Section outline

    • Give Reasons for the following:
      Q1. Why should we not to play with fireworks?

      Ans:

      We should not play with fireworks because:

      • Explosion of fireworks is a chemical change.
      • It produces heat, light, sound & polluting gases.
      Q2. Why does an apple turn brown when cut?

      Ans:

      • When apple is cut and kept for some time, it reacts with oxygen present in the air.
      • Due to this chemical reaction, a new substance is formed that is brown in colour.
      • This oxidation of apple is a chemical change.
      Q3. Why is the iron pillar near Qutub Minar special?

      Ans:

      • The iron pillar near Qutub Minar has not rusted for over 1600 years.
      • It shows advanced metal technology from ancient India.
      • For its rust resistance quality, it has been examined by scientists across the world. 
      Q4. Which type of change is shredding of paper & why?

      Ans:

      • Shredding of paper is a physical irreversible change.
      • This is because no new substance is formed.
      • There is change only in physical properties like the size & shape of the paper.
      • It is irreversible as the paper cannot be restored back to its original size or shape.
      Q5. Why does blue copper sulphate solution change into green when a iron nail is placed in it?
      Ans:

      The blue copper sulphate solution changes into green when a iron nail is placed in it because:

      • Iron is more reactive that copper.
      • Hence iron displaces copper forming iron sulphate which is green in colour.
      • This is a chemical change which changes the solution green and copper gets deposited on iron nail.
      • Chemical equation:

      Copper Sulphate [CuSO4] (blue solution) + Iron (Fe) → Iron Sulphate [FeSO4] (green solution) + Copper (Cu) (brown deposit)

      Q6. When you dissolve magnesium ash is water it turns red litmus blue, why?

      Ans:

      • Magnesium reacts with oxygen to form white ash which is magnesium oxide.
      • Magnesium oxide is a basic in nature.
      • When it mixes with water it forms a new substance magnesium hydroxide.
      • The solution is basic in nature, so it turns red litmus paper to blue.
      • Chemical equation:

      Magnesium (Mg) + oxygen (O2) → Magnesium oxide (MgO)

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

      Q7. Is breaking down of ozone a chemical change? Explain with reason.

      Ans:

      • Yes, breaking down of ozone a chemical change.
      • Ozone absorbs the harmful ultraviolet radiation coming from the sun and breaks into oxygen (new substance).
      • The chemical composition of ozone (O3) and oxygen (O2) is different.
      • Thus, as a new substance is formed and the reaction is irreversible, it is a chemical change.
      Q8. Why does stainless steel not rust?

      Ans:

      • Stainless steel is made by mixing iron with carbon and metals like chromium, nickel and manganese.
      • This prevents it from rusting. 

       

    • Answer the following in short:
      Q1. What is a chemical change? Give two examples.

      Ans:

      • A chemical change is a change in which one or more new substances are formed.
      • It is a irreversible chnage.
      • Examples: rusting of iron and burning of wood.
      Q2. What is a physical change? Give two examples.

      Ans:

      • A physical change is a change in the physical properties of a substance.
      • No new substances are formed in this change.
      • Examples: Melting ice & cutting vegetables
      Q3. What is crystallization & what type of change it is?

      Ans:

      • Crystallization is a process to obtain large crystals of pure substances from solutions.
      • It involves forming solid crystals from a liquid.
      • This is a physical change.

       

    • Answer the following:
      Q1. What is the effect of cooking oil on rusting?

      Ans:

      • Rusting is the process in which iron when exposed to moisture & air reacts with oxygen & water to form an brownish film of iron oxide called as rust.
      • This is harmful for the iron metal.
      • As oil and water don't mix cooking oil can form a film over water.
      • This acts as a protective barrier & prevents moisture from coming in contact with the metal surface.
      • This can slow down the process of rusting or prevent it entirely.
      Q2. How is ozone a boon for us?

      Ans:

      • Ozone absorbs the harmful ultraviolet radiation coming from the sun and breaks down to oxygen.
      • As oxygen is different from ozone, it is a chemical change.
      • If ultraviolet radiation were not absorbed by ozone, it would reach the earth’s surface.
      • This could cause harm to us and other life forms.
      • Thus, ozone layer acts as a natural shield & protects us from the harmful ultraviolet radiation.