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What is the gas that if formed when a reactive metal is placed in a salt solution of a less reactive metal? Theoretically, it should just be a simple displacement.

I know that this question has been asked on a different page, but there Hydrogen formation was the accepted answer, which I don't think is the case (see observations).

Observations:

When a strip of magnesium ribbon was placed in a concentrated copper sulfate solution, the magnesium ribbon piece started bubbling.

Magnesium didn't bubble as violently when placed in plain water of the same temperature.

The gas was bubbled through a soap solution and the bubble was passed near a candle flame. It just popped and nothing happened.

Pure hydrogen gas was obtained by placing a Mg ribbon in acid, and bubbles were obtained. These exploded with a bang when passed over a flame (learnt the hard way not to do this!).

Regular distilled water was used everywhere.

Thanks a lot in advance for any help.

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The bubbles are still hydrogen. Transition metal salts are generally much weaker acids than whatever you put your magnesium ribbon into for your hydrogen control. So you get less gas for the candle flame to react with when you use the transition metal salt, which I am sure you saw. That the decreased amount of gas still popped, which is basically a low-intensity bang, should be deemed a positive test for hydrogen.

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  • $\begingroup$ The bubble just popped due to the heat of the flame. The gas in it did not burn in any sort of way. Additionally, there was as much, if not more gas evolving from the Mg/CuSO4 mixture than the Mg/Acid mixture (I used acetic acid). It isn't possible for the gas to be a mixture of H2 and something else, is it? $\endgroup$
    – tt123
    Oct 25, 2019 at 1:16
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    $\begingroup$ I don't see any chance of this being anything other than hydrogen. Popping individual bubbles can be deceptive. Try to collect a whole beaker (with proper safety precautions, that is). $\endgroup$
    – Ivan Neretin
    Oct 25, 2019 at 7:12
  • $\begingroup$ Let me try collecting more gas. I'll post a video if I can. Is there any possibility of liberating any dissolved gases in the solution? I read this as an answer to another question... $\endgroup$
    – tt123
    Oct 25, 2019 at 11:19
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When you dip magnesium ribbon into a copper sulfate solution, theoretically (and realistically) you do get a simple replacement reaction. Then reality sets in: you have a metallic anode (magnesium) with little cathodes (copper) all over it. At that point, the magnesium just overreacts. Well, it reacts faster.

The size of the bubbles will cause some difference in the popping ability of the bubbles. Bubbles could pop differently if some were pure H2 and others were mixed with air.

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