Making Salts

Section 1 of 6What a salt is

Definition

• Definition A salt forms when the hydrogen in an acid is replaced by a metal ion or by the ammonium ion.
  • Replacing the hydrogen in HCl with potassium gives potassium chloride, KCl.

Uses

• Uses Salts are used in fertilisers, batteries, cleaning products, medicines and fungicides.
• Some can be mined, but many are made in the laboratory.

Section 2 of 6Choosing the starting material

Starting material	Suitable?	Reason or example
Copper (below hydrogen in the reactivity series)	No	It does not react with dilute acids.
Sodium or other very reactive metals	No	The reaction with acid is dangerously violent.
Zinc or magnesium (moderately reactive)	Yes	Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
Insoluble base or carbonate	Yes	CuO(s) + H2SO4(aq) → CuSO4(aq) + H2O(l)
Soluble alkali	Yes, by titration	HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)

Section 3 of 6Method: acid with an insoluble solid

1

Warm dilute acid in a beaker over a Bunsen burner, taking care not to boil it.

2

Add the metal, oxide or carbonate a little at a time, stirring, until some solid remains undissolved. This is the excess, and it shows all the acid has been used.

3

Filter the mixture into an evaporating basin to remove the unreacted solid; the filtrate is the salt solution.

4

Heat the solution gently to evaporate some water until crystals start to form on a glass rod dipped in and cooled.

5

Leave the solution to cool and crystallise, filter off the crystals, and dry them between filter paper.

Section 4 of 6Method: titration for a soluble alkali

1Measure the alkali

• Measure the alkali Use a pipette to place a fixed volume of alkali in a conical flask and add a few drops of indicator.

2Add acid to the end-point

• Add acid to the end-point Run acid from a burette until the indicator just changes colour, and note the volume.

3Repeat without indicator

• Repeat without indicator Mix the same volumes of acid and alkali again but leave the indicator out, so the salt stays pure.

4Crystallise

• Crystallise Evaporate part of the water, cool, filter and dry the crystals.

Section 5 of 6Key terms

Term	Meaning
Salt	A compound formed when the hydrogen of an acid is replaced by a metal or ammonium ion.
Excess	More of a reactant than is needed, so some is left over.
Filtrate	The liquid that passes through filter paper.
Crystallisation	Forming solid crystals from a solution as water is removed or the solution is cooled.
End-point	The moment in a titration when the indicator changes colour.

Section 6 of 6Exam technique

1

Using too little solid. The base must be in excess, otherwise acid is left and the salt is not pure.

2

Evaporating the solution to dryness. Crystallise from a hot, saturated solution instead, because drying out in the basin can spoil the crystals.

3

Adding indicator to the final titration mixture. The indicator would contaminate the crystals, so repeat without it.

4

Choosing copper or sodium to react with an acid. Copper does not react and sodium is far too dangerous.