Required Practical 17: Preparing A Soluble Salt

Section 1 of 9Aim and chemistry

Reaction used

• Reaction used Copper(II) oxide reacts with dilute sulfuric acid to give copper(II) sulfate and water.
• CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l)

Why this works

• Why this works Copper oxide is insoluble, so any excess can be filtered off, leaving a solution of the salt only.
• Gentle warming increases the rate of reaction, but the acid must not be boiled.

Section 2 of 9Method

1

Measure about 25 cm³ of dilute sulfuric acid into a beaker and warm it gently using a water bath or a low flame.

2

Add copper oxide powder a spatula at a time, stirring, until some black solid remains at the bottom.

3

Allow the mixture to cool slightly, then filter it into an evaporating basin to remove excess copper oxide.

4

Heat the filtrate gently until about half the water has gone and crystals begin to appear on a cold glass rod.

5

Leave the basin to cool and crystallise, then pour off the liquid.

6

Dry the crystals between filter paper or in a warm place.

Section 3 of 9Safety and hazards

Hazard	Precaution
Acid irritates skin and eyes	Wear eye protection and wash spills quickly
Copper compounds are harmful	Avoid contact and wash hands afterwards
Hot liquids and apparatus	Do not boil dry; let items cool before handling
Spitting when evaporating	Heat gently and stop before dryness

Section 4 of 9Why each stage matters

1Excess base

• Excess base Ensures that all of the acid is used up.

2Filtration

• Filtration Removes the unreacted solid and leaves a clear blue solution.
• Use a funnel and filter paper folded into a cone so the powder cannot pass through into the filtrate.

3Partial evaporation

• Partial evaporation Makes the solution saturated without driving off all the water.

4Slow cooling

• Slow cooling Allows large, well-formed copper sulfate crystals to grow.

Section 5 of 9Evaluating the method

Getting pure crystals

Purity

• Purity Washing the crystals with a very small volume of cold distilled water removes traces of solution without dissolving many crystals.

Yield

• Yield Some salt is lost on the filter paper and in the solution left behind, so the mass of crystals is less than the maximum calculated.

Dryness

• Dryness Pressing the crystals between sheets of filter paper removes surface water without heating, which could drive off water of crystallisation.

Section 6 of 9Observations to record

Stage	Expected observation
Adding copper oxide to acid	The black powder dissolves and the solution turns blue
Excess oxide present	Black solid remains on the bottom even after stirring
After filtering	A clear blue filtrate and black residue on the filter paper
On cooling	Blue crystals of hydrated copper sulfate form

Section 7 of 9Calculating the percentage yield

Theoretical yield

• Theoretical yield Use the balanced equation and the amount of acid to work out the maximum mass of crystals that could form.

Percentage yield

• Percentage yield Percentage yield = actual mass of crystals ÷ maximum theoretical mass × 100.
  • It is always below 100% because some product is lost during filtering, transfers and drying.

Section 8 of 9Key terms

Term	Meaning
Excess	More of a reactant than is needed to react with the other reactant.
Filtrate	The solution that passes through the filter paper.
Saturated solution	A solution that cannot dissolve any more solute at that temperature.
Evaporating basin	A shallow dish used to evaporate solvent from a solution.
Water bath	A beaker of hot water used to heat gently.
Residue	The solid left behind on the filter paper after filtering.
Crystallise	To form solid crystals as a saturated solution cools.

Section 9 of 9Exam technique

1

Heating the solution until it is completely dry, which spoils the crystals.

2

Not adding enough oxide, so acid remains in the final crystals.

3

Filtering before the reaction is complete.

4

Washing the crystals with lots of warm water, which dissolves them.

5

Filtering with a hole or tear in the filter paper, so unreacted oxide ends up in the final crystals.