The bubbles come from Sodium Carbonate, or NaCO3. It is the white chalky looking stuff on the tablets and the same as baking soda. When you drop them in water the CO3 becomes CO2 (Carbon Dioxide) and makes all the fizzing bubbles you see. The reaction is the same as when you mix baking soda with vinegar to make a fake volcano (it is very messy, but fun to do!).
H + CO3 -> CO2 + H2O
The extra oxygen molecule left over after the CO3 turns into CO2 combines with a hydrogen to make water (H20). The sodium (Na) does not react.
By itself, the tablets don't react much, since the sodium carbonate is very stable. However, there is also citric acid (same as lemon juice) in a separate area of the tablet. In solid form nothing happens, but when it is dropped in water, the citric acid releases hydrogen that can start the reaction going. They also put in a little aspirin, but that is for medical benefits, like if you have a headache.
