Physical and Chemical Changes CBSE Class 7 Science Revision Notes Chapter 6

 

Physical Changes

• Physical Properties of a substance are those characteristic of a substance that describes its physical nature.

• For Example colour, density, shape, size and volume are some physical properties.

• Physical Change is a change which occurs when there is an alteration in the physical properties of a substance.

• The physical change does not result in the formation of any new substance but can alter the shape and size of the existing substance.

Figure 1: Physical Properties

Figure 2: Physical Changes

Chemical Changes

• The chemical property of a substance are those characteristic of a substance that describes its chemical nature.

• For Example toxicity of a substance or how a substance reacts with other substances is its chemical property.

• A chemical change or chemical reaction is any change in the chemical properties of a substance.

• Whenever a substance undergoes a chemical change, a new substance is formed.

• Examples of Chemical Changes:

  • Rusting of iron occurs when iron gets in contact with moisture

  • When a magnesium ribbon is burnt it radiates white light and converts into ashes

  • Formation of Medicines

  • Extracting of iron from the iron ore

  • Formation of plastic

Figure 3: Chemical Properties

A chemical change is always accompanied by any one or all of the following way:

• Radiation or absorption of heat

• Production of sound

• Change in the colour of the substance

• Change in the smell of the substance

• Formation of a gas

• Formation of a solid as residue

Change in Property	Example
Radiation or absorption of heat	Burning of a substance such as coal, wood or candle results in production of heat and hence is a chemical change. Similarly, melting of ice results in absorption of heat and therefore it is a chemical change.
Production of sound	Bursting of fireworks is a chemical change. It results in production of sound, heat, radiation as well as gas.
Change in the colour of the substance	Raw fruits and vegetables when cut and left in open air start acquiring brown color due to a chemical change. Similarly, rusting of iron results in change of color of iron to reddish brown is also a chemical change.
Formation of a gas	When antacids are mixed in water bubbles are formed indicating the production of a gas and therefore it is a chemical change.
Change in the smell of the substance	When food gets spoiled, it produces foul smell. This is because of a chemical change in food. Rotten eggs often produce a bad odour due to production of sulphur.
Formation of Solids	Two liquids combine with each other and form a solid called precipitate. For Example, shells of animals are precipitates formed by chemical changes.

Figure 4: Chemical Changes

Rusting of Iron

When the iron comes in contact with oxygen and water, reacts and forms a red colored substance over it. It is called Rust.

Figure 5: Formation of Rust

How to prevent rusting of iron

• By applying paint on iron objects so that they cannot come in contact with oxygen and moisture in the environment

• Galvanization of iron which means applying a layer of zinc or chromium metals on the iron

Crystallization

It is a process of obtaining crystals of a pure substance from its solution. For Example, we can obtain the crystals of copper sulphate by boiling copper sulphate solution.

Figure 7: Crystallization of Copper Sulphate