When ethanoic acid (also known as acetic acid) reacts with magnesium, the gas given off is hydrogen gas (H₂).

Here's the chemical reaction:

2CH₃COOH (aq) + Mg (s) → (CH₃COO)₂Mg (aq) + H₂ (g)

* Ethanoic acid (CH₃COOH) is a weak acid.

* Magnesium (Mg) is a reactive metal.

* The reaction produces magnesium acetate ((CH₃COO)₂Mg) and hydrogen gas.

The hydrogen gas can be identified by its characteristic "pop" sound when ignited with a flame.