Chemistry AQA GCSE Core Revision Notes on Acids, Bases, Salts, PH

Acids, bases and salts

Soluble salts can be made from acids and insoluble salts can be made from solutions of ions. When acids and alkalis react the result is a neutralisation reaction.

It is necessary to know to:

■ select an appropriate method for making a salt, given appropriate information.

Making salts

a) The state symbols in equations are (s), ( l ) , (g) and (aq).

b) Soluble salts can be made from acids by reacting them with:

■ metals – not all metals are suitable; some are too reactive and others are not reactive enough

■ insoluble bases – the base is added to the acid until no more will react and the excess solid is filtered off

■ alkalis – an indicator can be used to show when the acid and alkali have completely reacted to produce a salt solution.

c) Salt solutions can be crystallized to produce solid salts.

d) Insoluble salts can be made by mixing appropriate solutions of ions so that a precipitate is formed.

Precipitation can be used to remove unwanted ions from solutions, for example in treating water for drinking or in treating effluent.

Acids and bases

a) Metal oxides and hydroxides are bases. Soluble hydroxides are called alkalis.

b) The particular salt produced in any reaction between an acid and a base or alkali depends on:

■ the acid used (hydrochloric acid produces chlorides, nitric acid produces nitrates, sulfuric acid produces sulfates)

■ the metal in the base or alkali.

c) Ammonia dissolves in water to produce an alkaline solution. It is used to produce ammonium salts. Ammonium salts are important as fertilisers.

d) Hydrogen ions, H+(aq), make solutions acidic and hydroxide ions, OH–(aq), make solutions alkaline.

The pH scale is a measure of the acidity or alkalinity of a solution.

e) In neutralisation reactions, hydrogen ions react with hydroxide ions to produce water. This reaction can be represented by the equation:

H+(aq) + OH–(aq) H2O(l)

You need to describe the steps of this experiment (practical):

■ the preparation of soluble salts:

− copper sulfate by adding copper oxide to sulfuric acid

− magnesium sulfate by adding magnesium oxide to sulfuric acid

− copper chloride by adding copper oxide to hydrochloric acid

− zinc nitrate by adding zinc oxide to nitric acid

− sodium chloride by adding sodium hydroxide to hydrochloric acid

− copper sulfate by adding copper carbonate to sulfuric acid

− investigation of the effect of conditions on the yield of the salt

■ the preparation of insoluble salts:

− lead iodide by mixing solutions of lead nitrate and potassium iodide

− barium sulfate by mixing solutions of barium chloride and sodium sulfate

− investigation of the effect of conditions on the formation of precipitates.

TIPS: 

• DO YOU KNOW THE STEPS FOR USING pH SENSORS TO INVESTIGATE NEUTRALIZATION? 

• Are you familiar with the pH scale from 0 to 14, and that pH 7 is a neutral solution?