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Does anybody know two liquids (or a solid and a liquid) that can react to produce a gas less dense than air (or similar) at room temperature?

This question is intended to make a chemical induced reaction when a lever is pulled to produce gas and decrease an object's mass.

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If the other products aren't significant, I'll recommend the all famous alkali metals' reaction with water: $$\ce{2Li(s) + 2H2O(l) -> 2LiOH(aq) + H2(g)}$$ Hydrogen gas is less dense than air, both at RTP and at STP, because the main components of air (by mole percent) are $\ce{N2}~(78\%)$, $\ce{O2}~(20.95\%)$ and $\ce{Ar}~(0.934\%)$ - all of which have a molar mass larger than hydrogen.

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    $\begingroup$ As a side note, basically any reaction that produces Hydrogen gas is what you need. I wrote this example because it's cheap enough to obtain. $\endgroup$ – M.A.R. Feb 20 '15 at 18:36
  • $\begingroup$ lithium isn't exactly cheap $\endgroup$ – permeakra Jun 24 '15 at 17:45
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    $\begingroup$ @permeakra FWIW, cheap is relative. They never mentioned what's cheap supposed to mean.' $\endgroup$ – M.A.R. Jun 24 '15 at 17:48
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In case you want a safe and reliable method, consider magnesium granules/foil and acetic acid. The reaction is quite exothermic, but the reagents are reasonably safe. Do not use magnesium powder for the reaction, it is likely to react violently.

In case you want a cheap and reliable method, consider aluminium granules and sodium hydroxide. The reaction is, again, quite exothermic, and again, I recommend to avoid aluminium powder. Note: sodium hydroxide is unstable on air.

The military option is ferrosilicon with sodium hydroxide, but ferrosilicon is not readily available and the reaction is, again, quite exothermic.

Standard lab option is to use zinc granules with hydrochloric acid, but zinc is quite costly.

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The other answers are very good, so purely in the interest of diversity I'll mention the methide (methanide?) carbides such as aluminum carbide or magnesium carbide. With aluminum, the reaction is:

$\ce{Al4C3(s) + 12 H2O(l) → 4 Al(OH)3(aq) + 3 CH4(g)}$

Methane ($\ce{CH4}$) is less dense than air, but not by nearly as much as $\ce{H2}$ which is generated by reactions mentioned in other answers. I've never tried the reaction myself but I believe it is comparatively less vigorous than hydrogen generation from alkali metals or alkali earths. But on the up-side, the relative change in mass of the system will be higher with methane evolution since it weighs more than hydrogen.

I'm not sure why you even need a gas less dense than air. If you're using a closed container (n.b. this is a bad, extremely dangerous idea unless it is a balloon or something), the mass of the system won't change. If you're using an open container, the mass of the system will change as the gas is evolved, whether or not it is less dense than air.

Calcium carbide is not a methanide, but an acetylide, so when mixed with water it generates acetylene gas. Acetylene is very nearly the same density as air (its molecular weight is 26 g/mol, compared to an average of 30 g/mol for air. The reaction of calcium carbide with water was formerly used widely in carbide lamps, so presumably it is relatively easy to control.

Calcium carbide is also cheap compared to alkali metals such as lithium, and acetylene evolution will cause an even higher relative change in mass of the system than methane evolution.

$\ce{CaC2(s) + 2H2O(l) -> HC#CH(g) + Ca(OH)2(s)}%edit$

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