Question

Salt ‘A’ commonly used in bakery products on heating gets converted into another salt ‘B’ which itself is used for removal of hardness of water and a gas ‘C’ is evolved. The gas ‘C’ when passed through lime water, turns it milky. Identify A, B and C. Write chemical equations involved in it.

Answer 1

Hint: To solve this we must know the salt used in bakery products. Salt used in bakery products is a salt of sodium. The sodium salt decomposes to another salt and a gas. The salt thus removes the hardness of water and the gas formed turns lime water milky.

Complete step by step solution:
We are given that salt A is used in bakery products. We know that the salt commonly used in bakery products is sodium bicarbonate. The molecular formula for sodium bicarbonate is ${\text{NaHC}}{{\text{O}}_{\text{3}}}$.
Thus, A is sodium bicarbonate - ${\text{NaHC}}{{\text{O}}_{\text{3}}}$.
We are given that salt A on heating gets converted into salt B and gas C. Sodium bicarbonate on heating undergoes a decomposition reaction. During the reaction, three products are formed: sodium carbonate which is a salt, carbon dioxide gas and water.
The molecular formula for sodium carbonate is ${\text{N}}{{\text{a}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}}$, for carbon dioxide is ${\text{C}}{{\text{O}}_{\text{2}}}$ and for water is ${{\text{H}}_{\text{2}}}{\text{O}}$.
Thus, the decomposition reaction of sodium bicarbonate is as follows:
${\text{2NaHC}}{{\text{O}}_{\text{3}}}\xrightarrow{{{\text{Heat}}}}{\text{N}}{{\text{a}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}} + {\text{C}}{{\text{O}}_{\text{2}}} + {{\text{H}}_{\text{2}}}{\text{O}}$
Thus, B is sodium carbonate - ${\text{N}}{{\text{a}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}}$ and C is carbon dioxide - ${\text{C}}{{\text{O}}_{\text{2}}}$.
We are given that salt B is used for removal of hardness of water.
Hard water contains calcium and magnesium salts. The reaction of calcium and magnesium ions with sodium carbonate produces calcium carbonate and sodium ions. As the calcium ions are used up, the water hardness gets removed. The reaction is as follows:
${\text{C}}{{\text{a}}^{2 + }} + {\text{N}}{{\text{a}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}} \to {\text{CaC}}{{\text{O}}_3} + 2{\text{N}}{{\text{a}}^ + }$
We are given that gas C when passed through lime water turns it milky. The reaction of carbon dioxide gas with lime water i.e. calcium hydroxide produces calcium carbonate and water. The calcium carbonate is a white precipitate which appears milky. The reaction is as follows:
${\text{C}}{{\text{O}}_{\text{2}}} + {\text{Ca}}{\left( {{\text{OH}}} \right)_{\text{2}}} \to {\text{CaC}}{{\text{O}}_{\text{3}}} + {{\text{H}}_{\text{2}}}{\text{O}}$

Thus, A is sodium bicarbonate ${\text{NaHC}}{{\text{O}}_{\text{3}}}$, B is sodium carbonate ${\text{N}}{{\text{a}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}}$ and C is carbon dioxide ${\text{C}}{{\text{O}}_{\text{2}}}$.

Note: The sodium bicarbonate which is used in bakery products reacts with acids and produces carbon dioxide gas. The carbon dioxide gas released causes expansion of batter and gives a spongy texture to the bakery products.