Cambridge IGCSE Chemistry · Syllabus 0620 · Acids, Bases and Salts
Alkali
What is Alkali?
A base that is soluble in water. An alkali dissolves to give an aqueous solution containing hydroxide ions, OH-(aq); sodium hydroxide and potassium hydroxide are alkalis, while insoluble bases such as copper(II) oxide are not.
This definition is part of the Acids, Bases and Salts chapter in Cambridge IGCSE Chemistry.
Common mistakes with Alkali
- “A base is the same thing as an alkali.” Repair A base is an oxide or a hydroxide of a metal. An alkali is a base that dissolves in water. Copper(II) oxide is a base — it neutralises acids — but it is insoluble, so it is not an alkali and no solution of it exists to turn litmus blue.
- “Neutralisation always ends at pH 7.” Repair Neutralisation is defined by what reacts — an acid with an alkali, producing water — not by a number. A mixture only sits at pH 7 when exactly enough of each has been added and the salt formed does not itself make the solution acidic or alkaline. Add one drop too much acid and the reaction was still a neutralisation.
- “Methyl orange is purple in alkali.” Repair Methyl orange is red in acid and yellow in alkali. Purple belongs to litmus in a neutral solution. Learn each indicator as a complete pair; half a pair is worth nothing in a multiple-choice question.
- “Thymolphthalein is the pink one.” Repair That is phenolphthalein, a different indicator and not one of the three this syllabus names. Thymolphthalein is colourless in acid and blue in alkali. The three named indicators are litmus, thymolphthalein and methyl orange.
- “Universal indicator just tells you acid or alkali.” Repair Litmus does that. Universal indicator gives a graded colour, so it lets you compare how acidic or alkaline two solutions are, and match the colour to an approximate pH. That is what Core statement 7.1.7 is asking for.
- “Mix the acid and the alkali, then filter off the salt.” Repair There is nothing solid to filter. Both reactants are dissolved and so is the salt. That is precisely why an acid–alkali preparation must be done by titration — you have to measure the right volume, because you cannot remove an excess afterwards.
- “Ammonium chloride + sodium hydroxide → sodium chloride + ammonium hydroxide.” Repair There is no such compound as ammonium hydroxide in an answer at this level. On warming, an alkali reacts with an ammonium salt to give a salt, water and ammonia gas. Write NH3(g) and H2O(l) as two separate products.
Examiner tips on Alkali
- The two sentences that carry the whole method “An excess of an alkali cannot be removed by filtration, because it is dissolved — so the volumes must be measured instead.” and “The titration is repeated without indicator so that the indicator does not contaminate the crystals.” Those two ideas are what separate a described method from a memorised one: each names the property of the reagents that forces the step.