Question

(a) What happens when $ C{O_2} $ gas is bubbled through lime water (i) in small amounts (ii) in excess?
(b) “Combination reaction is the reverse of decomposition reaction.” Justify this statement with the help of the appropriate chemical equation of each.

Answer 1

Hint: Specific test: It is a qualitative or quantitative test which is used to identify a chemical compound, gas or a functional group present in the organic compound. The most common specific test for carbon dioxide gas is the lime water test.

Complete answer:
(a) The chemical name of lime water is calcium hydroxide and its molecular formula is $ Ca{(OH)_2} $ . When $ C{O_2} $ gas is bubbled through lime water, following observations are made:
(i) When carbon dioxide gas is bubbled or passed through lime water in small amounts, it undergoes a chemical reaction which forms a milky or whitish colour solution. The appearance of milky colour in the solution is due to the formation of calcium carbonate which is insoluble in water. The reaction for the given process proceeds as follows:
 $ C{O_2} + Ca{(OH)_2} \to CaC{O_3} + {H_2}O $
(ii) When carbon dioxide gas is bubbled or passed through lime water in excess, then further reaction takes place and the milkiness of the solution disappears due to the formation of calcium bicarbonate that is colourless in nature. The reaction for the given process proceeds as follows:
 $ CaC{O_3} + {H_2}O + C{O_2} \to Ca{(HC{O_3})_2} $
(b) Combination reactions are the chemical reactions in which two or more than two compounds react together to form only a single product. These reactions are also termed as additional reactions. The formation of new bonds takes place in these types of reactions and are generally represented as follows:
 $ A + B \to C $
Example- Formation of ammonium chloride
 $ N{H_3} + HCl \to N{H_4}Cl $
Decomposition reactions are the chemical reactions in which a single compound breaks or dissociates into several products in the presence of heat or light. These reactions are also termed as dissociation reactions. The breaking of bonds takes place in these types of reactions and are generally represented as follows:
 $ AB \to A + B $
Example- Thermal decomposition of calcium carbonate.
 $ CaC{O_3} \to CaO + C{O_2} $
Therefore, we can conclude that in combination reactions, two or more compounds react to form a single product i.e., new bonds are formed whereas in decomposition reactions, one compound dissociates to form two or more products i.e., bonds are separated. Hence, combination reaction is the reverse of decomposition reaction.

Note:
The reactions which are performed in the presence of light are also known as photochemical reactions. When a molecule is present in its ground state, it absorbs light and electrons are excited to a higher energy level and hence photochemical reactions take place.
Example: Decomposition reaction of silver chloride.
 $ 2AgCl\xrightarrow{light}2Ag+C{{l}_{2}} $