Question

When an acid reacts with a metal carbonate or metal hydrogen carbonate, it gives the corresponding salt, gas and product. Name the gas and product?
(A) Carbon dioxide, Water
(B) Carbon dioxide, Salt
(C) Hydrogen, Salt
(D) All of the above

Answer 1

Remember that all metal carbonates and hydrogen carbonates are weak bases. So, when an acid reacts with base, ionic compounds are the main product of the reaction. Compare the chemical formula of metal carbonates and hydrogen carbonates to predict which gas they both will give with acid.

Complete step by step answer:
Let us go through each of the given reactions i.e. that of an acid with a metal carbonate and a metal hydrogen carbonate and analyse the products evolved before giving an informed answer to the given question.
i) When an acid reacts with metal carbonates, such as calcium carbonate (found in chalk, limestone and marble), the production of a metal salt, water and carbon dioxide is observed.
Acid + Metal carbonate $\to$ Salt + Water + Carbon dioxide
e.g.
$FeC{{O}_{3}}+{{H}_{2}}S{{O}_{4}}\to FeS{{O}_{4}}+C{{O}_{2}}+{{H}_{2}}O$

Here, we can see that Carbon dioxide gas evolves from the decomposition of carbonate ion and salt Iron sulphate is formed. It is also observed that the presence of carbon dioxide causes bubbling during the reaction. It can also be detected by passing the gas through lime water, which will cause the latter to go cloudy. This type of reaction can be used to test unknown solutions to see if they are acidic as upon the addition of a solution of sodium carbonate to the solution, if carbon dioxide gas is given off, then the solution is acidic in nature.

Now, let us observe the reaction of an acid with a metal hydrogen carbonate.
ii) At first the unstable carbonic acid is formed. If this happens at normal atmospheric pressure then large amounts of bubbling commences as carbon dioxide gas leaves the solution. This continues until the carbonate has been relieved of all carbon dioxide, leaving behind the positive metal ion and the negative ion group from the acid. Hydrogen ions from the initial acid are taken up by the bicarbonate ion to form carbonic acid or hydrogen carbonate. When carbon dioxide is released, only the product water remains along with the new metal salt.
e.g.

NaHC{{O}_{3}}~+\text{ }HCl\text{ }\to \text{ }NaCl\text{ }+\text{ }{{H}_{2}}C{{O}_{3}} \\\ {{H}_{2}}C{{O}_{3}}~\to \text{ }{{H}_{2}}O\text{ }+\text{ }C{{O}_{2}}\left( g \right) \\\ \end{array}$$ Here, we can see that initially, carbonic acid is formed by the reaction of acid with sodium bicarbonate. This carbonic acid not being stable gives carbon dioxide and water as a product. Thus, we can conclude that when metal carbonates and hydrogen carbonates react with acid, carbon dioxide, water and salt are always produced. _**Thus, the correct option is (A).**_ **Note:** Metal bicarbonates are amphoteric compounds and their aqueous solutions are mildly alkaline due to the formation of carbonic acid and hydroxide ion whereas metal carbonates are basic in nature with their aqueous solutions being strongly alkaline.