When Mg metal is added to Hydrochloric acid there is an immediate and vigorous reaction in which H2 gas is evolved. Those are the bubbles we see in the beaker. If we capture the H2 gas we can light it on fire and it should ignite (think Hindenburg Disaster). Note: don't try this at home!

The reaction that takes place is Mg (s) + HCl (aq) = MgCl2 (aq) + H2 (g).
(If we balance the equation we could have Mg + 2HCl = MgCl2 + 2H2).

Once the reaction is complete there is no more Mg metal we can test the H2 gas. Here we collect the H2 gas in a balloon. When brought near a flame we get a very pleasing explosion. The reaction for igniting H2 in air is H2 (g) + O2 (g) = H2O (g).
(If we balance the equation we get 2H2 + O2 = 2H2O).

You might ask were the Magnesium metal went. It is actually dissolved in the water as MgCl2(aq) which is a clear solution. If we evaporate the water away we would have MgCl2 (s) which is a white salt.

Chapters
0:00 Intro
0:15 Mg in HCl Solution
0:30 Mg in Concentrated HCl Solution
0:59 How to Identify H2 Gas
1:25 Collecting H2 Gas in a Balloon
1:45 Exploding H2 Gas