Science 7–10 · Years 9–10
Making a salt: copper(II) sulfate crystals from copper oxide and sulfuric acid
Reactions
School laboratory, not for home
In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.
The idea
An insoluble base neutralises an acid to make a soluble salt and water; using an excess of the base and filtering it off gives a pure salt solution from which crystals of the salt can be grown.
Safety card
Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.
Hazards
- sulfuric acid 0.5 mol/L is an irritant
- copper(II) oxide and copper(II) sulfate are harmful and toxic to aquatic life
- hot beaker; over-heating gives sulfur oxides
Controls
- eye protection
- heat over a water bath, never to dryness
- beaker tongs
- copper residues and filter paper to the labelled waste
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
- dilute sulfuric acid 0.5 mol/L, 20 mL
- copper(II) oxide about 1 g (an excess: the acid needs only 0.80 g)
- 100 mL beaker, tripod, gauze, Bunsen burner, glass rod, filter funnel and paper, evaporating basin, water bath, balance reading to 0.01 g
How to do it
- Warm 20 mL of the acid in the beaker to about 60 °C (do not boil) and add copper(II) oxide half a spatula at a time with stirring until black solid stops dissolving.
- Filter the warm blue solution to remove the excess oxide.
- Evaporate the filtrate over a water bath until crystals form at the surface, then leave to cool and crystallise overnight.
- Dry the crystals between filter papers, weigh them, compare the mass with other groups' and list the steps where product was lost.
What you should see
The black powder dissolves and the solution turns clear blue; blue crystals of copper(II) sulfate pentahydrate form on cooling. For the teacher: 20 mL of 0.5 mol/L acid reacts with 0.80 g of copper oxide and can give at most 2.50 g of CuSO4·5H2O. The mass of crystals is always below this because some solution stays on the filter paper, some salt stays dissolved in the cold liquid and some crystals are lost in drying. The learner knows it worked when unreacted black oxide is visible on the filter paper (proving the acid was fully used) and the crystals are blue and well formed.
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 copper oxide dissolves like sugar.
- Adding more copper oxide gives more crystals, even after all the acid has reacted.
- Boiling to dryness gives the best crystals.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Science 7–10 Syllabus (2023)SC5-RXN-01SC5-WS-04
- Australian Curriculum v9AC9S9U07AC9S10U07AC9S10I03AC9S9I03
Sources
The pages the author read to write this activity.
- curriculum.nsw.edu.au/learning-areas/science/science-7-10-2023/outcomes
- edu.rsc.org/experiments/reacting-copperii-oxide-with-sulfuric-acid/1917.article
- edu.rsc.org/experiments/preparing-salts-by-neutralisation-of-oxides-and-carbonates/1762.article
- education.nsw.gov.au/content/dam/main-education/asset-management/chemical-safety/5._Volume_2_Appendices.pdf