The apparatus below is used to investigate the products formed when a hydrocarbon fuel burns in air. Gases are drawn through the apparatus by a pump. A colo...

This question tests the two products of complete combustion of a hydrocarbon (water and carbon dioxide) and how you prove each one. Tube A is a cold trap that catches the water vapour; the remaining gas is bubbled through limewater to test for carbon dioxide.

(a) Tube A is surrounded by ice to cool the vapours so that the water condenses and collects as a liquid. [1] Without cooling, the water would stay as vapour and pass straight through, so none could be collected or tested.

(b) A chemical test for water, worth two marks: add the liquid to anhydrous copper(II) sulfate (or use anhydrous cobalt(II) chloride paper). [1] With water present the anhydrous copper(II) sulfate turns from white to blue (cobalt(II) chloride paper turns from blue to pink). [1] These colour changes are caused by water becoming chemically combined into the crystal structure.

(c) In tube B the limewater turns milky / cloudy. [1] This is the standard positive result for carbon dioxide (a white precipitate of calcium carbonate forms).

(d) The gas causing this change is carbon dioxide. [1]

(e) A hydrocarbon contains only two elements: carbon and hydrogen. [1] That is the definition of a hydrocarbon, and it is why complete combustion gives only CO₂ (from the carbon) and H₂O (from the hydrogen).

(f) The word equation for complete combustion, worth two marks:

hydrocarbon + oxygen → carbon dioxide + water [1] for the correct reactants and [1] for the correct products. Complete combustion needs a plentiful supply of oxygen; if oxygen is limited you would also get carbon monoxide and soot.

Exam tip: the water test and the carbon dioxide (limewater) test together confirm a hydrocarbon has burned completely. Learn both positive results word for word.