NCERT Solution class 7 Science Chapter-6

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NCERT Solution class 7 Science Chapter-6

Chapter – 6 Physical and Chemical Change

Chapter Exercise

Page no. – (65)

 

Q.1- Classify the changes involved in the following processes as physical or chemical changes:-

  1. Photosynthesis
  2. Dissolving sugar in Water
  3. Burning of coal
  4. Melting of Wax
  5. Beating aluminium to make aluminium foil
  6. Digestion of food

Ans.1-

  1. Chemical change
  2. Physical change
  3. Chemical change
  4. Physical change
  5. Physical change
  6. 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.

  1. Cutting a log of wood into pieces is a chemical change. (True/False)
  2. Formation of manure from leaves is a physical change. (True/False)
  3. Iron pipes coated with zinc do not get rusted easily. (True/False)
  4. Iron and rust are the same substances. (True/False)
  5. Condensation of steam is not a chemical change. (True/False)

Ans.2-

  1. False, Cutting a log of wood into pieces is a physical
  2. False, Formation of manure from leaves is a chemical change.
  4. False, Iron and rust are two different chemical substances.

 

Q.3- Fill in the blanks in the following statements:-

  1. When carbon dioxide is passed through lime water, it turns milky due to the formation of ____.
  2. The chemical name of baking soda is ____.
  3. Two methods by which rusting of iron can be prevented are ____ and ____.
  4. Changes in which only ____ properties of a substance change are called physical changes.
  5. Changes in which only new substances are formed are called ____ changes.

Ans.3-

  1. When carbon dioxide is passed through lime water, it turns milky due to the formation of __calcium carbonate__.
  2. The chemical name of baking soda is __Sodium hydrogen carbonate__.
  3. Two methods by which rusting of iron can be prevented are __painting__ and __galvanisation__.
  4. Changes in which only __physical__ properties of a substance change are called physical changes.
  5. Changes in which only new substances are formed are called __chemical__

 

Q.4- When baking soda is mix with lemon juice, bubbles are formed with the evolution of a gas. What type of change is it? Explain.

Ans.4- When some sour substance is added to milk or kept overnight, it turns into curd and the curd is never converted into milk. Hence, formation of curd is chemical change.

 

Q.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.

Ans.5- Burning of candle are both chemical and physical change:-

Chemical change:- The burning of gas with evolution with Co2.

Physical change:- melting of wax, vapourisation of melted wax.

 

Q.6- How would you show that setting of a curd is a chemical change?

Ans.6- The setting of curd is a chemical change because curd cannot convert into milk. The properties of milk and curd are different.

 

Q.7- Explain why burning of wood and cutting it into small pieces are considered as two different types of changes.

Ans.7- The cutting of wood is a physical change because the nature of wood is not change. But, burning of wood formed new substance.

 

Q.8- Describe how crystals of copper sulphate are prepared.

Ans.8-

  1. Take a cupful of water in a beaker and add a few drops of dilute sulphuric acid.
  2. Heat the water. When it starts boiling, add copper sulphate powder slowly.
  3. Continue to add copper sulphate powder till no more powder can be dissolved.
  4. During this process continuously stir the solution. Filter the solution.
  5. Leave it for cooling. Look it after some time, you can see the crystals of copper sulphate.

 

Q.9- Explain how painting of an iron gate prevents it from rusting?

Ans.9- Rusting of iron requires contact with moisture and oxygen. By painting an iron gate, we prevent the contact between iron, Oxygen and moisture, this helps in preventing rusting of iron.

 

Q.10­- Explain why rusting of iron objects is faster in coastal areas than in deserts.

Ans.10- The presence of moisture in the air promotes rusting of iron. The quantity of moisture in coastal areas is much higher than coastal area where moisture content in the air is very low.

 

Q.11- 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.

  1. Process – A is a chemical change.
  2. Process – B is a chemical change.
  • Both processes A and B are chemical changes.
  1. None of these processes is a chemical change.

Ans.11- ii. Process – B is a chemical change.

 

Q.12- 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.

  1. Process – A is a chemical change.
  2. Process – B is a chemical change.
  • Both processes A and B are chemical changes.
  1. None of these processes is a chemical change.

Ans.12- iii. Both processes A and B are chemical changes.