Cambridge O Level Chemistry · Syllabus 5070 · Acids, Bases and Salts
Base
What is Base?
A substance that acts as a proton acceptor. In this syllabus a base is a metal oxide or a metal hydroxide, and it reacts with an acid to give a salt and water. A base does not have to dissolve in water.
This definition is part of the Acids, Bases and Salts chapter in Cambridge O Level Chemistry.
Base 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.
Common mistakes with Base
- “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.
- “Neutralisation always ends at pH 7.” Repair Neutralisation is defined by what reacts — an acid with a base — 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.
- “Amphoteric means neutral — it sits in the middle.” Repair The opposite. An amphoteric oxide reacts with both acids and bases to give a salt and water. A neutral oxide reacts with neither. Aluminium oxide and zinc oxide are the two you need; they are defined by evidence of reaction, never by position on a table.
Questions students ask about Base
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.
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.