Lesson focus: A pure soluble saltA compound made when an acid reacts with a base, such as sodium chloride (table salt). can be prepared by selecting a suitable acid and base, removing excessMore than enough, so some is left over after the reaction. solid and crystallising carefully.
Plan a salt preparation, choose reagents from the target salt and explain each separation step.
Build on earlier learning
NeutralisationNeutralisation provides the reaction; this lesson isolates a useful pure salt product.
Practical
Preparing copper sulfate crystals
- Warm dilute sulfuric acid and add copper oxide until some is left over.
- Filter to remove the excess copper oxide.
- Heat the blue filtrate gently to evaporate some water, then leave it to crystallise.
Record: Observations at each step and why excess copper oxide is used.
Safety: Wear eye protection; do not boil the solution dry.
You will be successful when you can…
- Choose acid and base for a target salt.
- Explain use of excess insoluble base.
- Sequence filtration and crystallisation.
- Distinguish insoluble-base and titration methods.
Words that carry the science
- salt
- An ionic compound formed when acid hydrogen is replaced by a metal or ammonium ion.
- excess
- More reactant than the amount needed to consume the other reactant.
- filtrate
- Liquid passing through filter paper.
- crystallisation
- Formation of solid crystals from a solution.
- saturated solution
- A solution holding the maximum dissolved solute under stated conditions.
Choose ions from the target name
01Choose ions from the target name
The acid supplies chloride, sulfate or nitrate. The metal compound supplies the positive metal ion.
To make copper sulfate, use sulfuric acid and a suitable copper base such as copper oxide. Copper metal does not react usefully with dilute sulfuric acid.
02Excess insoluble base removes all acid
Warm dilute acid gently, add insoluble base in small portions while stirring, and continue until some solid remains. This shows the acid has been consumed.
Filter to remove excessMore than enough, so some is left over after the reaction. unreacted solid. The desired soluble salt remains in the filtrateThe liquid that passes through the filter paper.. Eye protection and controlled heating are essential.
03Concentrate, cool and dry crystals
Evaporate some water gently until the solution is near saturation, then cool so crystals form. Filter and dry the crystals.
Do not heat to dryness: strong heating can spit solution or decompose hydrated salts. If both reactants are soluble, use titration rather than adding a filterable excess.
Making a salt
A salt is made when an acid reacts with a base. The first part of the salt's name comes from the metal in the base; the second part comes from the acid. Excess base makes sure all the acid is used up.
Dots: solid metal oxide. Diamonds: salt crystals.
Controls
| Acid | Salt from copper oxide |
|---|---|
| Hydrochloric acid | copper chloride |
| Sulfuric acid | copper sulfate |
| Nitric acid | copper nitrate |
Plan copper sulfate preparation
Prepare copper sulfate crystals from copper oxide.
- Warm dilute sulfuric acid in a water bath or with controlled gentle heating.
- Add copper oxide with stirring until some remains in excess.
- Filter off excess copper oxide.
- Concentrate the blue filtrate, cool to crystallise, then filter and dry crystals.
How to make the reasoning visible: The method consumes the acid and separates pure soluble copper sulfate from excess insoluble base.
Salt preparation
Match each target to reagents and a method.
| Target salt | Acid | Second reagent | Method |
|---|---|---|---|
| copper sulfate | sulfuric acid | copper oxide | excess + filter |
| magnesium chloride | hydrochloric acid | magnesium oxide | excess + filter |
| sodium nitrate | nitric acid | sodium hydroxide | titration |
| potassium sulfate | sulfuric acid | potassium hydroxide | titration |
Why do sodium and potassium salts use titration here?
Which step separates unreacted copper oxide?
Check your understanding
Answer from memory, then check against the suggested answer.
Which acid makes sulfates?
What is filtrate?
Why cool concentrated solution?
Choose reagents for zinc nitrate crystals.
Why can washing crystals improve purity but reduce yield?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A student makes pure, dry copper sulfate crystals from dilute sulfuric acid and copper oxide.
Describe how to make a copper sulfate solution with no copper oxide left in it.
(3)Describe how to get dry crystals from the solution.
(3)Total for this question: 6 marks
Command word: For describeA command word: say what happens or what something is like, step by step, without needing to give reasons., give the method in order and state the purpose of separation steps.