Cambridge O Level Chemistry · Syllabus 5070 · 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 O Level Chemistry.
Alkali in context
Acids, bases and salts are three connected ideas: an acid is a proton donor that produces hydrogen ions, H+(aq), in solution; a base is a proton acceptor — a metal oxide or metal hydroxide — that neutralises an acid to form a salt and water; and a salt is the compound formed when the hydrogen ion of an acid is replaced by a metal ion or the ammonium ion. An alkali is simply a base that is soluble in water, so every alkali is a base but not every base is an alkali, since insoluble bases such as copper(II) oxide neutralise acids without ever forming a solution.
Preparing a pure salt follows one route every time: identify the particles present, choose the reacting partner, predict the product, then apply the six solubility rules to decide whether the salt is soluble. A soluble salt made from a dissolved alkali must be prepared by titration, because an excess alkali cannot be filtered off; a soluble salt made from an insoluble solid uses excess solid followed by filtration; and an insoluble salt is made directly by precipitation, filtered, washed and dried. Every soluble-salt route ends the same way — evaporate to the crystallisation point, cool, filter, wash and dry.
Common mistakes with Alkali
- “A base is the same thing as an alkali.” Repair A base is a metal oxide or metal hydroxide. 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.
- “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 requires. Thymolphthalein is colourless in acid and blue in alkali. The three required 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 statement 7.1.12 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.
Examiner tips on Alkali
- The two sentences examiners are looking for “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.
Questions students ask about Alkali
What is the difference between a base and an alkali?
A base is a metal oxide or metal hydroxide that neutralises an acid; an alkali is a base that is also soluble in water. Copper(II) oxide is a base — it reacts with acid to give a salt and water — but it is insoluble, so no solution of it exists to turn litmus blue, and it is not an alkali.
Why can't you make a soluble salt from an acid and an alkali by filtering?
Both the acid and the alkali are dissolved, and so is the salt formed, so there is no undissolved solid for a filter to remove. The exact volume of alkali that reacts with the acid must instead be found by titration, using an indicator, and then the same volumes are mixed again without indicator so the crystals are not contaminated.
How do you decide which method to use to prepare a given salt?
First check whether the target salt is soluble using the six solubility rules. An insoluble salt is made by precipitation. A soluble salt is made either by titration, if the other reactant is a dissolved alkali, or by adding an insoluble solid — a reactive metal, base or carbonate — in excess and filtering off the excess afterwards.
What are the colours of the three required indicators in acid and in alkali?
Litmus is red in acid and blue in alkali. Thymolphthalein is colourless in acid and blue in alkali. Methyl orange is red in acid and yellow in alkali. Each indicator must be learned as a complete pair, since half a pair — such as remembering only that methyl orange is red — is not enough to answer a question correctly.