FSc Notes Part 2 Chemistry Important Chemicals Glass, Silvering Of Mirror, Bleaching Powder and Corrosion

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Glass

Definition

Glass is a hard material that is usually transparent made by cooling certain molten materials in such a manner that they do not crystallize but remain in amorphous state.

Glass is considered to be a super cooled liquid, i.e. the solid in which the molecules are present as aggregates as in liquid, and are not present in any definite pattern.

Preparation of Glass

The principle ingredients of all types of glass are sand or silica (SiO2). Therefore, glass is one of the most important artificial silicates. For example, the ordinary soft glass or Soda Glass is a mixture of Sodium Silicate (Na2SiO3) and Calsium Silicate (CaSiO3).

Soda glass is manufactured by heating sand (SiO2), sodium carbonate (Na2CO3) and calsium carbonate (CaCO3) in a furnace at high temperature, i.e. 1400ºC

Types of Glass

Glass is classified in a number of ways on the basis of its chemical composition, properties, manufacturing process or it’s use. Some important types of glass are as follows.

1. Soft Glass or Soda Glass

Soft glass or Soda glass is an ordinary glass which is a mixture of sodium silicate and sodium calcium silicate. it is also known as window glass.

2. Refractory Potassium Glass

It is a mixture of potassium silicate and calcium silicate. It has high refractive index. This glass is used for making prism, lenses and decorative glass wear.

3. Pyrex Glass

It is boro silicated glass. The main constituents of Pyrex Glass are boroxide (B2O3) and silica (SiO2). This glass has no chemical durability and is soluble even in water.

4. Water Glass

Water glass is just a sodium silicate, which is prepared by the reaction of sodium oxide (Na2O) and silica (SiO2). This glass has no chemical durability and is soluble even in water.

5. Coloured Glass

Coloured glass is prepared by adding certain transition metal oxides. For example copper oxide (CuO) gives light blue coloured glass, where as cobalt oxide (CoO) gives dark blue colour, chromium oxide (Cr2O3) gives green colour and zinc oxide (ZnO) give red coloured glass.

6. Photochromic Glass or Photosensitive Glass

Photochromic glass produces darkness on exposure to bright sunlight but becomes clear again in absence of light. This glass contains silver chloride or silver bromide salts which is sensitive to light, in presence of light, the salt is decomposed to give finely divided black silver, in absence of sunlight, silver and chloride recombine to reform AgCl. This glass is used in sunglasses.

7. Optical Fibres

Optical fibres are thin fibres of silica glass of high purity. They have excellent optical transparency. This glass is used to transmit T.V Programs, Telephone conversion, Computer output etc. It is also used to make a design on glass. This process is called Etching

Silvering Of Mirror

Mirror

A mirror is a glass plate coated with silver film on one side.

Silvering of Mirror

Coating a glass plate with silver is called silvering of mirror.

Explanation

When ammonical silver nitrate solution is treated with an aldehyde or other organic reducing agent on the surface of a glass plate the silver (I) is reduced to silver (0). This metallic silver is deposited on the glass plate as a fine film.

The chemical reactions that occur are as follows.

AgNO3 + 2NH4OH —-> [Ag(NH3)2]+ + NO3- + 2H2O

[Ag(NH3)2]+ + RCHO + H2O —-> Ag0↓ + RCOOH + 2NH4+

Method

The process of silvering of mirror is carried out through the following steps.

1. A solution of silver nitrate and ammonia is prepared in distilled water.
2. The aqueous solution of ammonia is slowly added to the solution of silver nitrate until brown precipitates of silver oxide form and dissolve.
3. The mixture is ammonical silver nitrate solution. This solution is mixed with a solution of an organic reducing agent such as glucose.
4. After mixing all the compounds thoroughly, the solution is poured on to the centre of a clean glass surface.
5. The reaction immediately starts and thin film metallic silver deposited on the surface of the glass.
6. The silver film is then coated with either shellae of copal varnish. Finally it is painted with some colour, normally red.

Spraying Method

Silvering of mirror can also be carried out by spraying method. In this technique, the mixture of chemicals is sprayed onto the glass sheet.

Bleaching Powder

Definition

Bleaching powder is a white amorphous powder with smell of chlorine.

Formula of Bleaching Powder

On the basis of available percentage of chlorine, the formula of bleaching powder was suggested by Professor Odling. The formula is given by

Ca(OCl)Cl or CaOCl2

Preparation of Bleaching Powder

Bleaching powder is prepared on the large scale by Hasen Clever Process. The plant consist of a number of iron cylinders in which chlorine is brought in contact with slaked lime [Ca (OH)2].

Ca(OH)2 + Cl2 —-> CaOCl2 + H2O

Chemical Reactions of Bleaching Powder

1. In aqueous solution bleaching powder liberates chlorine.

CaOCl2 + H2O —-> Ca(OH)2 + Cl2

2. It reacts with acids to set free chlorine.

CaOCl2 + 2HCl —-> CaCl2 + H2O + Cl2

3. It reacts with atmospheric CO2 and moisture to give following reaction.

2CaOCl2 + CO2 + H2O —-> CaCO3 + CaCl2 + 2HOCl

Uses of Bleaching Powder

1. It is used for sterilization of drinking water.
2. It is used for bleaching of cotton, linen and paper pulp.
3. It is used for the preparation of Cl2 gas and chloroform (CHCl3).

Corrosion

Definition

Corrosion is the deterioration of a metal, as a result of its reactions with the environment or any chemical agent.

It is an oxidation process that occurs at the surface of the metal.

Causes of Corrosion
Corrosion may be regarded as the natural tendency of metals to return to their oxidized state. The main causes of corrosion are as follows.

1. The atmosphere
2. Submersion in water
3. Underground Soil Attack
4. Emersion in chemicals
5. Corrosive gases

The most important of these is the atmosphere.

Examples of Corrosion

Some familiar examples of corrosion are as follows.

1. Rusting of Iron
2. Detarnishing of silver
3. Development of green coating on copper, brass and bronze.

Types of Corrosion

There are two types of corrosion.

1. Atmospheric Corrosion

When corrosion in metal is due to the action of atmosphere, it is known as atmospheric corrosion.

2. Corrosion in Liquid

When corrosion in metal is due to the reaction of a liquid on a metal, it is known as corrosion in liquid.

Rate of Corrosion

Corrosion is a chemical process. Different metals corrode at different rates. Gold does not corrode at all. Iron corrodes slowly. Tin, lead, copper and silver corrode very slowly.

Prevention From Corrosion

Corrosion causes great damages to metallic articles such as bridges, ships and vehicles. It has been found out that due to corrosion, one fifth of iron is lost annually. Therefore, any one of the following methods are adopted to prevent corrosion.

1. Protective Metallic Coating

In this method, the metal is coated with a thin layer of less oxidizing metal, which reduces the rate of corrosion. For example, corrosion of iron is prevented by coating it with zinc, tin or chromium.

2. Non-Metallic Material Coating

In this method, the metal is coated with a thin layer of non metallic material such as paints, oils, grease, plastic emulsion, enamels etc. For example, red lead (Pb3O4) and zinc chromate (ZnCrO4) are often used for this purpose.

3. Alloying of Metals

Metals can be made more resistant against corrosion by making their alloys. For example, stainless steal is an alloy of Fe, Cr and Ni.

4. Electro-Plating

Noble and bare metals are used for Electro-Plating on any desired metal.