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I'm looking for a chemical reaction in order produce about 5 liter of harmless gas.

The goal is to start with something that takes as little volume as possible to start with, and reacts to fill up a bag, and also not producing too much heat to not harm the bag.

Every answer is appreciated!

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I'd go with carbon dioxide $\ce{CO2}$ as a rather convenient gas which is non-flammable and doesn't require special equipment or exothermic reaction to synthesize. Common ways of obtaining $\ce{CO2}$ gas are:

  • Dry ice + ambient temperature above $\pu{−78.5 ^\circ C}$;
  • Carbonate + acid. These reactions rarely have significant thermal effect. One of the best mixture would be sodium bicarbonate + citric acid + water. Assuming dissolving of citric acid is endothermic and takes some time, prepare the solution of citric acid beforehand.

$\ce{O2}$ and $\ce{H2}$ are rather dangerous in terms of flammability, also common synthetic methods require exothermic reactions.

$\ce{N2}$ is inert, but not easy to come by; the most effective way of obtaining nitrogen would be thermal decomposition of azides $\ce{M^IN3}$ and ammonium nitrite $\ce{NH4NO2}$.

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    $\begingroup$ There used to be little ‘bomb bags’ available. Sealed bags containing sodium hydrogencarbonate and a smaller internal bag with acetic acid. You broke the inner bag and after some time the pressure built up inside would make the bag explode. $\endgroup$ – Jan Dec 16 '17 at 6:07
  • $\begingroup$ @Jan Interesting. Must be local though; when I was a kid calcium carbide and some water in a tightly closed bottle used to be a trend:) $\endgroup$ – andselisk Dec 16 '17 at 6:12

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