Chemistry 11–12 · Year 12

Reversible or not: copper(II) sulfate hydration, iron(III) thiocyanate, burning magnesium and burning steel wool

Module 5: Equilibrium and Acid Reactions

PracticalMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

The idea

Some changes can be driven back to the starting substances by reversing the conditions and some cannot; the difference decides whether a system can reach equilibrium.

Safety card

Medium riskLearners carry it out, with a teacher supervising

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

copper(II) sulfate is harmful if swallowed and toxic to aquatic life; overheating it releases sulfur oxides
silver nitrate corrosive and staining
sodium hydroxide corrosive
hot tubes and burning metal

Controls

copper(II) sulfate heated gently and only until white; copper wastes collected, not poured down the sink
no looking directly at burning magnesium
eye protection
wash hands

Note

NSW Department of Education Chemical Safety in Schools (CSIS) package, 2021 Technical Update: Section 1.7 (risk assessment) and Volume 2 Appendix D (generic assessment advice and DoE chemical categories); record a RiskAssess (riskassess.com.au) risk assessment before the lesson and check the school's hazardous chemical register (CSIS Section 1.9) for local restrictions. The hydration step uses copper(II) sulfate, the RSC class practical, rather than cobalt(II) chloride, a classified carcinogen that this directory keeps to teacher demonstrations.

What you need

Copper(II) sulfate pentahydrate, powdered, about 2 g in a dry test tube; test-tube holder, Bunsen burner, dropper of water
0.1 mol/L iron(III) nitrate 5 mL and 0.1 mol/L potassium thiocyanate 5 mL, 100 mL beaker of water, dropper of 1 mol/L sodium hydroxide and of 0.5 mol/L silver nitrate
Magnesium ribbon 3 cm, tongs, balance to 0.01 g
Steel wool 2 g, balance, tongs, heat-resistant mat
Deionised water, universal indicator

How to do it

Copper(II) sulfate: warm the blue hydrated powder gently, moving the flame along the tube, until it turns white and water condenses near the mouth; stop before the solid darkens, which would release sulfur oxides. Cool, then add water dropwise, record the colour and feel the tube warm; heat again to show the change repeats.
Iron(III) thiocyanate: add 2 mL of each solution to 50 mL of water to give a pale red solution; split into three tubes. To one add 1 mol/L sodium hydroxide drop by drop only until the red colour has just gone (about four drops, which remove its Fe3+ as a precipitate), to another silver nitrate dropwise (removes SCN- as a precipitate); compare with the third and record the colour loss. Then add 0.1 mol/L iron(III) nitrate drop by drop to the tube treated with sodium hydroxide (its thiocyanate is still in solution), counting the drops until the colour returns: each drop of sodium hydroxide beyond what the iron needed precipitates about three drops of the iron(III) nitrate before any complex can form.
Magnesium: weigh, burn in tongs, collect the ash and weigh; attempt to reverse by heating the ash with a splint of charcoal and by adding water; record that neither returns the metal.
Steel wool: weigh, heat in the flame until it glows and sparkles, cool and reweigh; record the mass gain and that no treatment in the lab returns it to iron.
Classify each change as reversible or irreversible, write equations, and state which of the four could form an equilibrium in a closed container.

What you should see

Copper(II) sulfate: the pale blue hydrate turns white on heating (CuSO4.5H2O gives CuSO4 + 5 H2O) and returns to blue on adding water, the tube becoming noticeably hot, repeatably. Iron(III) thiocyanate: the red colour fades when either ion is removed and returns once the added iron(III) has used up any surplus hydroxide, showing the same reaction runs both ways. Magnesium gains mass on burning (1.00 g of metal becomes 1.66 g of oxide when fully burned, M(MgO)/M(Mg) = 40.30/24.31) and nothing done in the school laboratory returns the metal. Steel wool also gains mass as iron oxide forms, well below the 38.2 percent gain that full conversion to Fe3O4 would give because only the outer fibres burn, and the change is not reversed; these two are irreversible under laboratory conditions.

What changes

This activity lists no variables to change, measure and keep the same.

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Adding water to white anhydrous copper(II) sulfate is a new reaction (it is the reverse of the dehydration; the same equation runs backwards and gives out the heat the dehydration took in).
Every reaction can be reversed if you try hard enough (magnesium oxide needs an industrial electrolytic cell, not a change of laboratory conditions).
The red colour fading means the reaction stopped (it means the equilibrium shifted; adding iron(III) brings the colour back).

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Chemistry Stage 6 Syllabus (2017), NESA; the current syllabus, taught in 2026 (codes read from the syllabus document)CH12-12 CH11/12-4 CH11/12-5 CH11/12-7
Chemistry 11-12 Syllabus (2025), NESA; implemented from 2028, not yet taughtCH-12-01 CH-12WS-04 CH-12WS-05
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

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