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1. Is there anything else in the kit?

   Citric acid merely dissolves in water (it doesn’t react with it).

   I’m not sure how the kit is supposed to function, but can imagine two possibilities:

   1) If there’s an active metal like zinc in the kit, then it’d be possible to produce hydrogen from your citric acid solution (citric acid + zinc –> zinc citrate + hydrogen).

   2) It’s also possible to use the batteries to electrolyze the water (split it into hydrogen and oxygen) by electrolysis. In that case, the citric acid is just serving as an electrolyte.

   Without knowing more about the kit, though, it’s hard to say more. If it’s electrolysis, though, the equations can be had from Wikipedia:

   In the water at the negatively charged cathode, a reduction reaction takes place, with electrons (e−) from the cathode being given to hydrogen cations to form hydrogen gas:

   Cathode (reduction): 2H2O(l) + 2e− → H2(g) + 2OH−(aq);

   At the positively charged anode, an oxidation reaction occurs, generating oxygen gas and giving electrons to the anode to complete the circuit:

   Anode (oxidation): 2H2O(l) → O2(g) + 4H+(aq) + 4e−;

   Combining these two reactions yields the overall decomposition of water into oxygen and hydrogen:

   Overall reaction: 2H2O(l) → 2H2(g) + O2(g)

   The number of hydrogen molecules produced is thus twice the number of oxygen molecules. Assuming equal temperature and pressure for both gases, the produced hydrogen gas has therefore twice the volume of the produced oxygen gas. The number of electrons pushed through the water is twice the number of generated hydrogen molecules and four times the number of generated oxygen molecules.