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Class 10, Science, Chapter-3, Lecture-4, Corrosion (Notes)

CORROSION:  

The phenomenon of slow conversion of metal into its compounds by the action of air, water and atmospheric pollutants on the surface of the metal is called corrosion of metal.

RUSTING:

The phenomenon of slow conversion of iron into hydrated iron oxide or iron hydroxide by the action of air, water and atmospheric pollutants on the surface of iron is called rusting.

RUST:  

A mixture of hydrated iron oxide and iron hydroxide formed by the action of air, water and atmospheric pollutants on the surface of iron is called rust

                                     ${\rm{Rust }} = {\rm{ F}}{{\rm{e}}_2}{{\rm{O}}_3} \cdot x{\rm{ }}{{\rm{H}}_2}{\rm{O  }} + {\rm{  Fe(OH}}{{\rm{)}}_3}$.

CONDITIONS NECESSARY FOR RUSTING:

1. Presence of air.
2. Presence of moisture.

EXPERIMENT to Show That Air and Water both are Necessary for Rusting:

Three test tubes containing water, oil, nail to show air and moisture are both necessary for rusting

Procedure:

  1. Clean iron nails are put in three test tubes labelled A, B, and C.
  2. Test tube A is half filled with water.
  3. Test tube B is filled with pure boiled distilled water to completely submerge the nails and a little amount of oil is poured to form a layer of oil to prevent air from entering.
  4. Anhydrous calcium chloride is put in test tube C to absorb the moisture.
  5. Mouth of each tube is closed by corks.

Observation:  

Rusting takes place in the nails in the test tube A only.

Conclusion:

Air and water (moisture) both are necessary conditions for rusting to take place.

PREVENTION FROM CORROSION:  

Corrosion of metals can be prevented by

  1. Painting
  2. Oiling
  3. Greasing
  4. Galvanising
  5. Chrome plating
  6. Alloying

ALLOY:

A homogeneous mixture of metal with another metal or non-metal is called an alloy.

AMALGAM:

 A homogeneous mixture of mercury with other metals or non-metals is called an amalgam.

PREPARATION OF ALLOY:

Steps involved:

  1. The main metal is melted.
  2. Other elements are dissolved in a definite proportion in the molten metal.
  3. The mixture is then cooled down to room temperature.

REASON FOR MAKING ALLOYS

Alloys are made to change the properties of the metal in desired way.

EXAMPLES:

  1. Pure iron is very soft, but when a small amount of carbon is mixed, it becomes hard and strong.
  2. Pure gold is unsuitable for making jewellery due to its softness. So, it is hardened by mixing a small amount of silver or copper.

DIFFERENCES BETWEEN:

Properties

Ionic Compounds

Covalent Compounds

Nature of bond

Formed by transfer of electrons

Formed by sharing of electrons

Constituent elements

Formed by reaction of metals with non-metals.

Formed by reaction of non-metals with non-metals.

Solubility

Are more soluble in water

Are less soluble in water

Conductivity

Are good conductors of electricity

Are bad conductors of electricity

Shape

Have definite shape

Do not have a definite shape

Melting/Boiling Point

Have high melting and boiling point

Have low melting and boiling point.