Notes of Ch 6 Physical and Chemical Changes Class 7th Science

Study Material and Notes of Ch 6 Physical and Chemical Changes Class 7th Science

  • Types of changes
  • Physical change
  • Chemical change
  • Copper Sulphate Reaction
  • Rusting
  • Crystallization

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


Physical Change
Chemical Change
1. The chemical composition of a
substance does not change.
1. The chemical composition of a
substance changes.
2. Most changes are reversible. 2. Most changes are irreversible.
3. No new substances are formed. 3. No new substances are formed.
For example:
Ice → Water → Steam
 For example,
Paper → Ashes

→ 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.


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