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Science 7–10 · Year 8

Blue to white and back: water of crystallisation in copper(II) sulfate

Change

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

Heating drives water out of blue copper(II) sulfate crystals, leaving a white solid of lower mass, and adding water turns the powder blue and warm again, so the water was part of the crystals rather than dampness on their surface.

Safety card

Medium riskA teacher supervises

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

Hazards

  • hydrated copper(II) sulfate is harmful if swallowed and harmful to aquatic life
  • over-heating releases toxic sulfur oxides
  • hot crucible

Controls

  • eye protection
  • gentle heating only, stop at white
  • tongs for the crucible; cool on the mat
  • copper residues to the labelled waste, not the sink

Note

NSW Department of Education, Chemical Safety in Schools (CSIS) 2021 Technical Update: Section 1.7 (risk assessment before use) and Volume 2 Appendix D (generic assessment advice, user codes and DoE categories). Complete a RiskAssess or CSIS site-specific risk assessment before the lesson.

What you need

  • hydrated copper(II) sulfate 2.50 g, weighed to 0.01 g
  • crucible and lid, pipe-clay triangle, tripod, Bunsen burner, tongs, heat-resistant mat
  • balance reading to 0.01 g, dropping pipette of water, thermometer

How to do it

  1. Weigh the empty crucible, add 2.50 g of the blue crystals and record the total.
  2. Heat gently with the lid ajar for about 10 minutes, stirring once with a glass rod, until the solid is fully greyish-white. Do not heat to redness.
  3. Cool on the mat, weigh, heat again for 2 minutes, cool and weigh; repeat until two masses agree within 0.02 g.
  4. Calculate the mass lost and the percentage of the starting mass it represents.
  5. Add three drops of water to the white powder with the thermometer bulb touching it; record the colour and any temperature change.

What you should see

The crystals turn from blue to greyish-white and lose mass. The data value the teacher supplies for the water in these crystals is 36.1 percent of their mass (from the formula CuSO4·5H2O), so 2.50 g loses 0.90 g; a smaller loss means water is left in the solid, and over-heating beyond white gives a black tinge and a larger loss as the sulfate itself decomposes. Adding water turns the powder blue again and the thermometer rises by several degrees, showing the reverse change releases energy. The learner knows it worked when the mass reaches a constant value, the loss is close to the supplied 36 percent of the starting mass, and the white powder turns blue and warm again with water.

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.

  • The blue colour is a dye that burns off.
  • Water in a crystal is just dampness on the surface.
  • A change that can be reversed cannot be a chemical 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.

Sources

The pages the author read to write this activity.

  1. curriculum.nsw.edu.au/learning-areas/science/science-7-10-2023/outcomes
  2. edu.rsc.org/experiments/a-reversible-reaction-of-hydrated-copperii-sulfate/437.article
  3. edu.rsc.org/experiments/finding-the-formula-of-hydrated-copperii-sulfate/436.article
  4. education.nsw.gov.au/content/dam/main-education/asset-management/chemical-safety/5._Volume_2_Appendices.pdf

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