude they are interchangeable terms. The distinction — solubility in water — is a detail they overlook.

Why it is wrong

The definitions are precise and the difference matters:

• Base: A substance that reacts with (neutralises) an acid to form a salt and water. Bases include metal oxides (CuO, MgO, Fe2O3), metal hydroxides (NaOH, Cu(OH)2), and ammonia.
• Alkali: A base that is soluble in water and produces hydroxide ions (OH-) in aqueous solution. Examples: NaOH, KOH, Ca(OH)2 (slightly soluble).

The relationship is a subset: all alkalis are bases, but not all bases are alkalis.

Copper(II) oxide (CuO) is a base because it neutralises sulfuric acid to form copper sulfate and water: CuO + H2SO4 goes to CuSO4 + H2O. But CuO does not dissolve in water, so it is not an alkali. It cannot produce OH- ions in solution.

Sodium hydroxide (NaOH) is both a base and an alkali. It neutralises acids, and it dissolves in water to give Na+ and OH- ions.

What the mark scheme actually wants

A typical question: “Explain the difference between a base and an alkali.” [2]

Marking points:

1. A base neutralises an acid / a base reacts with an acid to form salt and water (1 mark)
2. An alkali is a base that is soluble in water / dissolves in water to produce OH- ions (1 mark)

An answer that treats them as identical scores 0.

Worked example: wrong vs right

Question: Classify the following as a base, an alkali, or both: (a) sodium hydroxide, (b) copper(II) oxide. [2]

Wrong answer: “(a) Alkali. (b) Alkali.” This scores 1/2 at most. NaOH is correctly an alkali (and a base), but CuO is not an alkali.

Right answer: “(a) Both a base and an alkali: it neutralises acids and dissolves in water to produce OH- ions (1). (b) A base only: it neutralises acids but is insoluble in water (1).” This scores 2/2.

How to avoid this mistake

Ask the solubility question: does this base dissolve in water?

• If yes: it is a base AND an alkali (e.g., NaOH, KOH)
• If no: it is a base only (e.g., CuO, Fe2O3, Cu(OH)2)

At IGCSE level, the main alkalis you need are NaOH, KOH, Ca(OH)2, and ammonia solution. Everything else that neutralises acid but does not dissolve is “just a base”.

For the full treatment of acid-base reactions, see acids, bases and indicators. Metal oxide reactions with acids are covered at oxides.