Core practical - Salt preparation (AQA GCSE Chemistry): Revision Notes
Core practical - Salt preparation
What is salt preparation?
Salt preparation is a method used to make pure, dry samples of salts in the laboratory. When we talk about salts in chemistry, we don't just mean table salt - we mean compounds that form when acids react with metals, metal oxides, alkalis, or metal carbonates.
In chemistry, a salt is any compound formed from the reaction between an acid and a base. This includes many different compounds beyond just table salt (sodium chloride). Examples include copper sulphate, magnesium chloride, and calcium nitrate.
The main idea is simple: acids + bases = salts + water
Aim of the practical
The goal is to prepare a pure, dry sample of a salt - in this case, copper sulphate crystals.
Equipment you need
Safety equipment:
- Eye protection
- Beaker
- Measuring cylinder
Lab equipment:
- Tripod and gauze
- Philtre funnel and philtre paper
- Spatula and glass rod
- Bunsen burner
- Evaporating basin
Safety First! Always wear eye protection when working with acids. Sulfuric acid is corrosive and can cause serious burns to skin and eyes.
The method - step by step
Worked Example: Preparing Copper Sulphate Crystals
Step 1: Measure 50cm³ of dilute sulfuric acid using a measuring cylinder. Pour it into a beaker.
Step 2: Put the beaker on a tripod with gauze and warm it gently with a Bunsen burner.
Step 3: Using a spatula, add small amounts of black copper oxide powder. Stir with a glass rod.
Step 4: Keep adding copper oxide until some black powder remains unreacted in the beaker. This means all the acid has been used up.
Step 5: Philtre the mixture using philtre paper and a funnel to remove the excess copper oxide.
Step 6: Heat the blue solution gently using a water bath or electric heater until crystals start to form.
Step 7: Pour the solution into an evaporating basin and leave it in a warm place until crystals form completely.
Step 8: Scrape the crystals onto philtre paper and pat them dry.
The chemical reaction
The equation for this reaction is:
\text{CuO}_{(s)} + \text{H}_2\text{SO}_4_{(aq)} \rightarrow \text{CuSO}_4_{(aq)} + \text{H}_2\text{O}_{(l)}
This means: copper oxide + sulfuric acid → copper sulphate + water
Key understanding points
Why do we heat the acid?
- Heating makes the reaction happen faster
- Warm acid dissolves the copper oxide more easily
Temperature and Reaction Rate
Heating increases the kinetic energy of particles, making them move faster and collide more frequently. This leads to more successful reactions and faster dissolution of the copper oxide.
Why do we add excess copper oxide?
- We need to make sure all the acid has reacted
- The excess copper oxide can be filtered out easily
- This leaves us with pure copper sulphate solution
Critical Step: Adding excess copper oxide ensures complete neutralisation of the acid. If any unreacted acid remains, your final salt sample will be contaminated and potentially dangerous to handle.
What happens during crystallisation?
- As water evaporates, the copper sulphate becomes more concentrated
- When it's concentrated enough, blue crystals start to form
- Slow evaporation gives larger, purer crystals
Visual clues:
- Black copper oxide powder goes in
- Blue copper sulphate solution forms
- Blue crystals appear when water evaporates
Types of salts you can make
The type of salt depends on the acid you use:
- Sulfuric acid makes sulphates (like copper sulphate)
- Hydrochloric acid makes chlorides
- Nitric acid makes nitrates
Pattern Recognition
Notice the pattern: the name of the salt comes from the acid used. Sulfuric acid → sulphates, hydrochloric acid → chlorides, nitric acid → nitrates. This helps predict what products you'll get from different acid-base reactions.
Remember!
Key Points to Remember:
- Safety first - always wear eye protection when working with acids
- Add excess base to make sure all the acid reacts completely
- Philtre carefully to remove unreacted solid
- Heat gently during crystallisation to get good crystals
- The black copper oxide turns the solution blue when it reacts with acid