Before battery-powered lamps became available in the early 1900s, carbide lamps that made use of a chemical reaction to produce a flammable hydrocarbon gas that burned with a bright, sooty flame were commonly used on bicycles and motor cars - Leaving Cert Chemistry - Question 1 - 2021

Gas B is acetylene, a flammable hydrocarbon gas produced from calcium carbide.

When liquid A (water) is added to calcium carbide, a vigorous reaction occurs, producing acetylene gas and calcium hydroxide. The reaction can be represented by the equation:

$CaC_2 + 2H_2O \rightarrow C_2H_2 + Ca(OH)_2$

This reaction generates heat and bubbles of gas.

The method of collection indicates that gas B (acetylene) is not very soluble in water, as it is collected over water rather than dissolved in it.

Additionally, the first few test-tubes of gas produced may contain impurities or water vapor, and thus, are not used in tests.

The term 'unsaturated' refers to a compound that contains at least one double or triple bond between carbon atoms. Acetylene, which has a triple bond between the carbon atoms in its molecular structure, is classified as an unsaturated hydrocarbon.

To test for unsaturation, you can perform a bromine water test. Add a few drops of bromine water to a sample of gas B (acetylene). If the solution changes from brown or reddish-brown to colorless, it indicates the presence of unsaturation due to the addition reaction of bromine across the double or triple bonds in the hydrocarbons.

When acetylene burns in pure oxygen, the flame is very hot and nearly colorless. It produces a bright, luminous flame suitable for welding and gives off less soot compared to burning in air.