NCERT solution //Class 7// Science //Chapter 6- Physical and chemical changes

 NCERT solution > Class 7 > Science > Chapter 6 -Physical and chemical changes 

NCERT solution //Class 7// Science //Chapter 6- Physical and chemical changes 

Exercise 

Q1. Classify the changes involved in the following process as a 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 

a) Chemical change 

b) Physical change 

c) Chemical change 

d) Physical change 

 e) Physical change 

f )Chemical change 

Q2. State whether the following statements are true or false in case of statement is false write the correct it is to told in your notebook ?

a) Cutting a log of wood into pieces is a chemical change 

Answer - False 

Correct statement - cutting a log of wood into pieces is a physical change 

b) Formation of manure from leaves is a physical change 

Answer - False 

Correct statement - formation of manure from leaves is a chemical change 

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

Answer- True 

d) Iron and rust are the same substance 

Answer - False

 Correct statement  - Iron is a metal while rust is the metal oxide 

e) Condensation of steam is not a chemical change 

Answer - True 

Q3. Fill in the blanks in the following statements 

a) When carbon dioxide is passed through the lime water it turns milky due to the formation of Calcium carbonate (CaCO3)

b) The chemical name of baking soda is Sodium bicarbonate (NaHCO3)

c) Two methods by which rusting of iron can be prevented are greasing and galvanization 

d) Changes in which only physical properties of a substance change is called physical change 

e) Changes in which new substances are formed are called chemical changes 

Q4. When baking soda is mixed with lemon juice bubbles are formed with the evolution of gas what type of change is it explain ?

Answer-  Carbon dioxide gas is evolved in this process and this type of change is a chemical change because new substance is formed in this process

Q5. When a candle burns both physical and chemical changes takes place identify these changes give another example of familiar process in which both the chemical and physical changes takes place ? 

Answer- 

Physical change - On heating candle wax get melted is a physical change it can again turns into solid wax on cooling 

Chemical change -  The wax near to the flame burns and give the new substance in the form of gas like carbon dioxide water vapour heat light

Cooking of food is the another example where both physical and chemical changes occur 

Q6. How would you show that setting of curd is a chemical change? 

Answer - Setting of curd is done with the help of milk and after setting. It  totally change the properties of milk lactic acid is form and the original taste of the milk is changed so it is a chemical change

Q7. Explain why burning of wood and cutting it into small pieces are considered as a two different types of changes ? 

Answer - Burning of wood is a chemical change while cutting of wood into small pieces is the physical change because in first condition new substance is formed in the form of ash while in second condition only length of the wood it's minimize

Q8. Describe how crystals of copper sulphate are prepared ? 

Answer. - 

• Crystals of copper sulphate are prepared in following way 
• Take of water in a beaker and add a few drops of dilute sulphuric acid in it 
• Heat the water when it starts boiling at copper sulphate powder slowly and continuously stir it 
• Continue adding copper sulphate powder in the saturation level .
• Filter the solution and left for cooling in undisturbed condition 
• After cooling we can observe do crystals of copper sulphate at the bottom of beaker

Q9. Explain how painting of an iron gate prevent it from rusting ? 

Answer - When the iron get comes in contact with atmospheric Oxygen and water vapour it it it become rusted the court of the paint prevent the direct contact of iron with higher air and does it prevent gate from rusting

Q10. Explain why rusting of iron object is faster in coastal areas than in desert ? 

Answer - For the rusting the presence of oxygen in air and water vapour is required in coastal areas the quantity of moisture is more in air as compared to desert region so the rusting becomes faster in coastal area 

Q11. The gas used in the kitchen is called liquefied petroleum gas (LPG) in the cylinder it exist as a liquid when it comes out from the cylinder it become a gas( change A )then it was burns (change B) the following statement pertain to these changes to the correct one 

a) Process is a chemical change 

b) Process B is a chemical change 

c) Both process A and B are chemical change 

 d) None of this process is a chemical change 

Answer - Process B is a chemical change 

Q12. Anaerobic bacteria digest animal waste and produce biogas (change A)if the biogas is then  burn as a fuel (change B) the following statement pertain to these changes choose the correct one 

a) Process a is a chemical change 

b) Process B is a chemical change 

c) Both process A and B are chemical changes 

d) None  of these processes is a chemical change  

Answer - Both processes A and B chemical changes