Testing candle wax
Activity 14.1 tests whether candle wax, a fuel, contains carbon and hydrogen.
Activity 14.1 — confirming that wax contains carbon and hydrogen
You will need: connecting tubes, a beaker, a suction pump, lime water, copper sulphate, a U-tube, a test tube
Set up the apparatus as in the diagram. Light the candle, connect the suction pump and draw the gases from the flame through the apparatus.
Figure to be added
The observations
- The anhydrous copper sulphate in the U-tube turns from white to blue. The change is caused by water produced as the candle burns. The hydrogen needed to make that water comes from the wax, so wax contains hydrogen.
- The lime water in the tube on the right turns milky. Lime water is turned milky by carbon dioxide, so carbon dioxide (CO2) was given off as the candle burned. The source of the carbon in that CO2 is the wax.
So wax contains carbon (C) and hydrogen (H).
Why each part of the apparatus is there
The water from the flame leaves as steam, so the U-tube stands in cold water to condense it where the copper sulphate can meet it. The water test comes first on purpose: lime water is itself mostly water, so testing for water after the lime water would find it whatever the candle did. The suction pump keeps the gases moving the right way. Oxygen from the air is needed for burning, so the test cannot prove the wax contains oxygen — only that the carbon and hydrogen came from it.
Almost every country meets its fuel needs with petroleum fuels, obtained by the fractional distillation of crude oil. All the compounds in those fuels are hydrocarbons. By their structure, hydrocarbons are classified as alkanes, alkenes and alkynes.
Blue copper sulphate, milky lime water
Burning wax gives water (anhydrous copper sulphate turns blue) and carbon dioxide (lime water turns milky), so wax contains hydrogen and carbon.