Topic outline

    • Changes Around Us

      Q15. A bag of cement lying outside in the open got wet due to heavy rain. Can we reverse the change occurred to the cement by placing it under the sun?

      Ans. No, the change cannot be reversed as it is an irreversible change.


      Q16. How iron articles of different shapes and sizes are made?

      Ans. A piece of iron is heated till it becomes red and hot and then it becomes soft and is beaten into desired shape and size.


      Q17. How setting of curd an irreversible change?

      Ans. Setting of curd is an irreversible change because new substance (curd) formed cannot be converted back into milk again.


      Q18. Is burning of incense stick an irreversible change?

      Ans. Burning of incense stick is an irreversible change because when incense stick are burnt, new substances are produced in the form of ash and smoke which cannot be converted back into incense stick again.

      Q20. Why a candle reduces in size on burning? What kind of change it is?

      Ans. A candle reduces in size on burning because wax near the wick melts into a liquid and changes into ash and fumes. It is an irreversible change as new substance formed cannot be converted back to candle again.


      Q21. Why is metal rim heated before fixing on to wooden cart wheel?

      Ans. Metals expand on heating and contract on cooling. Iron rims are made slightly smaller than the wheels. Rim is heated so that it expands and lies easily on the wheels. The same rim, on cooling contracts and get firmly fixed on the wheels.


      Q22. Differentiate between slow and fast change.


      Slow Change

      Fast Change

      1. The changes that take place in long period of time are called Slow change.

      1. The changes that take place in short period of time are called Fast change.

      2. Examples: germination of seed, curdling of milk, mountain formation, rusting of iron etc.

      2. Examples: burning of paper, burning of candle, thunderstorm, bursting of crackers etc.

      Q23. What kind of change is the formation of clouds?

      Ans. Formation of clouds is a physical change because water evaporates by the process of evaporation and goes into air and form clouds by the process of condensation. This water again comes back to the earth by precipitation. This process is repeated again and again.


      Q24. How is frying a puri different from rolling a puri?


      Frying a puri

      Rolling a puri

      1. It is an Irreversible change.

      1. It is reversible change.

      2. Taste and color will change.

      2. Shape and size will change.


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