Making Soluble Salts

Question 1

A soluble salt can be made by reacting an acid with an insoluble reactant.

Give some examples of a suitable insoluble reactant.

Answer 1

• A metal
• A metal oxide
• A carbonate

(following solubility rules)

Question 2

Describe a method to make a soluble salt from an acid and an insoluble reactant.

Answer 2

• Add dilute hydrochloric acid to beaker
• Add powdered insoluble reactant to acid in beaker, one spatula at a time, stirring to mix - the mixture will effervesce
• Continue adding powder until some unreacted powder is left over (is in excess)
• Filter the mixture in the beaker to remove the excess powder
• Allow the water in the solution to evaporate by heating in order to obtain pure dry crystals of the salt

Question 3

Why is an excess of the reactant added when preparing a soluble salt from an acid and an insoluble reactant?

Answer 3

To make sure all of the acid has reacted

Question 4

Why do we filter out the unreacted insoluble reactant from the salt solution after the acid and the reactant have reacted?

Answer 4

To leave behind only the soluble salt and water in the solution

Question 5

What is the purpose for having only the soluble salt and water left in the solution?

Answer 5

So you can heat the solution which evaporates the water, leaving behind the pure salt to crystallise

Question 6

Describe how to carry out an acid-alkali titration, using a burette, pipette and a suitable indicator, to prepare a pure, dry salt.

Answer 6

• Measure an amount of acid into conical flask using pipette - add few drops of indicator
• Use a burette to add alkali to acid until the acid has been neutralised and the indicator changes colour
• Repeat reaction using same volumes of acid and alkali but with no indicator, so salt won’t be contaminated with indicator
• Remaining solution after reaction done contains only salt and water
• Evaporate water and leave solution to crystallise - filter off and dry the solid, leaving you with a pure, dry salt

Question 7

Why must titration be used when preparing a soluble salt from reacting an acid and an alkali?

Answer 7

To determine the volumes of acid and alkali that must be mixed to obtain a solution containing only salt and water

Question 8

Why is it important that the acid and the alkali are mixed in the correct proportions in an acid/alkali titration?

Answer 8

To neutralise the acid

Question 9

Suggest why universal indicator must not be used in titration experiments.

Answer 9

The colour change is too gradual