NCERT CLASS 7 SCIENCE CHAPTER 6 PHYSICAL AND CHEMICAL CHANGES
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NCERT CLASS 7 SCIENCE CHAPTER 6 PHYSICAL AND CHEMICAL CHANGES

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NCERT CLASS 7 SCIENCE CHAPTER 6 PHYSICAL AND CHEMICAL CHANGES

Question Answer

Q.1 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 
Ans.(a) Chemical change.     (b) Physical change
        (c) Chemical change.     (d) Physical change
        (e) Physical change.       (f) Chemical change
Q.2 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.     (T/F)
 (b) Formation of manure from leaves is a physical change.     (T/F)
 (c) Iron pipes coated with zinc do not get rusted easily.       (T/F)
 (d) Iron and rust are the same substances.    (T/F)
  (e) Condensation of steam is not a chemical change.    (T/F)
Ans. (a) False (F), (b) False (F), (c) True (T), (d)True (T), (e)True (T).
Q.3 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………….
(b) The chemical name of baking soda is …………..
(c) Two methods in which rusting of iron can be prevented are ……..and……… .
(d) Changes in which only ……….. properties of a substance change are called physical changes.
(e) Changes in which new substances are formed are called…………..changes.
Ans ( a) calcium carbonate,(b) Sodium hydrogen carbonate, (c) coating, galvanization, (d) Physical,(e) chemical.
Q.4 when baking soda is mixed with lemon juice, bubbles are formed with the evolution of a gas . What type of change is it? Explain.
Ans. It is a chemical change.
When baking soda is mixed with lemon juice, bubbles are formed with the evolution of a gas called carbon dioxide.
Lemon juice + Baking soda → Carbon dioxide + Lime water.
Q.5 When a candle burns, both Physical and chemical changes take place. Identify these changes. Give another examples of of a familiar process in which both the chemical and physical changes take place.
Ans . Melting of wax is a physical change while burning of candle is a chemical change.
Lighting torch bulb using dry cell is another example where  both physical and chemical changes take place. The lighting of the bulb is physical change while current from the dry cell is obtained by the chemical substances inside it . This is a chemical change because the chemical in the cell get converted into new substances and hence the cell ultimately become useless.
Q.6 How would you show that setting of curd is a chemical changes?
Ans . The conversation of milk in to curd, i.e., setting of curd is a permanent as well as irreversible and lead to the production of a new substance. The curd is not to be converted into milk.
Thus, the formation of curd is a chemical change.
Q.7 Explain why burning of wood and cutting it into small pieces are considered as two different types of changes.
Ans. Burning of wood is chemical change because in addition to new products burning is always accompanied by production of heat .
Cutting of wood into small pieces is a physical change because pieces of wood under went changes it’s size and no new substances is formed .
Q.8 Describe how crystals  of copper sulphate are prepared? 
Ans. Take a cup, full 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.
Crystals of Copper Sulphate              
   (Figure Crystals of Copper Sulphate)
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. Can you see the crystals of copper sulphate?  If not , wait for some more time . Crystals of copper sulphate slowly form at the bottom of the beaker. 
Q.9 Explain how painting of an iron gate prevents it from rusting? 
Ans. For rusting , iron must be in contact with both air and moisture . When iron gate is painted, the layer of paint cuts the contact between air, moisture and iron .Thus, it prevents rusting.
Q.10 Explain why rusting of iron objects is faster in coastal areas than in deserts.
Ans. The water of coastal areas contain many salts. The salt water makes the process of rust formation faster. Thus, rusting of iron objects is faster in coastal areas than in deserts. 
Q.11 The gas we use in the kitchen is called Liquefied Petroleum Gas (LPG). In the cylinder, it exists 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 process – A and B are chemical changes.
(iv) None  of these processes is a chemical change.
Ans. (iii) Both process – A and B are chemical changes.
Q.12.  Anaerobic bacteria digest animal waste and produce biogas (Change – A). The  biogas is then burnt as fuel (Change -B). The following statement pertain to these changes. Choose the correct one.
(i) Process – A is a chemical change
(ii) process – B is a chemical change
(iii) Both process – A and B are chemical changes
(iv) None of these processes is a chemical change
Ans. (iii) Both processes – A and B are chemical changes.
 
 
         
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