This video demonstrates how to make pure, dry crystals of copper(ii) sulfate (CuSO4). A typical 5-mark exam question would go as follows:
“Explain how you would use this reaction to prepare a pure, dry sample of cupper(ii) sulfate.”
A model answer is given below, with indexes linked to the video.
Step 1: Mix sulfuric acid with EXCESS copper oxide (1:27)
Step 2: Stir using a glass stirring rod (1:45)
Step 3: Filter off the excess (left over) copper oxide (7:48)
Step 4: HEAT the solution to drive off SOME of the water (9:42)
Step 5: Leave to crystallise. (You can encourage crystal formation by dipping a glass rod into the solution) (12:08)
Step 6: Dry the crystals. This can be done either in an oven; by dabbing with a paper towel, or simply by leaving the crystals to dry naturally. The latter was used in this video and leads to larger, more well-formed crystals. You can see an initial time-lapse showing crystal formation (12:28); after 40 mins (12:57); after 24 hours (13:07) and after 72 hours (13:19).
As you can see from the video, copper(ii) sulfate forms parallelogram-shaped blue crystals.
Technically, the blue crystals are copper(ii) sulfate pentahydrate (CuSO4·5H2O). It was once called ‘blue vitriol’ and often met at school. For example, anhydrous copper(ii) sulfate absorbs water to form hydrated copper(ii) sulfate. This is accompanied by a colour change from white to blue. This is used as the chemical test for water. The hydration can be reversed by heating the blue crystals to 200 degrees Celsius. This is often given as an example of a thermal decomposition reaction at prep school. Students may also have seen the blue colour in Benedict’s, or Fehling’s, solution, used to test for aldehydes or reducing sugars like glucose. The resulting brick red precipitate is the insoluble solid copper(i) oxide (Cu2O).
Outside the classroom, copper(ii) sulfate is often used to control fungal growth. It is part of ‘Bordeaux mixture’ which is used to help keep wine grapes fungus-free.
Copper(II) sulfate pentahydrate decomposes before melting. It loses two water molecules upon heating at 63 °C, followed by two more at 109 °C and the final water molecule at 200 °C. At 650 °C, copper(II) sulfate decomposes into copper(II) oxide (CuO) and sulfur trioxide (SO3).
