Preparing salts by neutralisation with oxides and carbonates

Read our standard health & safety guidance

Lesson organisation

These are well-tried standard class experiments, which should take no more than 30 minutes each to the point at which the solution produced has been filtered. If reagent solutions are provided in ready-measured quantities in small labelled bottles, and the solid powder reagents provided in roughly 1 g quantities in labelled specimen tubes, the experimental work can start immediately.

The experiments could be worked through in sequence, or allocated so that each group attempts only one preparation.

Most classes should be able to perform this experiment as a class experiment, but if there are real doubts about safe behaviour or adequate manipulative skills while solutions are being heated, or hot solutions being poured into the filter paper, then student-aided demonstrations of these may be more sensible.

Apparatus and chemicals

Eye protection

Each working group requires:

Beaker (100 cm³)
Conical flask (100 cm³)
Spatula
Glass stirring rod (15 cm)
Evaporating basin (50–100 cm³ capacity)
Crystallising dish
Filter funnel (65 mm diameter)
Filter paper
Bunsen burner
Tripod
Gauze
Heat resistant mat
Pipeclay triangle

Sulfuric acid, 0.5 mol dm^-3 (Irritant at this concentration), 20 cm³ (in a small bottle)

One or more of the following, according to the lesson organisation:

Copper(II) oxide (Harmful, Dangerous for the environment), about 1 g (in a specimen tube)
Magnesium carbonate (Low hazard), about 1 g (in a specimen tube)

Technical notes

Sulfuric acid (Irritant at concentration used) Refer to CLEAPSS Hazcard 98A and Recipe card 69
Copper(II) oxide (Harmful, Dangerous for the environment) Refer to CLEAPSS Hazcard 26
Magnesium carbonate (Low hazard) Refer to CLEAPSS Hazcard 59B

Stage 1

Stage 2

Procedure

HEALTH & SAFETY: Wear eye protection

Stage 1

Preparation 1: copper(II) sulfate

a Add 20 cm³ of 0.5 mol dm^-3 sulfuric acid to the 100 cm³ beaker and heat carefully on the tripod with a gentle blue flame until nearly boiling. Be very careful not to knock the tripod while the beaker is supported by it.

b When hot enough, use a spatula to add small portions of copper(II) oxide to the beaker, stirring gently for up to half a minute after each addition. When adding the solid to the beaker, move carefully and slowly to avoid knocking the beaker over. You should not be sitting down.

c When all the solid has been added, continue to heat gently for one to two minutes to ensure the reaction is complete. If the mixture is a clear solution, add a little more copper(II) oxide and stir. Continue adding copper(II) oxide until you are sure there is an excess of solid. Then turn off the Bunsen burner.

d Allow the beaker to cool slightly while you set up the next experiment.

Preparation 2: magnesium sulfate

e Add 20 cm³ of 0.5 mol dm^-3 sulfuric acid to a clean 100 cm³ beaker. Use a spatula to add small portions of magnesium carbonate to the beaker, stirring gently for up to half a minute after each addition.

f When all the solid has been added, heat the beaker gently for one to two minutes on a tripod over a small flame to ensure the reaction is complete. Remove the heat. If the resulting mixture is a clear solution, add a little more magnesium carbonate and stir. Continue adding the solid until you are sure there is an excess of solid.

Stage 2 - both preparations

a Place the filter funnel in the neck of the conical flask.

b Fold the filter paper to fit the filter funnel, and put it in place.

c Make sure the beaker is cool enough to hold at the top, but the contents should still be hot.

d Swirl the contents gently to mix, and carefully pour into the filter paper in the funnel. Allow to filter through.

e A clear solution should collect in the flask. If the solution is not clear, and any solid remains in it, you will need to repeat the filtration.

Stage 3

a Pour the clear solution into an evaporating basin and place on a pipeclay triangle or gauze on the tripod.

b Heat the solution gently over a medium Bunsen flame so that water boils steadily.

c When about half the water has boiled away, take a drop of the hot solution on the end of a glass rod and let it cool. If the drop crystallises on cooling, the solution is ready for the next stage; if it does not crystallise, keep boiling and repeat the testing until the drop does crystallise on cooling. Do not boil dry.

Stage 4

a Pour the hot solution carefully into a crystallising dish.

b Set it aside to cool, with a label attached giving the name of the salt being prepared and the names of the group.

c Leave the crystallising dish in a warm place, safe from interference, until it has produced a good crop of crystals.

d If necessary, filter the solution, collect the crystals from the filter paper onto a paper towel and allow to dry.

Teaching notes

There may problems associated with younger students heating beakers perched on tripods, and with lifting hot glassware off a hot tripod after heating. Student should not be sitting down on lab stools whilst whilst handling hot solutions. Using tongs of suitable size is a good solution for lifting the hot beakers, but some schools may not have these. If there is any doubt about the safety of this step, you should lift each beaker down onto the heat resistant mat first before the students pour the contents into the funnel.

In Stage 1 students should note any colour change in the solution (in Preparation 1 from colourless to blue, at the same time as the black powder ‘disappears’; no colour change but ‘fizzing’ in Preparation 2).

In Stages 2–4 of Preparation 1, younger students are expected to use their previous experience of blue solutions/crystals to recognise the familiar colour of copper sulfate. No such visual help is available in Preparation 2! Older students already familiar with acid/base reactions should use that understanding to predict the identities of the compounds formed.

The symbolic equations for the reactions are only relevant for older students:

Preparation 1: CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l)

Preparation 2: MgCO₃(s) + H₂SO₄(aq) → MgSO₄(aq) + CO₂(g) + H₂O(l)

('Magnesium carbonate' is unlikely to be pure MgCO₃, which only occurs as such in certain minerals, but a basic salt also containing Mg(OH)₂)

In Preparation 2, the product, magnesium sulfate, is commonly known as Epsom salts, though general familiarity with this substance and its uses is decreasing. However, it can help to broaden the interest in what many students find to be obscure substances with funny names if they do some web research into their wider occurrence and uses (see Web links)

Student questions

Stage 1

1 What change(s) do you see as you add the solid?

Stage 2

2 What do you think is present in the clear filtered solution? Give a reason for your answer.

Stage 4

3 What substance do you think these crystals are? Give a reason for your answer.

4 Write word and symbol equations for the reaction you have done.

Health and safety checked, February 2008

Web links

The Copper Development Association provides a wealth of information about copper and its compounds on its website.

www.cda.org.uk/megab2/general/general.htm

This website has the historical background to the spa at Epsom.

www.epsom.townpage.co.uk/salts.htm

For the sheer variety of what you can do with Epsom salts, just put ‘Epsom salts’ into Google!