Question

When excess of gas A was passed through solution B a white precipitate was obtained which turned colourless.
(a) Name A and B
(b) Name the product formed after each reaction
(c) State balanced chemical equation involved

Answer 1

Try to think of a reaction in which a gas reacts with a colourless solution to form white ppt. Remember Calcium carbonate$\left( {CaC{O_3}} \right)$has milky appearance and Calcium bi-carbonate $Ca(HC{O_3})$ is colourless.

Complete step by step answer:
When ($C{O_2}$ ) carbon dioxide gas is passed through a solution of lime water the solution turns milky. This is due to the formation of$\left( {CaC{O_3}} \right)$Calcium carbonate. The chemical reaction is as follows:
$C{O_2} + Ca{(OH)_2} \to CaC{O_3} + {H_2}O$
When excess carbon dioxide ($C{O_2}$) is passed through a solution of Calcium carbonate $\left( {CaC{O_3}} \right)$the milkiness disappears. This is due to the formation of Calcium bi-carbonate$Ca{(HC{O_3})_2}$. The chemical reaction is as follows:
$CaC{O_3} + {H_2}O + C{O_2} \to Ca{(HC{O_3})_2}$
(a) Hence gas A is$C{O_2}$ and solution B is lime water .
(b ) $C{O_2} + Ca{(OH)_2} \to CaC{O_3} + {H_2}O$
The product formed when Carbon dioxide reacts with Lime water is Calcium Carbonate.
$CaC{O_3} + {H_2}O + C{O_2} \to Ca{(HC{O_3})_2}$
The product formed when excess of Carbon dioxide ($C{O_2}$) is passed through a solution of Calcium Carbonate $\left( {CaC{O_3}} \right)$ and water ($C{O_2}$) is of Calcium bi-carbonate ($Ca{(HC{O_3})_2}$).
(c) The balanced chemical equation for each step is as follows:
$C{O_2} + Ca{(OH)_2} \to CaC{O_3} + {H_2}O$
$CaC{O_3} + {H_2}O + C{O_2} \to Ca{(HC{O_3})_2}$

Note: (1) Calcium bicarbonate - Calcium bicarbonate is also known as Calcium hydrogen carbonate. Calcium bicarbonate is white in colour. It can be used as a colouring agent and in food. This term does not refer to a known solid compound because it is found only in aqueous solution. A solution is known as an aqueous solution in which the solvent is water. Calcium bicarbonate containing ions such as calcium ($C{a^{2 + }}$),bicarbonate ($HCO_3^ -$), and carbonate ($CO_3^{2 - }$).
(2) Calcium Carbonate - Calcium Carbonate is found on mineral rocks. Calcium Carbonate is used for refining of sugar, added to the swimming pool to maintain pH level. This is also used as antacid.