Tuesday, 21 June 2011

Physical and Chemical Changes: Lesson 6. CBSE NCERT Class VII (7th) Science Exercises Solution

Exercises : Questions & Answers
1.Question :  Classify the changes involved in the following processes as physical or chemical changes:
(a) Photosynthesis
(b) Dissolving sugar in water
(c) Burning of coal
(d) Melting of wax
(e) Beating aluminium to make aluminium foil
(f ) Digestion of food
Answer :
Physical Changes
  • Dissolving sugar in water
  • Melting of wax
  • Beating aluminium to make aluminium foil

Chemical Change
  • Photosynthesis
  • Burning of coal
  • Digestion of food

2.Question :  State whether the following statements are true or false. In case a statement is false, write the corrected statement in your notebook.
(a) Cutting a log of wood into pieces is a chemical change. (True/False)
(b) Formation of manure from leaves is a physical change. (True/False)
(c) Iron pipes coated with zinc do not get rusted easily. (True/False)
(d) Iron and rust are the same substances. (True/False)
(e) Condensation of steam is not a chemical change. (True/False)
Answer :
(a) Cutting a log of wood into pieces is a chemical change. (False)
Correct Statement : Cutting a log of wood into pieces is a physical change as it does not involve change of chemical properties.

(b) Formation of manure from leaves is a physical change. (False)
Correct statement : Formation of manure from leaves is a chemical change as it has different chemical properties and  can not be reversed.

(c) Iron pipes coated with zinc do not get rusted easily. (True)

(d) Iron and rust are the same substances. (False)
Correct statement : Iron and rust are two different substance.

(e) Condensation of steam is not a chemical change. (True)


3. question :  Fill in the blanks in the following statements:
(a) When carbon dioxide is passed through lime water, it turns milky due to the formation of calcium carbonate.(b) The chemical name of baking soda is Sodium hydrogencarbonate.
(c) Two methods by which rusting of iron can be prevented are coating of paint or grease and galvanisation with chromium or zinc.
(d) Changes in which only physical properties of a substance change are called physical changes.
(e) Changes in which new substances are formed are called chemical changes.

4.Question :  When baking soda is mixed with lemon juice, bubbles are formed with the evolution of a gas. What type of change is it? Explain.
Answer : When baking soda is mixed with lemon juice, bubbles are formed with the evolution of a Carbon Dioxide. This is a chemical change as citric acid present in lemon juice combines with baking soda (Sodium hydrogencarbonate) to form sodium citrate, carbob oxide and water. This can be shown as follows :

C6H8O7(aq) + 3NaHCO3(aq) ---> Na2C6H6O7(aq) + 3CO2(g) + 3H2O(l)
 Citric Acid
+ Baking Soda  ---->> Sodium citrate+ Carbon Dioxide + Water

5. Question. 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.
Physical changes in candle burning .
  1. The Wax near the wick melts from solid to liquid state due to Heat.
  2. The melted liquid wax as it flow down, turns back into solid wax again on cooling.
Chemical Changes in Candle burning : 
As the wax, near the burning wick, melts and vaporises, the  vapours (Hydrocarbon) of wax combines with surrounding oxygen, burn to form heat and  candle light. In this burning process wax (Hydrocarbon)  together with oxygen gets converted into new substance like carbon dioxide (CO2) , carbon mono oxide( CO), Carbon (C) soot, and water vapour (H2O)  vapor along with generation of heat and light

Cooking of food on gas stove is an another example  of a familiar process in which both the chemical and physical changes take place.

6.Question : How would you show that setting of curd is a chemical change?
Answer : The setting of milk into curd is an example of chemical change as new substance are formed which have different taste , form,  properties and can not be converted back to milk. Also apart from solid pieces cheese, resulting left behind liquid called milk plasma has chemical properties different from milk.

7. Question . Explain why burning of wood and cutting it into small pieces are considered as two different types of changes.
Answer : Burning of wood and cutting it into small pieces are  two different types of changes as burning of wood is a chemical process in which    new substance along with heat are produced. Where as cutting of wood in to small pieces  is merely a physical change as it does not result into making a new substance with different chemical properties.

8.Question.  Describe how crystals of copper sulphate are prepared.

Answer :Preparation of copper sulphate Crystal : For making crystals of copper sulphate, take a cupful of 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 while stirring continuously. Continue adding copper sulphate powder till no more powder can be dissolved. Filter the solution. Allow it to cool. Do not disturb the solution when it is cooling. Look at the solution after some time. In the bottom of beaker we can see the fine deposite of solid crystals of copper sulphate.
crystals of Copper Sulphate

9.Question : Explain how painting of an iron gate prevents it from rusting.
Answer : For rusting, the presence of both oxygen and water (or water vapour) is essential. In fact, if the content of moisture in air is high, which means if it is more humid, rusting becomes faster.Therefor painting of  an iron gate prevent iron  from coming in contact with oxygen, or water, or both hence prevent it from rusting.

10.Question :  Explain why rusting of iron objects is faster in coastal areas than in deserts.
Answer :The content of  of moisture in air  is high in costal area where as air is almost dry in desert. For rusting, the presence of both oxygen and water (or water vapour) is essential. In fact, rusting becomes faster,  if the content of moisture in air  (humidity) is high.  Hence, the rusting of iron objects is faster in coastal areas than in deserts

11. Question. The gas we use in the kitchen is called liquified petroleum gas (LPG). In the cylinder it exist as a liquid. 
When it comes out from the cylinder it becomes a gas (Change – A) then it burns (Change – B).
The following statements pertain to these changes. Choose the correct one.
(i) Process – A is a chemical change.
(ii) Process – B is a chemical change.
(iii) Both processes A and B are chemical changes.
(iv) None of these processes is a chemical change.
Answer : (i) Process – A is a chemical change. (ii) Process – B is a chemical change
12. Question . Anaerobic bacteria digest animal waste and produce biogas (Change – A). 
The biogas is then burnt as fuel (Change – B).
 The following statements pertain to these changes. Choose the correct one.
(i) Process – A is a chemical change.
(ii) Process – B is a chemical change.
(iii) Both processes A and B are chemical changes.
(iv) None of these processes is a chemical change.

Answer : (iii) Both processes A and B are chemical changes.
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Extended Learning — Activities and Projects
1. Describe two changes that are harmful. Explain why you consider them
harmful. How can you prevent them?
2. Take three glass bottles with wide mouths. Label them A, B and C. Fill about half of bottle A with ordinary tap water. Fill bottle B with water which has been boiled for several minutes, to the same level as in A. In bottle C, take the same boiled water and of the same amount as in other bottles. In each bottle put a few similar iron nails so that they are completely under water. Add a teaspoonful of cooking oil to the water in bottle C so that it forms a film on its surface. Put the bottles away for a few days. Take out nails from each bottle and observe them. Explain your observations.
3. Prepare crystals of alum.
4. Collect information about the types of fuels used for cooking in your area. Discuss with your teachers/parents/others which fuels are less polluting and why

Did you know?
Iron Pillar of Qutub Minar
Near the Qutub Minar in Delhi stands an iron pillar  which is more than 7 metres high. It weighs more than 6000 kg. It was built more than 1600 years ago. After such a long period it has not rusted. For its quality of rust resistance it has been examined by scientists from all parts of the world. It tells something about the advances India had made in metal technology as back as 1600 years ago.
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Physical and Chemical Changes: Lesson 6. CBSE NCERT Class VII (7th) Science Exercise

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