Chemical and Physical Changes

PHYSICAL AND CHEMICAL CHANGE

CHANGES AROUND US.

PHYSICAL CHANGE.

If we find any difference in shape, size, colour and state of a substance we say its a physical change. A change, in which only the physical properties of any substance get changed and no new substance is formed, is called a physical change.

EXAMPLES OF PHYSICAL CHANGE

Formation of Clouds Melting of Ice

Melting of ice

•Grinding of a substances.•Magnetisation of a piece of Iron.•Stretching of a rubber band.

Characteristics of Physical Changes During a physical change, only physical properties(such as size, shape, colour and state)changes.During a physical change, no new substances is formed.A physical change can be either reversible or irreversible(Dissolving sugar in water is a reversible change).No very small amount of energy is either absorbed or evolved during a physical change.

CHEMICAL CHANGE

A change in which composition and chemical properties of substance are changed and formed, is called Chemical change.

EXAMPLES OF CHEMICAL CHANGE

Burning of incense stick Burning of coal

Rusting of iron

•Spoilage of food•Digestion of food•Burning of dry leaves•Explosion of fireworks

Characteristics of Chemical Changes

During a chemical change, one or more new substance are formed.Chemical changes are permanent. Thus, a chemical change cannot be reversed by simple physical methods.During a chemical change, relatively larger amount of heat, light or any other radiation may be given out or absorbed.During a chemical change, both the physical and chemical properties of a substance are changed.

Difference Between Physical and Chemical Change

S.No. Parameters Physical change Chemical change

1 New substance No new substance is formed. One or more substance are formed.

2 Nature of change It is a temporary change. It is a permanent change.

3 Reversibility It is reversible by simple physical methods.

It is irreversible.

4 Physical and chemical properties

Only physical substance are changed.

Both physical and chemical properties of substance are

changed.5 Amount of energy(in

the form of heat, light, etc.)

Very little amount of energy is absorbed or given out.

A very large amount of energy is absorbed or given

out.

RUSTING OF IRON Iron objects like iron nails, iron pipes, iron grills and iron railings, when kept in moist air for a few days, get covered with powdery substance called rust. This process is called rusting of iron.The two conditions, necessary for the rusting of the iron are: Presence of oxygen(of air)Presence of water or water vapour(moisture)

PROCEDURE OF RUSTING Iron combines with the oxygen (of air) in the presence of water (or moisture) to form hydrated iron hydroxide.This hydrated iron hydroxide is called rust.

IRON(METAL)

(DAMPED AIR)

OXYGEN+WATER

HYDRATED IRON

OXIDE(RUST:RED-

BROWN)

Rust is soft and porous. The layer of rust formed falls of from the surface, exposing fresh iron which starts rusting. This continues rusting makes the iron weak and ultimately useless.Iron is the cheapest& most easily available metal. Rusting causes a great loss & damage. It is an irreversible chemical change

RUSTING DAMAGES IRON OBJECT

PREVENTION OF RUSTING

1.Coating the iron surface with paint or grease.2.Coating iron with other metals(Galvanisation):

The process of depositing a thin layer(coating) of zinc metal on iron object is called galvanisation.

3.By converting iron into stainless steal: Molten iron consist of carbon, chromium and nickel. On cooling, it forms an alloy called stainless steal. The stainless steal does not rust at all in moist air

CRYSTALLISATIONThe salt obtained from the sea water by the process of evaporation is not pure and its crystals are very small and shape cannot be seen clearly. Crystals are the purest solid form of a substance with a definite geometrical shape. Crystals of different substances have different shape. Large crystals of pure substances can be easily form their saturated solutions by the process of crystallisation. By obtaining crystals, we are able to purify certain substances.