Making salts required practical

Aim: To prepare a pure, dry sample of a soluble salt, such as copper(II) sulfate, by reacting an insoluble metal oxide or carbonate with a dilute acid.

Equipment

• Dilute sulfuric acid (H₂SO₄)
• Copper(II) oxide powder (or copper(II) carbonate)
• Measuring cylinder
• Beaker (100 cm³) and glass stirring rod
• Spatula
• Bunsen burner, tripod, gauze and heatproof mat
• Filter funnel, filter paper and conical flask
• Evaporating basin and water bath (or beaker of boiling water)
• Crystallising dish and paper towels

Method

1. Measure about 40 cm³ of dilute sulfuric acid into a beaker using a measuring cylinder.
2. Warm the acid gently over a Bunsen burner, without letting it boil, so the reaction goes faster.
3. Add copper(II) oxide a small spatula at a time, stirring after each addition.
4. Keep adding until some black solid is left over and will not dissolve; this shows the oxide is in excess and all the acid has reacted.
5. Filter the warm mixture into a conical flask to remove the excess copper(II) oxide.
6. Pour the blue filtrate into an evaporating basin and heat it over a water bath until about half the water has evaporated or crystals start to appear at the edge.
7. Pour the concentrated solution into a crystallising dish and leave it somewhere cool for a day or more so crystals can form slowly.
8. Remove the crystals and pat them dry between paper towels (or leave them in a warm place to dry).

Safety

• Wear eye protection: dilute sulfuric acid is an irritant and copper compounds are harmful.
• Do not boil the acid or heat the solution to dryness, as it can spit hot liquid and decomposes the crystals.
• Let the evaporating basin and tripod cool before touching them, or use tongs.
• Wash hands after handling copper compounds and do not taste the product.

Results and calculations

Describe the appearance of the crystals (blue crystals of copper(II) sulfate) and, if asked, record the mass obtained. The word equation is copper oxide + sulfuric acid → copper sulfate + water, and percentage yield = (actual mass ÷ theoretical mass) × 100.

Common mistakes and improvements

• Not adding the base in excess, leaving unreacted acid in the product; keep adding until solid remains undissolved.
• Heating the solution to dryness, which gives a powder rather than crystals and can decompose the salt; stop when crystals first appear and let it crystallise slowly.
• Letting solid get into the filtrate by overfilling the filter paper; pour slowly and keep the level below the top of the paper.
• Getting a low mass because some solution is left in the beaker or on the filter paper; rinse carefully and do not spill when transferring.

Exam tips

• Explain why the oxide or carbonate is added in excess: to make sure all the acid is used up so the salt is not contaminated with acid.
• Explain why filtration is used: to remove the unreacted insoluble base, not to collect the salt.
• Know how to name the salt from the acid: sulfuric acid gives sulfates, hydrochloric acid gives chlorides, nitric acid gives nitrates.
• With a carbonate, fizzing stops when the acid is used up; say that carbon dioxide is also produced.
• For 6-mark method questions, give the steps in a logical order with named equipment: react in excess, filter, evaporate, crystallise, dry.