How do you do the making salts required practical?

What does the making salts practical investigate?

It shows how to make a pure, dry sample of a soluble salt from an acid and a solid base that does not dissolve in water. The usual version reacts black copper(II) oxide with dilute sulfuric acid to make blue crystals of copper(II) sulfate: copper(II) oxide + sulfuric acid → copper(II) sulfate + water, or CuO + H₂SO₄ → CuSO₄ + H₂O.

It is a required practical on both AQA GCSE Chemistry and Combined Science: Trilogy, so questions on it can come up whichever course you take.

Strictly, it is a preparation rather than an investigation, because nothing is changed and measured. What the exam tests is the method and the reason behind each step. You can also start from a carbonate, such as copper(II) carbonate, which fizzes as it reacts because it gives off carbon dioxide.

What equipment do you need?

Besides the acid and the copper(II) oxide, you need a Bunsen burner to warm the acid and a filter funnel to remove the leftover solid. A water bath does the gentle evaporating at the end.

• Dilute sulfuric acid and a measuring cylinder
• Copper(II) oxide powder and a spatula
• A glass rod for stirring
• A 100 cm³ beaker for the reaction
• A Bunsen burner, tripod, gauze and heatproof mat
• A filter funnel, filter paper and a small conical flask
• An evaporating basin, with a 250 cm³ beaker of water to use as a water bath
• A crystallising dish (any shallow dish will do)
• Eye protection

What is the method?

In short: warm the acid, stir in copper(II) oxide until some is left over, filter off the extra, then evaporate the solution gently and leave it to crystallise.

• Measure 40 cm³ of dilute sulfuric acid into the 100 cm³ beaker.
• Heat the acid gently on the tripod and gauze until it is almost boiling, then turn off the Bunsen burner.
• Add a small amount of copper(II) oxide and stir. The black powder disappears and the solution turns blue.
• Keep adding small amounts, stirring each time, until some black powder stays in the beaker after stirring. The oxide is now in excess, so all the acid has reacted.
• Let the mixture cool, then filter it into the conical flask to remove the leftover copper(II) oxide.
• Pour the blue filtrate (the liquid that came through the filter paper) into the evaporating basin and heat it gently over the water bath.
• Stop heating as soon as crystals start to form at the edge of the solution.
• Pour what is left into the crystallising dish and leave it somewhere cool for at least 24 hours.
• Lift out the crystals and pat them dry between two sheets of filter paper.

What are the variables?

Nothing is changed on purpose, because this practical makes a product rather than testing an idea, so there is no independent variable. Instead, think about what you keep under control and what you watch for.

How do you work out and present the results?

Your result is the crystals themselves, so there is no graph to draw. Describe them (blue, with flat faces and straight edges) and write word and symbol equations for the reaction. If you started from copper(II) carbonate, the equation has an extra product: CuCO₃ + H₂SO₄ → CuSO₄ + H₂O + CO₂.

The name of the salt comes from the metal in the base and the acid you used. Sulfuric acid makes sulfates, hydrochloric acid makes chlorides and nitric acid makes nitrates, so copper(II) oxide and hydrochloric acid would give copper(II) chloride.

Separate Chemistry students can also be asked for the percentage yield, once the crystals are weighed: the mass you made ÷ the most you could have made × 100. For example, if you could have made 5.0 g and made 3.5 g, the percentage yield is 3.5 ÷ 5.0 × 100 = 70%. It is always below 100%, because some salt stays dissolved in the leftover solution and some is lost on the filter paper and glassware.

How do you make it accurate and safe?

Wear eye protection throughout. Hot acid can spit, and dilute sulfuric acid irritates the eyes and skin. Copper(II) sulfate is harmful if swallowed, so wash your hands when you finish. Turn the Bunsen burner off before you add the powder, and add it a little at a time: tipped into hot acid all at once, it can froth up and over the top of the beaker.

Let the mixture cool before you filter it. Then evaporate over a water bath, not a direct flame, and stop when crystals start to form rather than heating to dryness. Heating it dry can make the solid spit out of the basin, and too much heat drives the water out of the crystals, turning blue copper sulfate into a white powder. Keep an eye on the water bath too, so it does not boil dry.

For a pure product, make sure the oxide really is in excess, so no acid ends up in the crystals. Leaving the solution to cool slowly gives bigger, better-shaped crystals, and patting them dry with filter paper means they are never heated again.

What do exam questions ask about it?

Most questions pick one step and ask why it is there. Why add the oxide in excess? So all the acid reacts. How do you know it is in excess? Some solid stays undissolved. Why filter? To remove the leftover solid. Why warm the acid? To make the reaction faster. You may also be asked to name the salt, write the equation, or explain why a student made fewer crystals than expected.

The longest question is often a 6-mark method for a different salt. The steps stay the same; only the chemicals change. For example:

"A student wants to make pure, dry crystals of zinc sulfate from zinc carbonate and dilute sulfuric acid. Describe a method the student could use. (6 marks)"

A strong answer puts the steps in order and gives the reason for the important ones: add zinc carbonate to warm acid until it stops fizzing and some is left over (so all the acid reacts), filter off the extra carbonate, evaporate gently until crystals start to form, then leave it to crystallise and pat the crystals dry. Another question worth being ready for: why not start from copper metal? Copper is less reactive than hydrogen, so it does not react with dilute acids.

Common mistakes

Each of these is easy to fix once you know why the step is there.

• Stopping before the oxide is in excess. If no solid is left over, some acid may still be there and the crystals will not be pure. Keep adding until powder stays behind after stirring.
• Saying you filter to remove the acid. Filtering removes the leftover solid; the salt is in the solution that comes through.
• Heating the solution to dryness. Stop when the first crystals appear and let the rest crystallise as it cools.
• Mixing up filtrate and residue. The filtrate is the liquid that passes through the filter paper; the residue is the solid left on it.
• Starting from copper metal. It does not react with dilute sulfuric acid, so use the oxide or the carbonate.