GCSE · Chemistry · Edexcel · Spec 1CH0

Preparing soluble salts from an insoluble base (excess method)

Here's an odd instruction for a practical: add too much on purpose. The leftover powder you can't get to react is exactly what makes this method work.

Making copper sulfate crystals

Stage 1 of 6: Add & stir. paused

Add & stir · 1/6follow the salt →

Add a small spatula of powdered copper oxide to warm sulfuric acid and stir. Stirring helps the acid and oxide particles collide and react; warming speeds it up. Where's the salt? Forming — and dissolving in the liquid as it's made.

Scrub through the method and keep asking: where is the salt right now?

Before you start

Pick the right acid and oxide

Which reactants make each salt? Copper sulfate is worked for you — then you plan magnesium chloride.

Copper sulfate: the ending 'sulfate' comes from the acid, so use sulfuric acid.Hydrochloric acid makes chlorides, sulfuric acid makes sulfates, nitric acid makes nitrates.
The first name 'copper' comes from the metal oxide, so use copper oxide.
copper oxide + sulfuric acid → copper sulfate + waterIt follows the pattern metal oxide + acid → salt + water. This is a neutralisation.
missing step

Which line is step 4?

Magnesium chloride: the ending 'chloride' comes from the acid, so use hydrochloric acid.Magnesium chloride: use sulfuric acid, the same acid as in the copper sulfate example.Magnesium chloride: use nitric acid, because any acid can make any salt.

The excess trick

Reason it through

Why add the copper oxide until some is left over?

Link 1 of 4

First link · your turn

Unreacted copper oxide is sitting at the bottom and the pH paper shows neutral. What does that tell you about the acid?

Locked — reveal the link above first

After filtering

Which part do you keep?

You've added excess copper oxide to sulfuric acid and poured the mixture through filter paper. There's solid left on the paper, and a solution has collected in the flask below.

Which part do you keep to make your copper sulfate crystals?

The solid on the filter paper — that's the salt, like when you make an insoluble salt.Throw away the liquid — it's only leftover acid and water.Keep both and mix them back together — some salt is in each part.The liquid that came through — the salt is dissolved in it.

How sure are you?

I'm sureI think soI'm guessing

The solid on the filter paper — that's the salt, like when you make an insoluble salt.

This is a common misconception

When the salt you want is insoluble, the product is the solid you collect — so this feels right. But copper sulfate is soluble: it's dissolved in the liquid and passes straight through the paper. The solid on the paper is the excess copper oxide you added on purpose. Keep it, and you've kept the leftover reactant.

Throw away the liquid — it's only leftover acid and water.

This is a common misconception

There's no acid left — adding the oxide in excess made sure every bit of it reacted. The liquid is your salt dissolved in water. Pour it away and your copper sulfate goes down the sink.

Keep both and mix them back together — some salt is in each part.

This is a common misconception

All of the salt is dissolved, so all of it went through the paper into the filtrate. The solid is just unreacted copper oxide. Mixing it back in would undo the filtering and put the impurity straight back.

The liquid that came through — the salt is dissolved in it.

This matches the evidence

Exactly. Copper sulfate is soluble, so it's in the filtrate with only water. The residue on the paper is the excess copper oxide. Next job: get the salt out of the water by crystallising it.

In the excess method, the salt is in the filtrate. The residue is only the excess metal oxide.

Predict, then check

Your copper sulfate filtrate is in an evaporating dish.

What happens if you heat the filtrate directly until all the water has gone?

Why crystals appear as it cools

Cooling stage: 1 of 4. State: Hot and saturated. static

Cooling stage1 of 4

Hot and saturated: the water is holding as much salt as it can at this temperature — no more can dissolve. Every salt particle is still in solution.

Salt particles in a saturated copper sulfate solution: dissolved in the water on the left, built into crystals on the right.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

metal oxide + acid → salt + water

Add the insoluble oxide until it won't react any more, filter off the extra, then let the salt crystallise.

What you need to know

A soluble salt can be made by reacting an acid with an insoluble base, such as a metal oxide: metal oxide + acid → salt + water.
Choose the pair from the salt's name: the metal oxide gives its first name, the acid gives its ending.
Three main steps: neutralise the acid with excess metal oxide, filter off the excess, then crystallise the salt.

The big picture

You can make a soluble salt by reacting an acid with an insoluble metal oxide. Add the oxide in excess so every bit of acid reacts, filter off the leftover oxide, then crystallise the salt from a saturated solution and dry the crystals.

Key points

1Metal oxide + acid → salt + water is a neutralisation reaction.

2Hydrochloric acid makes chlorides, sulfuric acid makes sulfates and nitric acid makes nitrates.

3Add small portions of the powdered oxide to a set volume of acid and stir, so the acid and oxide particles collide and react; warming the mixture speeds the reaction up.

4Keep adding the oxide until unreacted solid collects at the bottom and pH paper shows neutral — then all the acid has reacted.

5Filtering removes the excess oxide as residue; the filtrate contains only the salt and water.

6Warm the filtrate over a water bath to a saturated solution, but don't heat to dryness — the crystals would spit out of the dish.

7Solubility falls as the temperature falls, so crystals form as the saturated solution cools. Dry them with filter paper, in the air or in a warm oven.

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Worked example

Problem

A student has some copper oxide powder and wants to make pure, dry crystals of copper nitrate. (a) Which acid should they use? (b) Write the word equation. (c) How will they know they have added enough copper oxide?

⚠ Watch out

Stopping too soon when adding the metal oxide. If there's no unreacted solid left at the bottom, some acid may still be there — and it ends up in the filtrate mixed with your salt.

🧠

Memory hook

Too much, then filter, then cool. The leftover powder proves the acid's gone — and the salt travels beaker → filtrate → crystals.

✓

Check yourself

Without looking back: name the three main steps in order, and say where the salt is after each one.

Flashcards

(16)

What reacts with an acid to make a soluble salt in this method?

An insoluble base, such as a metal oxide.

Finish it: metal oxide + acid → ?

salt + water

What type of reaction is metal oxide + acid?

Neutralisation.

Which acid makes a chloride?

Hydrochloric acid.

Which acid makes a sulfate?

Sulfuric acid.

Which acid makes a nitrate?

Nitric acid.

Symbol equation for copper oxide + sulfuric acid, with state symbols

CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l)

Why stir each small portion of powdered oxide into the acid?

So the acid particles collide and react with the metal oxide particles.

Why warm the acid while adding the oxide?

Warming increases the rate of reaction.

Two signs that all the acid has reacted

Unreacted metal oxide collects at the bottom, and pH (universal indicator) paper shows neutral against the chart.

After filtering, what is the residue?

The excess, unreacted metal oxide.

After filtering, what is in the filtrate?

Only the soluble salt and water.

What is a saturated solution?

A solution in which no more solute can dissolve at that temperature.

Why do crystals form as a saturated solution cools?

Solubility decreases as the temperature goes down, so the salt crystallises out of solution.

Why use a water bath and stop before the dish is dry?

Heating to dryness makes the crystals spit out of the dish.

Three ways to dry the crystals

Dab gently with filter paper, leave in the air, or put in a warm oven.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Making copper sulfate crystals

Pick the right acid and oxide

Which part do you keep?

Why crystals appear as it cools

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