JANDKNCERT | Science | Class 7th |

Physical and Chemical Changes | Science | Chapter 6 | Class 7th | Questions and Answers |

 

Chapter 6: Physical and Chemical Changes

EXERCISES

Q1. 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

 

Q2. 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) The 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)

Ans.

(a) Cutting a log of wood into pieces is a chemical change. (False)

(b) The formation of manure from leaves is a physical change. (False)

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

(d) Iron and rust are the same substances. (False)

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

 

Q3. 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 by 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) 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 hydrogen carbonate.

(c) Two methods by which rusting of iron can be prevented are painting and galvanization.

(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.

 

Q4. 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. When baking soda is mixed with lemon juice, a chemical reaction takes place. In this reaction, carbon dioxide gas is released that gives rise to the formation of bubbles. This is a chemical change.

 

Baking soda + Lemon juice ---à Carbon dioxide + other substances

(NaHCO₃)             (Citric acid)                   (CO₂)

 

Q5. 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. During the burning of a candle, the melting of wax is a physical change. The formation of gas during this process is a chemical change.

Digestion of food is an example of the same process, in which the breakdown of larger food particles into smaller ones is a physical change and the action of enzymes and acids on the food that helps it to digest is a chemical change.

 

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

Ans. We know that a chemical change is not a reversible change. The setting of curd is not a reversible process, therefore the change is a chemical change.

 

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

Ans. Burning of wood is a chemical change because in this process formation of gas takes place. On the other hand, the cutting of wood into pieces is a physical change because no new substance is formed in this process.

 

Q8. Describe how crystals of copper sulphate are prepared.

Ans. Crystals of copper sulphate can be prepared by the process of crystallization. Let us do an activity to get crystals of copper sulphate by the method of crystallization.

Take a cupful of water in a beaker and add a few drops of dilute sulphuric acid in it. Heat the mixture. When it starts boiling add copper sulphate powder till no more powder can be dissolved. Filter the solution and allow it to cool for some time. After some time, you will see crystals of copper sulphate.

 

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

Ans. We know that rusting of iron takes place when it comes in contact with humid air. Painting of iron gate prevents it from coming in contact with humid air, i.e., oxygen and water vapour thus prevents it from rusting.

 

Q10. Explain why rusting of iron objects is faster in coastal areas than in deserts.

Ans. Rusting is faster when the content of moisture in the air is very high. In coastal areas, the content of moisture in the air is very high than in deserts, where the air remains totally dry. That is why rusting takes place faster in coastal areas.

 

Q11. The gas we use in the kitchen is called liquified 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 processes A and B are chemical changes.

(iv) None of these processes is a chemical change.

Ans. (ii) Process – B is a chemical change.

 

Q12. 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.

Ans. (iii) Both processes A and B are chemical changes.

 

Choose the correct one

 

Q13. Rust is ……...

(a) Carbon dioxide

(b) Iron

(c) Oxygen

(d) Oxygen oxide

Ans. (d) Oxygen oxide

 

Q14. Which of the following is not a physical change ……………?

(a) Rusting of iron

(b) Melting of ice

(c) Freezing of water

(d) Dissolving sugar in water

Ans. (d) Dissolving sugar in water

 

Some Important Definitions

Also Read: More about your body structure

·  Properties such as shape, size, colour, and state of a substance are, its Physical Properties.

·  A change in which a substance undergoes a change in its physical properties is called a Physical Change. A physical change is generally reversible. In such a change no new substance is formed.

· A change in which one or more substances are formed is called a Chemical Change. A chemical change is also called a chemical reaction.

·  Rusting is faster, if the content of moisture in the air is high. The process of rusting can be represented by the following equation.

Iron (Fe) + Oxygen (O₂) + Water (H₂O) ----à Rust (Iron oxide, Fe₂O₃)

· The process of depositing a layer of zinc on iron is called Galvanization. It prevents iron from rusting.

·   Crystallization is a process by which a chemical or substance is converted from a liquid solution into a solid crystalline state.