Physical and Chemical Changes Class 7 Science Notes Chapter 6

Learn from School Connect Online in this chapter we will learn about Physical and Chemical Changes  as t here are several changes we come across daily e.g dissolving sugar in water, or flattening a metal rod by beating it. These involve changes in the form of the substance.Physical and Chemical Changes Class 7 Science Notes Chapter 6

The different topics covered in CBSE Class 7 Science Chapter 6 are tabulated below:

Ex 6.1Physical Changes
Ex 6.2Chemical Changes
Ex 6.3Rusting of Iron
Ex 6.4Crystallisation

Physical and Chemical Changes Class 7 Science Notes Chapter 6

Ex. 6.1 – Physical Changes

Physical vs. Chemical Changes

There are several changes we come across daily e.g 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:

(i). Physical

(ii). Chemical

Physical properties

Physical properties include 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.

Ex. 6.2 – Chemical Changes

Chemical change

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

Metallic Oxides

Formation of metal oxides are examples of chemical changes. They are formed by the reaction of oxygen in 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

  • Reaction of metal oxides with water form metal hydroxides.
  • 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

Ex. 6.3 – Rusting of Iron

  • 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, quicker the formation of rust.


  • 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.

Ex. 6.4 – 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.

CBSE Notes for Class 7 Science Free Download for All Chapters

CBSE Class 7 Science Study NotesCBSE Class 7 Science Study Notes
Nutrition in Plants Class 7 Notes Chapter 1Respiration in Organisms Class 7 Notes Chapter 10
Nutrition in Animals Class 7 Notes Chapter 2Transportation in Animals and Plants Class 7 Notes Chapter 11
Fibre to Fabric Class 7 Notes Chapter 3Reproduction in Plants Class 7 Notes Chapter 12
Heat Class 7 Notes Chapter 4Motion and Times Class 7 Notes Chapter 13
Acids,Bases and Salts Class 7 Notes Chapter 5Electric Current and its Effects Class 7 Notes Chapter 14
Physical and Chemical Changes Class 7 Notes Chapter 6Light Class 7 Notes Chapter 15
Weather,Climate and Adaptations of Animals to Climate Class 7 Notes Chapter 7Water;A precious resource Class 7 Notes Chapter 16
Winds Storms and Cyclones Class 7 Notes Chapter 8Forests;Our life line Class 7 Notes Chapter 17
Soil Class 7 Notes Chapter 9Waste water story Class 7 Notes Chapter 18

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