Physical and Chemical Changes science(chap-6)

                                                      Physical and Chemical Changes               science(chap-6)

Ø  Every day we come across many changes in our surroundings, such as, souring of milk, rusting of iron, melting of ice, breaking of glass etc.

Ø   Change of substances are broadly  divided into Physical and Chemical Changes. These changes may involve one or more substances.   

Ø  Properties such as shape, size, colour and state of a substance are called physical properties.

Ø  A change in which a substance undergoes a change in its physical properties is called a Physical Change. It is generally reversible.

Ø  As Physical changes are reversible, no new substance is formed.

Ø  Change of water into solid to liquid, or from gas to liquid, the process of tempering steel to form a knife blade, salt dissolved in water etc. are few examples of Physical changes.

Ø  In physical changes, changes affect the form of a chemical substance, but not its chemical composition.

Ø  A change in which one or more substances are formed is called a Chemical change. It is occurred when the substance combines with another reaction to form a new substance called chemical decomposition into two or more different substances.

Ø   Digestion of food in our body, ripening of fruits, fermentation of grapes, burning of wood, souring of milk, baking of cake, rusting of iron etc are few examples of  Chemical changes  which can not be reversible.

Ø  If we put a wet piece of iron in an open place for some time, it acquires a film of brownish substance, called rust. The process of becoming rust is called rusting.

Ø  For rusting, the presence of both oxygen and water or water vapour is essential. The process of rusting can be represented by the following equation;

               Iron(Fe)+ Oxygen(O2  from the air) + water(H2O)------Rust (Iron oxide Fe2O3)

Ø  When the content of moisture or humidity in air is high, rusting becomes faster.

The way we can prevent Iron from rusting are: (i)prevent iron articles from coming in contact with oxygen, or water or both. (ii)apply a coat of paint or grease unto it regularly. (iii)deposit a layer of a metal like chromium or zinc on iron.

Ø  Chemical reaction is a process in which one or more substances(reactants) are converted to one or more different substances(products). Medicines are the end product of a chain of chemical reaction.

Ø  The Ozone layer in our atmosphere protects us from the harmful Ultraviolet radiation coming from sun. Ozone absorbs this radiation and breaks down to oxygen. The breaking down of ozone is a chemical change.

Ø  If ultraviolet radiation were not absorbed by ozone, it would reach the earth’s surface and can cause harm to us and other life forms. So, ozone is known as natural shield against sun radiation.

Ø  The process of depositing a layer of zinc on iron is called galvanization.

Ø  The iron pipes we use in our homes to carry water are galvanized to prevent rusting.

Ø  The salt water makes the process of rust formation faster. So, the fraction part of ship’s iron is replaced each year.

Ø  Stainless steels do not rust because stainless steels are made by mixing iron with carbon and metals like chromium, nickel and manganese.

Ø  The salt obtained by the process of evaporation is not pure and the shape of its crystals can not be seen clearly. However, large crystals of pure substances can be formed from their solutions by the process of crystallization.

Ø  Crystallization is an example of chemical change.