Copper(II) sulfate recrystallization
Recommended age: 11+. Adult supervision required! We prepare a saturated solution from blue copper(II) sulfate, then grow sparkling, deep-blue crystals by slow cooling — one of the loveliest school chemistry classics about solubility and crystallization.
Chemicals
- 25 g copper(II) sulfate pentahydrate (blue vitriol) (at least 98%, school or garden-store grade) — at least 98%, school or garden-store grade
- 100 ml distilled water (boiled and cooled tap water also works) — boiled and cooled tap water also works
Procedure
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Put on your goggles and lab coat! Copper(II) sulfate is harmful if swallowed, so never taste it, and wash your hands thoroughly when you finish. Heating and hot filtration must be done by the adult only.
Safety! -
Weigh out 25 g of copper(II) sulfate on the kitchen scale and tip it into the 250 ml beaker.
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Add 100 ml of distilled water and mix with the glass stirring rod.
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The adult heats the solution to about 60 °C and keeps stirring until every crystal has dissolved. No need to boil!
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While still warm, the adult filters the solution through the filter paper in the funnel into the crystallizing dish — any impurities stay behind on the paper.
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Place the dish somewhere calm and free of vibrations, and cover it loosely with a sheet of paper so no dust falls in.
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The slower the solution cools, the bigger the crystals grow. If you want a few large, beautiful pieces instead of many tiny ones, wrap the dish in a towel — that gives the slowest cooling.
Technique -
Wait 2 days. Once the crystals have grown, the adult pours the leftover solution off them into the waste jar — never down the drain!
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Dry the crystals by pressing them gently between two sheets of filter paper, then admire them — take a photo and compare their shapes!
Apparatus
- 1× beaker — 250 ml
- 1× glass stirring rod
- 1× hot plate or spirit burner
- 1× funnel
- 3× filter paper
- 1× crystallizing dish or shallow glass bowl
- 1× kitchen scale
Frequently asked questions
- Why does it work?
- Much more copper(II) sulfate dissolves in warm water than in cold. As the saturated solution slowly cools, the 'excess' can no longer stay dissolved and comes out as regular crystals — the particles arrange themselves into an ordered lattice. This is crystallization, which chemists also use for purification.
- What should I do if no crystals grow?
- The solution was probably not saturated. Pour it back into the beaker, have the adult dissolve one more tablespoon of copper sulfate while warming it, and start again. Dropping in a tiny 'seed crystal' also kick-starts crystallization.
- Can I touch the finished crystal with my hands?
- Briefly yes, but wash your hands right afterwards and never put it in your mouth. Store it in a closed, labelled box out of reach of younger siblings and pets.
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Gallery — every version of this photo
Every version of this image is kept. Nothing here was photographed in a lab — the illustrations are generated on ATAILA hardware, and when the models or the prompt improve we regenerate and keep the old one so the difference is visible.