(a) In the laboratory preparation of crystals of CuSO\(_4\), a green powder Q was added to dilute H\(_2\)SO\(_4\), and stirred. Effervescence occurred and a...

Answer Details

Preparation of copper(II) sulphate crystals

A green powder Q reacts with dilute H\(_2\)SO\(_4\) with effervescence, giving off a gas R that turns lime water milky (so R is carbon dioxide). This identifies Q as a copper(II) carbonate.

• (i) Substance Q: copper(II) carbonate (basic copper carbonate), CuCO\(_3\). \[CuCO_3 + H_2SO_4 \to CuSO_4 + H_2O + CO_2\uparrow\]
• (ii) Gas R: carbon dioxide, CO\(_2\) (it turns lime water milky).
• (iii) Why excess Q was used: to make sure that all the acid is used up (neutralised), so that no free/unreacted sulphuric acid is left to contaminate the copper(II) sulphate.
• (iv) How you know the reaction is complete: effervescence stops and some of the green solid Q remains undissolved (unreacted), showing the acid has all been used up.
• (v) Method used to remove excess Q: filtration (the undissolved excess Q is left on the filter paper as residue and the copper(II) sulphate solution passes through as filtrate).
• (vi) Why the CuSO\(_4\) solution was not heated to dryness: heating to dryness would drive off the water of crystallisation and could decompose the salt, giving a white anhydrous powder instead of the blue hydrated crystals CuSO\(_4\).5H\(_2\)O. It is therefore evaporated only to the point of crystallisation and allowed to cool so that good crystals form.

(b) Reagents for testing

• (i) Water: anhydrous copper(II) sulphate (turns from white to blue), or anhydrous cobalt(II) chloride paper (turns from blue to pink). A confirmatory test is that pure water boils at 100 degrees Celsius.
• (ii) Primary alkanol (primary alcohol): acidified potassium tetraoxomanganate(VII) or acidified potassium dichromate(VI). On warming, the alkanol is oxidised, so acidified KMnO\(_4\) is decolourised (purple to colourless) and acidified K\(_2\)Cr\(_2\)O\(_7\) turns from orange to green.