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Bleaching Powder (CaOCl2)

Bleaching powder is calcium oxychloride. The chemical formula of bleaching powder is CaOCl2. It is also called chloride of lime.

Manufacture of Bleaching Powder

Bleaching powder is manufactured (prepared on a large scale) by passing chlorine gas over dry slaked lime:

The manufacture of bleaching powder can be done by using two types of manufacturing plants: Hasenclever plant and Bachmann plant. We will describe the manufacture of bleaching powder by using Hasenclever’s plant.

The Hasenclever plant for manufacturing bleaching powder consists of a number of hollow horizontal cylinders made of cast iron which are fitted with shafts and connected to one another. The shafts have a large number of blades fixed along their one end to the motor. The uppermost cylinder has a hopper as its top. This hopper is for putting in slaked lime. It has also an outlet for the waste gases to go out. The lowermost cylinder has an inlet for passing in chlorine gas (see Figure). It has also an outlet for the powder formed to come out.


We will now describe the working of the Haseclever plant. Dry slaked lime is put into the hopper. From the hopper, slaked lime slowly drops into the uppermost cylinder (See figure). This slaked lime is moved forward by the revolving blades of the rotating shaft. In this way, the slaked lime moves slowly from the upper cylinders to the lower cylinders by the motion of the revolving blades of the rotating shafts. Chlorine gas is passed in through the lowermost cylinder. This chlorine gas gradually moves up from the lowermost cylinder to the upper cylinders. As the slaked lime moves down from one cylinder to another, it meets the upcoming chlorine gas. The slaked lime reacts with chlorine gas to form bleaching powder. By the time slaked lime reaches the end of the lowermost cylinder, it gets thoroughly saturated with chlorine gas and gets converted completely into bleaching powder. This bleaching powder comes out through the outlet at the bottom of the lowermost cylinder and gets collected in a vessel.

Properties of Bleaching Powder

  1. Bleaching powder is a yellowish white powder which gives a strong smell of chlorine.
  2. Bleaching powder is soluble in cold water. The small insoluble portion always left behind is the lime present in it.
    Bleaching powder reacts with carbon dioxide and produce chlorine gas. This is discussed below:
    Bleaching powder always smells of chlorine because the carbon dioxide present in air decomposes it slowly to produce chlorine gas.
    When bleaching powder is treated with an excess of a dilute acid, all the chlorine present in it is liberated. The chlorine, produced by the action of a dilute acid on bleaching powder, acts as a bleaching agent (a substance which decolourises coloured substances is called a bleaching agent). The bleaching action of chlorine is due to its oxidizing property. The reactions of bleaching powder with dilute sulphuric acid and dilute hydrochloric acid are given below:
  3. Reaction with dilute sulphuric acid: When bleaching powder is treated with an excess of dilute sulphuric acid, all the chlorine present in it is liberated:
    The amount of chlorine thus set free is called "available chlorine" . Greater the amount of available chlorine, more superior the bleaching powder will be (because then it will have more bleaching power). Bleaching powder is, therefore, priced and sold according to the amount of available chlorine in it. A good sample of bleaching powder contains 35 to 38 per cent of available chlorine. It should be clear by now that bleaching powder is an arrangement for storing chlorine.
  4. Reaction with Dilute Hydrochloric Acid: When bleaching powder is treated with an excess of dilute hydrochloric acid, all the chlorine present in it is liberated:

Uses of Bleaching Powder

  1. Bleaching powder is used for bleaching cotton and linen in textile industry and for bleaching wood pulp in paper industry. It is also used for bleaching washed clothes in laundry (Laundry is a place where clothes are washed and pressed). The bleaching action of bleaching powder is due to the chlorine released by it.
  2. Bleaching powder is used for disinfecting drinking water supply. That is, for making drinking water free from germs.
  3. Bleaching powder is used for the manufacture of chloroform (CHCl3).
  4. Bleaching powder is used as an oxidizing agent in many chemical industries.
Let us solve one problem now.
Sample Problem

How will you obtain bleaching powder from lime-stone? Write the equations of chemical reactions involved.


First of all we will have to convert lime-stone into lime, then lime is converted into slaked-lime, and finally slaked-lime is converted into bleaching powder.

  1. Limestone is heated strongly to obtain lime:
  2. Lime is then reacted with water to obtain slaked-lime.
  3. Dry slaked lime reacts with chlorine gas to obtain bleaching powder:
In this way bleaching powder can be prepared from lime-stone.

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