NCERT Class VII Science Solutions: Chapter 6-Physical and Chemical Changes Part 3

Glide to success with Doorsteptutor material for IEO : fully solved questions with step-by-step explanation- practice your way to success.

Question 5:

When a candle burns, both physical and chemical changes take place. Identify these changes. Give another example of a familiar process in which both the chemical and physical changes take place.

Answer:

Physical Changes: On heating, candle wax gets melted is a physical change. Since it again turns into solid wax on cooling.

Chemical Changes: The wax near to flame burns and gives new substances like Carbon Dioxide, Carbon soot, water vapour, heat and light.

Cooking of food, is another example where both physical and chemical changes occurs.

As showing in images is a physical and chemical changes

Physical and Chemical Changes

Loading image

Question 6:

How would you show that setting of curd is a chemical change?

Answer:

Setting of curd is a chemical change since a new substance (lactic acid) is formed which differs in taste from the original substance (fats and proteins) i.e. milk. Also the change is irreversible (curd cannot be converted into milk).

1. In a chemical change, a new substance is formed. Here, curd is the new substance obtained from milk.

2. Both milk and curd have different properties.

3. Milk cannot be re-obtained from curd.

Question 7:

Explain why burning of wood and cutting it into small pieces are considered as two different types of changes.

Answer:

Burning of wood is a chemical change because wood on burning converted to new substances like ash (carbon), carbon dioxide gas, water vapour, heat and light. The change is irreversible.

While cutting of wood in smaller pieces is a physical change because the original composition of wood does not change. No new substances are formed.

As showing in images is a wood and cutting it into small pie …

Cutting Wood into Small Pieces and Burning It

Loading image

Question 8:

Describe how crystals of copper sulphate are prepared.

Answer:

Crystals of copper sulphate are prepared as follows:

  • Take a water in a beaker and add a few drops of dilute sulphuric acid.

  • Heat the water. When it starts boiling add copper sulphate powder slowly with continuous stirring.

  • Continue adding copper sulphate powder till saturation level.

  • Filter the solution and left for cooling.

  • Leave undisturbed the solution when it is cooling.

  • We can observe the crystals of copper sulphate after few hours.

As showing in images is a result for copper sulphate

Result for Copper Sulphate

Loading image