Physical and Chemical Changes Class 7 Science Notes - Chapter 6

According to the CBSE Syllabus 2023-24, this chapter has been renumbered as Chapter 5.

Physical vs Chemical Changes

Introduction
There are several changes we come across daily, for example, dissolving sugar in water or flattening a metal rod by beating it. These involve changes in the form of the substance. Changes can be classified as follows:
(i) Physical
(ii) Chemical

Physical properties
Physical properties include the size, shape, colour and state (solid/liquid/gas) of a substance.

Physical change

• Any change to the physical properties of a substance is called a physical change.
• Physical changes are usually reversible as no new substance is formed. It is the same substance but with changed physical properties.

Chemical change

• A change in which one or more new substances are formed is called a chemical change.
• Usually, a chemical change involves a chemical reaction, which forms new products.
• Example: Rusting of Iron or burning wood.

To know the Difference between Physical and Chemical Change, visit here.

Metallic Oxides

The formation of metal oxides is an example of chemical changes. They are formed by the reaction of oxygen in the air.
– Burning of Magnesium ribbon:
– 2Mg + O₂ → 2MgO
– The product formed is the oxide of magnesium, which is in the form of ash. It does not look anything like the magnesium ribbon used for burning.

Reaction of metallic oxides with water

• The reaction of metal oxides with water forms metal hydroxides.
• For example, dissolving Magnesium oxide in water by stirring the ash very well with water.
• MgO + H₂O → Mg(OH)₂
• The product formed is basic in nature and turns red litmus paper → blue

Reaction between baking soda and vinegar

When a pinch of baking soda is added to vinegar, we hear a hissing sound and observe the formation of bubbles.

– Vinegar (Acetic Acid) + Baking Soda (Sodium bicarbonate)  → CO₂ (Carbon dioxide) + Other products
The carbon dioxide produced during the reaction of Vinegar and baking soda, when passed through lime water, gives calcium carbonate as follows:
– CO₂ + Ca(OH)₂ (lime water) → Calcium Carbonate (CaCO₃) + H₂O
– The calcium carbonate turns lime water milky.

Observations that indicate a chemical change

• Heat or light is absorbed or given out during a chemical reaction.
• Production of sound
• Production of gases or precipitates
• Production of smell
• A colour change may occur

Rusting

• When substances made of Iron are exposed to oxygen and moisture in the atmosphere, it forms a red layer, which is called rust.
• The formation of rust can be represented by the following reaction:
• 4Fe + 3O₂ → 2Fe₂O₃. The chemical formula for rust is Fe₂O₃.nH₂O. More the moisture in the air, the quicker the formation of rust.

For more information on the Corrosion of Metals, watch the below video

To know more about Rusting of Iron, visit here.

Galvanization
– The process of depositing zinc on the surface of Iron to prevent rusting is called galvanisation.
– Example: Iron water pipes are galvanised. Ships are made out of iron which is galvanised. Due to the presence of salts in seawater, the process of rusting is hastened. Hence ships need to replace their iron body every year.

Crystallisation
– The process of separation of salts from their solution is called crystallisation. It is a purification technique that purifies seawater or separates crystals from impure samples. It is a physical change.

To know more about Crystallisation, visit here.