Physical & Chemical Changes Summary Class 7 NCERT Summary

Types of Changes

• Changes can broadly be classified into two types − physical and chemical.

Physical Change

• Properties such as shape, size, colour and state of a substance are called its physical properties.
• A physical change in which a substance undergoes a change in its physical properties is called a  physical change.

Chemical Change

• A change in which one or more new substances are formed is called a chemical change.
• A chemical change in which a substance undergoes a change in its chemical properties is called a chemical change.

Burning of Magnesium Ribbon

• Magnesium ribbon burns with a brilliant white and leaves behind a powdery ash when completely burnt.
  Magnesium (Mg) + Oxygen (O₂) → Magnesium oxide (MgO)

Dissolving magnesium ash in water

Magnesium oxide (MgO) + Water (H₂O) → Magnesium hydroxide [Mg(OH)₂]

Difference between Physical and Chemical Change

• Burning a candle is a combination of physical and chemical change.

Copper Sulphate Chemical Reaction

• When copper sulphate reacts with iron, the colour of the solution changes from blue to green due to the formation of iron sulphate.
• Copper Sulphate solution (blue) + Iron → Iron Sulphate solution (green) + Copper (brown deposit)

Rusting

• Rusting is an example of chemical change.
  Iron (Fe) + Oxygen (O₂, from the air) + water (H₂O) → rust (iron oxide Fe₂O₃)
• Presence of both air and water is essential for rusting to take place.
• Rusting can be prevented by cutting the contact of either air or water or both with iron.

Protection from Rusting

• The same can be done by greasing, oiling, painting, and galvanizing iron.
• The process of depositing a layer of zinc on iron is called galvanization.
• Rusting can also be checked by alloying iron with other elements. Stainless steel is an alloy of iron with carbon, chromium, nickel, and manganese.

Crystallization

• Crystallization is an example of physical change. The process of crystallization is used for purification of some substances.