Making a soluble salt from an acid and an insoluble base
Definition
Base
A substance that neutralises an acid, such as a metal oxide, a metal hydroxide or a metal carbonate.
Definition
Insoluble
An insoluble substance does not dissolve, or dissolves only very slightly, in a particular solvent.
- You can make a soluble salt by reacting an acid with an insoluble base, such as a metal oxide, hydroxide or carbonate.
- Add the base in excess so that all of the acid is used up and none is left over.
- The leftover solid base is insoluble, so it can be filtered off, leaving only the salt solution.
- For copper sulfate the base is black copper(II) oxide and the acid is dilute sulfuric acid.
- CuO(s)+H2SO4(aq)→CuSO4(aq)+H2O(l)\text{CuO}(s) + \text{H}_2\text{SO}_4(aq) \rightarrow \text{CuSO}_4(aq) + \text{H}_2\text{O}(l)CuO(s)+H2SO4(aq)→CuSO4(aq)+H2O(l)
Practical
Investigation: preparing pure, dry copper sulfate crystals
- Measure 40 cm3\text{cm}^3cm3 of dilute sulfuric acid into a beaker and warm it gently over a Bunsen burner until it is almost boiling, then turn the Bunsen off.
- Use a spatula to add a small amount of black copper(II) oxide to the hot acid and stir; the acid reacts and the solution turns blue.
- Keep adding copper(II) oxide a little at a time until some solid stays unreacted, which shows all the acid has now reacted.
- Filter the mixture so the excess copper(II) oxide stays on the filter paper as the residue and the blue copper sulfate solution passes through as the filtrate.
- Pour the filtrate into an evaporating basin and heat it gently on a water bath until crystals just begin to form at the edges.
- Stop heating and leave the solution in a cool place so that copper sulfate crystals grow slowly over about a day.
- Remove the crystals with a spatula and pat them dry between two pieces of filter paper.
- Wear eye protection throughout, and take care with the hot acid and hot apparatus.
Filtering and crystallising give a pure, dry salt
Definition
Filtration
A separation method that uses filter paper to separate an insoluble solid from a liquid.
Definition
Filtrate
The liquid that passes through the filter paper during filtration.
Definition
Crystallisation
Forming solid crystals from a solution by evaporating some of the water so the dissolved solid is left behind.
- Filtration separates the unreacted base (the residue) from the salt solution (the filtrate).
- Evaporating some water makes the solution saturated, so crystallisation begins.
- Slow cooling grows larger, purer crystals than fast evaporation to dryness.
- Patting the crystals with filter paper removes surface water without dissolving them.
Common Mistake
- Do not evaporate the solution all the way to dryness, because strong heating can spoil the crystals.
- Do not skip the excess base, because leftover acid would contaminate the salt.
Why each step is needed
Definition
Residue
The insoluble solid that stays on the filter paper after filtration.
- Adding the base in excess makes sure the acid is fully neutralised.
- Filtering removes the insoluble excess so only the dissolved salt remains.
- Gentle evaporation on a water bath avoids overheating and keeps the crystals well formed.
- Slow crystallisation and drying give a pure, dry sample of the salt.
Self review
- Why is the insoluble base added in excess?
- What is the residue and what is the filtrate in this preparation?
- Why is the solution not evaporated to dryness?
- Write the equation for copper oxide reacting with sulfuric acid.
- Why are the crystals dried by patting rather than by strong heating?
