ENO is an antacid (manufactured by GSK) which contains a mixture of sodium bicarbonate (`NaHCO_3` ), citric acid (`H_3C_6H_5O_7` ) and sodium carbonate (`Na_2CO_3` ). Initially, citric acid is inert when dry (or anhydrous). Upon the addition of water, the citric acid will start to dissociate and eventually becomes reactive. It can convert the carbonates into Carbon dioxide and water.
The general reaction for this can be written as:
ENO (s) + water (l) -> citrate (salt) + carbon dioxide (g) + water (l)
We can see that among the products, carbon dioxide is the gas that will be produced in the reaction. To confirm the presence of carbon dioxide in the solution, effervescence should be noticed. You can also bubble the gas produced over lime water. If the lime water turns cloudy, carbon dioxide is present.