Chemistry 11–12 · Year 11

Heating hydrated copper(II) sulfate: water of crystallisation and the molar mass of a compound

Module 2: Introduction to Quantitative Chemistry

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

The mass lost on heating a hydrate is the water of crystallisation, and the mole ratio of water to anhydrous salt fixes the formula and the molar mass.

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 irritant
hot crucible
over-heating gives sulfur oxide fumes

Controls

gentle heating only, stop at grey-white
tongs
eye protection
wash hands
collect copper waste for disposal, not the sink

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.

What you need

Hydrated copper(II) sulfate crystals, 2.5 g
Porcelain crucible, pipe-clay triangle, tripod, Bunsen burner, heat-resistant mat
Crucible tongs
Electronic balance reading to 0.01 g
Dropper and deionised water for the reverse test

How to do it

Weigh the empty crucible, then weigh in 2.00 to 3.00 g of blue crystals to 0.01 g.
Heat gently for about 10 minutes, stirring once with a glass rod, until the solid is uniformly pale grey-white; avoid strong heating that blackens the solid.
Cool on the mat until the crucible is cold, then weigh; heat for a further three minutes, cool and reweigh until the mass is constant.
Calculate the mass of water lost and the mass of anhydrous salt; convert both to moles (CuSO4 159.60 g/mol, H2O 18.015 g/mol) and find the ratio.
Add two drops of water to a small portion of the cooled white solid and record the colour and temperature change.
State the formula, the molar mass of the hydrate and the percentage of water by mass.

What you should see

The blue crystals turn grey-white and lose about 36.1 percent of their mass (2.50 g gives about 0.90 g of water and 1.60 g of anhydrous salt); the mole ratio water to CuSO4 comes out close to 5, so the formula is CuSO4.5H2O with molar mass 249.7 g/mol. Adding water to the white solid restores the blue colour and the solid warms noticeably.

What changes

What you change: mass of hydrate heated
What you measure: mass of water lost (g)
What you keep the same: heating to constant mass without decomposition
cooling fully before weighing

Common misconceptions

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

Water of crystallisation is just damp crystals (the water is bonded into the lattice in a fixed ratio).
Heating to a black solid means fully dry (black means decomposition to copper(II) oxide, which spoils the ratio).
The white solid is a different compound (it is the same salt without its water; adding water reverses the change).

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)CH11-9 CH11/12-4
Chemistry Stage 6 Syllabus (2017), NESA; the current syllabus, taught in 2026 (codes read from the syllabus document); a Working Scientifically outcome not among those the syllabus targets in Module 2, which it allows in any moduleCH11/12-3
Chemistry 11-12 Syllabus (2025), NESA; implemented from 2028, not yet taughtCH-11-02 CH-11WS-03
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

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