What additional product is formed when a metal carbonate reacts with an acid?

When a metal carbonate reacts with an acid, one of the key products formed is carbon dioxide. This reaction typically follows the general equation where a metal carbonate (for example, calcium carbonate) reacts with an acid (such as hydrochloric acid) to produce a salt, water, and carbon dioxide. The carbon dioxide is a gas that often escapes into the atmosphere, which is observed as bubbles during the reaction.

The formation of carbon dioxide can be attributed to the breakdown of the carbonate ion, which releases carbon dioxide along with other products. This characteristic reaction is common to all metal carbonates when they interact with acids, making carbon dioxide a consistent product in these reactions.

In contrast, the other substances listed do not form through this specific reaction. Hydrogen gas is produced in reactions with metals and acids, but not from carbonates. Oxygen gas is not typically a product of carbonate reactions with acids, and although water is produced, it is combined with the other products rather than being the recognizably unique product of the reaction.