Question

Why does bleaching powder smell strongly of chlorine and does not dissolve completely in water?

Answer 1

Bleaching powder formed by passing $C{l_2}$ gas into $Ca{(OH)_2}$ . These are used to bleach wood pulp and fabrics, and as disinfectants. The formula of bleaching powder is $CaOC{l_2}$ . Its bleaching action is shown by oxidising property.

Complete solution
Bleaching powder can be defined as salt of Calcium hypochlorite.
It is sold in the market as a mixture of $Ca{(OH)_2}$ land $CaC{l_2}$ .
It can be prepared by passing $C{l_2}$ gas into der slaked lime i.e. $Ca{(OH)_2}$ . It is represented by the chemical formula- $CaOC{l_2}$ but the actual formula is very complex to remember.
$Ca{(OH)_2} + C{l_2} \to CaOC{l_2} + {H_2}O$
When allowed to stand for long, bleaching powder undergoes some chemical reactions which are as follows-
$6Ca(OCl)Cl \to Ca{(Cl{O_3})_2} + 5CaC{l_2}$
Also on reaction with catalyst-
$2Ca(OCl)Cl + {H_2}O \to 2CaC{l_2} + {O_2}$
On reacting with water it forms calcium hydroxide and chlorine gas evolves.
$Ca(OCl)Cl + {H_2}O \to Ca{(OH)_2} + C{l_2} \uparrow$
Here are some reactions which shows the oxidising nature of bleaching powder-
$Ca(OCl)Cl + {H_2}S \to S + {H_2}O + CaC{l_2}$
In this process sulphur is oxidised as the oxidation state of sulphur changed from $- 2$ to $0$ .
$Ca(OCl)Cl + 2KI + HCl \to KCl + {H_2}O + CaC{l_2} + {I_2}$
In the above reaction iodine is oxidised as the oxidation state of iodine changed from $- 1$ to $0$ and thus iodine gas evolved which is violet in colour.
It can be used-
a) For bleaching linen and cotton in the textile industry
b) For bleaching wood pulp in paper industries and also washed clothes in laundry
c) As an oxidising agent in industries and purifying drinking water from germs.
Bleaching powder has the smell of chlorine due to the presence of chlorine as the constituent. The compound has different salts and complexes which contain chlorine and thus on standing, smells of chlorine.
From the reaction: $Ca(OCl)Cl + {H_2}O \to Ca{(OH)_2} + C{l_2} \uparrow$
We can see that on dissolving in water, it forms calcium hydroxide which is not soluble in water. So when the heavier complexes present in bleaching powder dissolves only calcium hydroxide is left behind.

Note:
Sodium hypochlorite is called a household bleach as it is mostly used for the disinfection of households, hospitals, roads and also in water treatment plants to kill particular harmful germs. Bleaching powder can have severe effects on living organisms as the chlorine liberated can cause irritation in respiration. Also it can be cancer causing as well.