Changes around us (part -2) class 6

Changes around us

Q.5. Give some examples of reversible and irreversible changes involving in same materials.
Ans. 
S.No.
Example
Reversible
Irreversible
1.
Folding a paper and cutting the paper
Folding a paper
Cutting the paper
2.
Rolling and baking of roti
Rolling of roti
Baking of roti
3.
Shaping wet clay into clay pot and baking a clay pot
Shaping wet clay into clay pot
Baking a clay pot
4.
Inflating and bursting a balloon
Inflating a balloon
Bursting a balloon
5.
Melting of wax and burning of wax
Melting of wax
Burning of wax


Q.6. What do we mean by terms 'Expansion' and 'Contraction'? 
Ans. The increase in size on heating is called expansion while the decrease in size on cooling is called contraction. For example:
  • In thermometers, mercury(which is a liquid metal at room temperature) is used. When the bulb of the thermometer is kept in hot objects, the mercury expands and its level increases in glass tube. Similarly, when the bulb of the thermometer is kept in cold objects, the mercury contracts and its level decreases in glass tube.
  • The gaps are left between railway tracks for smooth expansion and contraction, other railway tracks can deform and disbalance the train.
  • The iron blade in digging tools is fixed to wooden handle through expansion and contraction of iron rings of blade.
Q.7. Why are tools often heated before fixing wooden handles?
Ans. Normally, the metal ring of tool is made slightly smaller in size than the wooden handle. To fix the handle, the ring is heated and it becomes slightly larger in size (expands). Now, the handle easily fits into the ring. When the ring cools down it contracts and fits tightly on to the handle.

Q.8. How does the metal rim made to fix on a  wooden wheel of a cart?
Ans. The metal rim is made slightly smaller than the wooden wheel. On heating, the rim expands and fits onto the wheel. Cold water is then poured over the rim, which contracts and fits tightly onto the wheel.

Q.9. What are physical changes? Give some examples.
Ans. The properties of substances like volume(capacity), state (solid, liquid and gas), temperature, shape, size, colour, weight, textures are called Physical properties.
The changes in physical properties are called Physical changes. For example:
  • Breaking of glass
  • Inflating a balloon
  • Melting of wax
  • Rolling of dough
  • Cutting of fruits, paper, wood etc.
  • Sharpening of pencil
  • Mixing of salt or sugar in water
  • Stretching of rubber band
  • Plucking flowers
  • Expansion of metals on heating
  • Evaporation of water from ponds
Q.9. What are chemical changes? Give some examples.
Ans. A change in which a new substance is formed that has entirely different composition from the original substance is called chemical change. For example:
  • Changing of milk into curd
  • Fresh vegetable gets rotten after few days
  • burning of paper, wax or fuel
  • digestion 
  • photosynthesis
  • ripening of fruits
  • blooming of flowers
  • explosion of fire cracks
  • cooking of food
  • rusting of iron
  • mixing cement and water

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