Alkali

Summary: A soluble base that releases hydroxide ions (OH⁻) when dissolved in water. Tags: igcse chemistry definition acids-bases Created: 2026-07-14 Last Updated: 2026-07-14

An alkali is a soluble base that produces hydroxide ions (OH⁻) in aqueous solution.

Soluble Bases

All alkalis are bases, but not all bases are alkalis. For example:

• NaOH is an alkali (soluble base)
• CuO is a base but NOT an alkali (insoluble in water)

An alkali dissolves in water to release OH⁻ ions:

• NaOH(s) → Na⁺(aq) + OH⁻(aq)
• KOH(s) → K⁺(aq) + OH⁻(aq)

OH⁻ Ions in Aqueous Solution

Aqueous solutions of alkalis contain OH⁻ ions (hydroxide ions). These are what make a solution alkaline. The higher the concentration of OH⁻ ions:

• The higher the pH
• The stronger the alkalinity

Alkalis and Indicators

Effect on Litmus

Alkalis turn red litmus blue. This is the simplest test for alkalinity.

Effect on Thymolphthalein

Alkalis turn thymolphthalein blue. (Thymolphthalein is colourless in acid and neutral; blue in alkali — colour change at pH 9.3–10.5.)

Effect on Methyl Orange

Alkalis turn methyl orange yellow. (Methyl orange is red in acid; yellow in alkali.)

Key properties of alkalis:

• Feel soapy/slippery to touch
• Turn red litmus blue
• pH greater than 7
• React with acids to form salt + water
• React with ammonium salts to produce ammonia gas

Common alkalis: NaOH, KOH, Ca(OH)₂ (limewater), aqueous ammonia

See also: Base, Acid, Acids and Bases, Indicators and pH, Litmus, Methyl Orange, Thymolphthalein

This is a key term definition page — part of the IGCSE Chemistry (0620/0971) keyword glossary.