Physical and Chemical Change

Chemistry class 7th

Physical Change

A change where the reactants undergoes changes only in its physical properties such as shape, size, colour and state.

Chemical Reaction

A reaction where the reactants react chemically to form new substances with different chemical properties.

Reactants

Original substances participating in a chemical reaction.

Products

The new substances formed during or after the chemical reaction.

Rust

A substance formed when iron is left exposed to moist air.

Rust II

It is known as iron oxide(Fe2O3)

Rust formation

4Fe+3O2+H2O———————2Fe2O3•H2O

The effect of rust

It corrodes the iron

Prevention of rusting

It can be prevented by not allowing the iron to come in contact with moisture and air.

Methods of prevention of rusting

1) Painting the surface

2) Greasing the surface

3) Galvanization

Galvanization

The process of transferring a layer of zinc on the surface of iron

Types of characteristics of chemical reactions

1) Evolution of a gas

2) Change of colour

3) Formation of precipitate

4) Changes in energy

5) Change of state

Evolution of a gas

A gas may evolve during a chemical reaction

Effervescence

Bubbling of a gas during a chemical reaction

An arrow pointing upwards

The sign of the evolution of a gas

Reactants: Calcium carbonate and Hydrochloric acid

Products: Carbon Dioxide, Calcium Chloride and Water

Limewater turns milky

Reason: Presence of Carbon Dioxide

Reactants: Limewater(Calcium hydroxide) and Carbon Dioxide

Products: Calcium carbonate and water

Calcium carbonate

A substance which is white in colour and not soluble in water

Reactants: Zinc and Sulphuric acid

Products: Zinc sulphate and Hydrogen

To test hydrogen

Put a burning matchstick near the hydrogen and hydrogen will burn making a pop sound