An alkali is a base that is soluble in water; it ionises in water to produce hydroxide ions, OH⁻.
Malay: Alkali · Chinese: 碱液
An alkali is a base that is soluble in water, and when it dissolves it ionises to produce hydroxide ions, OH⁻. Both halves matter: an alkali is always a base, but it is the special kind of base that dissolves and fills the solution with OH⁻ ions. Those hydroxide ions are what give an alkaline solution its properties, turning red litmus blue, feeling soapy, and neutralising acids.
A worked example shows the ionisation. Sodium hydroxide dissolves fully, NaOH → Na⁺ + OH⁻, so its solution is a strong alkali with a high concentration of hydroxide ions and a pH near 14. Ammonia dissolves in water and reacts with it to give some hydroxide ions, NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, so it is a weaker alkali. Common alkalis you should recognise are sodium hydroxide, potassium hydroxide, calcium hydroxide and aqueous ammonia.
The confusion to avoid is treating “base” and “alkali” as the same word. Every alkali is a base, but not every base is an alkali, only the soluble ones qualify. Copper(II) oxide and magnesium oxide are bases because they neutralise acids, yet they are insoluble, so they are not alkalis. In an exam, calling an insoluble metal oxide an “alkali” is a straight error. The clean test is solubility: if the base dissolves and releases OH⁻ into solution, it is an alkali; if it does not dissolve, it is a base but not an alkali.
Within this chapter the alkali is the direct partner of the acid: acids release H⁺, alkalis release OH⁻, and mixing them gives neutralisation, H⁺ + OH⁻ → H₂O. Our teachers make sure you keep the base–alkali distinction sharp, because it decides how a salt is prepared later in the chapter.
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