Required practical - Salt preparation (AQA GCSE Chemistry Combined Science): Revision Notes
Required practical - Salt preparation
What is salt preparation?
Salt preparation is a practical method used to make pure, dry salt crystals. This process involves reacting an acid with an insoluble substance like a metal oxide, metal carbonate, or metal hydroxide through chemical reaction and purification techniques.
When an acid reacts with these substances, it creates a salt plus water. The type of salt you get depends on which acid you use:
- Sulfuric acid makes sulphates
- Hydrochloric acid makes chlorides
- Nitric acid makes nitrates
The relationship between acids and salts is fundamental in chemistry. Each type of acid produces a specific family of salts, which is why choosing the right acid is crucial for preparing the desired salt compound.
Aim of the practical
The main goal is to create a pure, dry sample of a salt through chemical reaction and purification techniques.
Equipment you'll need
- Eye protection (safety goggles)
- Beaker
- Measuring cylinder
- Spatula and glass stirring rod
- Bunsen burner
- Tripod and gauze
- Philtre funnel and philtre paper
- Evaporating basin
Safety Equipment Priority
Eye protection is listed first because acids can cause serious damage to eyes. Always put on safety goggles before handling any chemicals or starting the practical work.
Step-by-step method
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Measure the acid: Use a measuring cylinder to measure 50cm³ of dilute sulfuric acid and pour it into a beaker.
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Heat the acid: Place the beaker on a tripod with gauze underneath. Heat gently with a Bunsen burner to speed up the reaction.
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Add the metal oxide: Using a spatula, add small amounts of copper oxide powder to the warm acid. Stir with a glass rod.
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Keep adding until excess remains: Continue adding copper oxide and stirring until some black powder remains unreacted in the beaker. This shows all the acid has been used up.
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Filter the mixture: Pour the mixture through philtre paper in a funnel to remove the excess copper oxide that didn't react.
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Start crystallisation: Heat the filtered solution gently using a water bath or electric heater until crystals start to form.
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Complete crystallisation: Pour the solution into an evaporating basin and leave it in a warm place until crystals have fully formed.
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Collect the crystals: Scrape the dry crystals onto philtre paper and pat them dry.
Critical Step - Adding Excess Metal Oxide
Step 4 is crucial for success. You must continue adding copper oxide until some remains unreacted. If you stop too early, leftover acid will contaminate your salt crystals and make them impure.
What happens during the reaction?
The copper oxide (which appears as black powder) reacts with the sulfuric acid to form copper sulphate and water. During the reaction, the black copper oxide dissolves to create a blue solution of copper sulphate. When water evaporates from this solution, blue copper sulphate crystals form.
Chemical Reaction: Copper Oxide + Sulfuric Acid
The complete chemical equation for this reaction is:
Copper oxide + Sulfuric acid → Copper sulphate + Water
This shows that one molecule of copper oxide reacts with one molecule of sulfuric acid to produce one molecule of copper sulphate and one molecule of water.
Why we heat the acid
Warming the acid makes the reaction happen faster. This saves time during the practical and ensures the reaction goes to completion.
Why we add excess copper oxide
Adding extra copper oxide ensures that all of the acid gets used up completely. If any acid was left over, it would make the salt crystals impure. The excess copper oxide that doesn't react gets filtered out.
Results you should see
- The black copper oxide powder will dissolve in the acid
- The solution turns blue as copper sulphate forms
- After filtering, you get a clear blue solution
- Blue crystals form when the water evaporates
Color Changes as Progress Indicators
The colour changes you observe are excellent indicators of your progress. The initial black powder disappearing shows the reaction is occurring, the blue colour confirms copper sulphate formation, and the final blue crystals indicate successful salt preparation.
Results analysis
The formation of blue crystals indicates that the practical was successful. The crystals should be:
- Pure (no black residue mixed in)
- Dry (no water droplets visible)
- Well-formed (clear crystal structure)
- Blue in colour (confirming copper sulphate formation)
Key Points to Remember:
- Safety first - always wear eye protection when handling acids
- Heat gently - this speeds up the reaction without causing dangerous splashing
- Add excess metal oxide - this ensures all the acid reacts completely
- Filter carefully - this removes unreacted solid and gives you pure salt solution
- The type of salt depends on the acid used - sulfuric acid makes sulphates, hydrochloric acid makes chlorides, nitric acid makes nitrates