Question

Give reasons: When the gas formed on heating limestone is passed through freshly prepared lime water, the lime water turns milky.

Answer 1

The carbonates are the salts form of carbonic acid which is formed when metal ions with positive charge react with the oxygen atom present in the carbonate ion. Carbonates are usually insoluble in water and thus form precipitate. They are mostly basic in nature.
Complete step by step answer:
Limestone is a rock which primarily contains calcium carbonate which has the chemical formula $CaC{O_3}$.
Thermal decomposition is a type of reaction where the single reactant is heated so that it breaks down into two or more than two products.
The thermal decomposition of limestone results in the release of a colorless gas called carbon dioxide.
The reaction for the thermal decomposition of limestone is shown below.
$CaC{O_3}\xrightarrow{\Delta }CaO + C{O_2}$
In the above reaction, calcium carbonate on heating breaks down into calcium oxide and carbon dioxide.
The lime water is prepared by dissolving calcium hydroxide in water. On passing the carbon dioxide gas to the lime water, the solution turns milky.
The reaction for the above statement is shown below.
$C{O_2} + Ca{(OH)_2} \to CaC{O_3}$
This happens because the carbon dioxide reacts with the calcium hydroxide to form calcium carbonate. Calcium carbonate is insoluble in water therefore it forms a white precipitate and due to this the solution appears milky.

Note: The calcium carbonate which is obtained as precipitate during the reaction of carbon dioxide and calcium hydroxide is in much purer state than the calcium carbonate obtained from the sedimentary rocks.