Acids and alkalis · Investigation
Making a pure dry salt
Blue crystals big enough to hold, grown from a beaker of clear acid and a spoonful of black powder. Every step of getting there is a decision you can get wrong.
Start here
You are asked for a jar of pure copper sulfate crystals. You are given sulfuric acid and black copper oxide powder.
The reaction itself is easy — the acid neutralises the oxide and the solution turns blue. The hard part is what comes after: getting a pure, dry, crystalline solid out of a beaker of blue liquid, with no leftover acid and no black powder in it.
How much copper oxide should you add?
More than you need, and then stop when no more will react. Copper oxide is a base that does not dissolve in water, so the excess just sits on the bottom where you can filter it off. Leftover acid could not be removed like that — it would be dissolved in with your product. Adding too much of the insoluble one is a problem with a solution; adding too much acid is not.
Every neutralisation makes a salt, and the salt's name is built from the two things that made it. The metal gives the first word. The acid gives the second: hydrochloric makes chlorides, sulfuric makes sulfates, nitric makes nitrates.
So the name tells you the recipe backwards. Copper sulfate had to come from copper — as the metal, its oxide, its hydroxide or its carbonate — and sulfuric acid. Nothing else could have made it.
Your turn · the naming bench
Pick an acid and a base. Name the salt before you check.
Twelve are possible — three is enough to see the rule.
The acid
The base
Hydrochloric acid + copper oxide
Hydrochloric acid + magnesium oxide
Hydrochloric acid + sodium hydroxide
Hydrochloric acid + calcium carbonate
Sulfuric acid + copper oxide
Sulfuric acid + magnesium oxide
Sulfuric acid + sodium hydroxide
Sulfuric acid + calcium carbonate
Nitric acid + copper oxide
Nitric acid + magnesium oxide
Nitric acid + sodium hydroxide
Nitric acid + calcium carbonate
Which salt does this make?
Copper chloride
hydrochloric acid + copper oxidemakescopper chloride + water
Copper oxide is insoluble, so you can add it in excess and filter off what is left.
Magnesium chloride
hydrochloric acid + magnesium oxidemakesmagnesium chloride + water
Magnesium oxide is insoluble, so you can add it in excess and filter off what is left.
Sodium chloride
hydrochloric acid + sodium hydroxidemakessodium chloride + water
Sodium hydroxide dissolves, so excess cannot be filtered off — this salt has to be made by titration instead.
Calcium chloride
hydrochloric acid + calcium carbonatemakescalcium chloride + water + carbon dioxide
A carbonate gives carbon dioxide as well, so this one fizzes while it reacts. Calcium carbonate is insoluble, so you can add it in excess and filter off what is left.
Copper sulfate
sulfuric acid + copper oxidemakescopper sulfate + water
Copper oxide is insoluble, so you can add it in excess and filter off what is left.
Magnesium sulfate
sulfuric acid + magnesium oxidemakesmagnesium sulfate + water
Magnesium oxide is insoluble, so you can add it in excess and filter off what is left.
Sodium sulfate
sulfuric acid + sodium hydroxidemakessodium sulfate + water
Sodium hydroxide dissolves, so excess cannot be filtered off — this salt has to be made by titration instead.
Calcium sulfate
sulfuric acid + calcium carbonatemakescalcium sulfate + water + carbon dioxide
A carbonate gives carbon dioxide as well, so this one fizzes while it reacts. Calcium sulfate barely dissolves either, so it coats the chips and the reaction stops almost at once. This is a salt you can name by the rule but not make by this method.
Copper nitrate
nitric acid + copper oxidemakescopper nitrate + water
Copper oxide is insoluble, so you can add it in excess and filter off what is left.
Magnesium nitrate
nitric acid + magnesium oxidemakesmagnesium nitrate + water
Magnesium oxide is insoluble, so you can add it in excess and filter off what is left.
Sodium nitrate
nitric acid + sodium hydroxidemakessodium nitrate + water
Sodium hydroxide dissolves, so excess cannot be filtered off — this salt has to be made by titration instead.
Calcium nitrate
nitric acid + calcium carbonatemakescalcium nitrate + water + carbon dioxide
A carbonate gives carbon dioxide as well, so this one fizzes while it reacts. Calcium carbonate is insoluble, so you can add it in excess and filter off what is left.
Your turn · build the method
Six steps, shuffled. Put them in the order you would do them.
Three of the six carry a decision that could ruin the crystals, and the order is what makes each of them possible.
That is the order, and every step earns its place.
Not the order that works. Here is the sequence and what each step is for.
- Warm the acid gently in a beaker. — Warming speeds the reaction up. It is not boiled — that would drive acid off before it has reacted with anything.
- Add the copper oxide a spatula at a time, stirring, until no more will react. — Excess is deliberate. The black powder settling out means every last bit of acid has been used up, which is the only way to be sure none is left in your product.
- Filter the mixture to remove the leftover copper oxide. — The excess base is insoluble, so it stays on the filter paper while the blue copper sulfate solution passes through. This is the step that pays for adding too much.
- Heat the filtrate to evaporate about half the water. — Concentrating the solution, not drying it. You stop while there is still plenty of liquid — the test is a drop on a cold glass rod forming crystals.
- Leave the concentrated solution to cool slowly. — Slow cooling is what grows large, regular crystals. Fast cooling gives a mass of tiny ones, and boiling to dryness gives powder.
- Pat the crystals dry between filter papers. — The last of the mother liquor is dissolved salt, and it would dry as a crust on the surface. Patting rather than rubbing keeps the crystals whole.
Key fact
The metal names the salt and the acid names its ending. Use excess insoluble base, filter it off, then crystallise — because excess solid can be removed and excess acid cannot.
Think again
“Boiling a solution dry gives the best crystals.”
You do get a solid, and quickly. Commit before you read on.
You get a solid, but not crystals. Boiling to dryness throws thousands of tiny crystals out of solution at once and leaves a caked powder, with whatever else was dissolved in the water trapped in it. Crystals need time: heat the solution until it is concentrated, stop while there is still liquid, and let it cool slowly.
This is the same rule you met when you crystallised salt from solution — fast drying gives powder, slow cooling gives crystals. Big clean crystals are a product of patience, not heat.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
Nitric acid is neutralised with magnesium oxide. What salt is made?
Rung 2 · The one that catches people
Why is excess copper oxide added rather than an exactly measured amount?
Rung 3 · Explain
Describe how you would make pure, dry crystals of magnesium sulfate from sulfuric acid and magnesium oxide, and explain the reason behind each of the three decisions that could ruin it.
Rung 4 · Take it somewhere new
A student needs to make sodium chloride crystals from hydrochloric acid and sodium hydroxide. Explain why the method above will not work, and what they must do instead.
Key note
A salt is named after the metal and the acid that made it: sulfuric acid gives sulfates, hydrochloric gives chlorides, nitric gives nitrates. To make a pure sample, react the acid with excess insoluble base, filter off what is left over, then evaporate part of the water and let the solution cool slowly so crystals grow.
Going further
Making a salt with a soluble alkali instead of an insoluble base is a much harder job, and it is worth knowing why. Sodium hydroxide dissolves, so excess cannot be filtered off — there is no way to see when you have added enough. The answer is to do the reaction twice: once with indicator to find the exact volume that neutralises the acid, then again with the same volumes and no indicator, so the crystals are not stained with dye.
Salts are not laboratory curiosities. Ammonium nitrate and ammonium sulfate, made by neutralising acids with ammonia, are the fertilisers that feed a large fraction of the world; the process that makes the ammonia consumes about one per cent of all energy generated on Earth. Copper sulfate goes on vines as a fungicide, calcium sulfate is plasterboard, and sodium chloride is on the table.
Before this lesson
Next in this unit
At GCSE this becomes
- Preparing soluble salts by titration, calculating percentage yield, and predicting solubility from the rules.
Where to next
Ask Mr Badmus AI
Still not sure why you add too much of the base on purpose?
Copper compounds are harmful if swallowed and the warm acid stage spits if it is allowed to boil. Eye protection throughout, the crystals are never tasted, and the evaporating basin is left to cool before it is handled.
Lesson content © MrBadmusAI.