Ch 6. Changes around Us
Objective type questions
1. Multiple choice questions
(a) Which of the following is not an example of an irreversible physical change? i. crushing of wood into sawdust ii. cutting of grass
iii. heating an iron rod iv. grinding of wheat into flour Ans: heating an iron rod
(b) In which of the following examples, a physical and a chemical change is occurring simultaneously?
i. rusting of iron ii. burning of candle
iii. dissolving salt in water iv. adding baking soda to vinegar Ans: burning of candle
2. State whether true or false. Correct the false statements.
(a) Application of heat can bring about both physical and chemical changes. Ans: True
(b) Reversible physical changes are the changes in which the original form of the substance cannot be restored.
Ans: False
Reversible physical changes are the changes in which the original form of the substance can be restored.
3. Give two examples of :
(a) Physical changes
Ans: breaking of glass, tearing of paper, freezing of water, melting of ice
(b) Man-made changes
Ans: Cutting of wood, making of paper, growing food in fields, burning fuels and making dams
(c) Slow changes
ans: Growth of plants and animals, Formation of rocks, soil and fossil fuels, Formation of mountains, Soil erosion
4. In the following questions, two statements are given- one labelled Assertion (A) and the other labelled Reason (R). Read the statements carefully and choose the correct alternative (A), (B), (C) and (D) as given below.
(A) Both A and R are true and R is correct explanation of the assertion. (B) Both A and R are true but R is not the correct explanation of the assertion. (C) A is true but R is false.
(D) A is false but R is true.
(a) Assertion: Rotting of food is an example of an undesirable change. Reason: Changes that are useful to us are known as undesirable changes. (C) A is true but R is false.
(b) Assertion: Adding baking soda to vinegar results in a chemical change with the evolution of carbon dioxide gas.
Reason: When two or more substances are mixed they may react with each other to form a new substance.
(A) Both A and R are true and R is correct explanation of the assertion.
Theory type questions
5. Answer in brief:
(a) What is a change? Explain why water cycle is considered as an example of reversible change.
Ans: ∙ The transformation either in the appearance, properties or functions of a substance is called a change.
∙ The water cycle is based on the interconversion of the three physical states of water.
∙ Processes such as melting, boiling, evaporation, freezing, condensation that are part of the water cycle are reversible changes.
∙ In these changes, there is a change in the state of matter but a new substance is not formed.
(b) ‘ The property of solids to expand and contract is used in making the wooden wheel of a cart. Explain.
Ans: ∙ The diameter of the metal rim is kept slightly smaller than the diameter of the wooden wheel.
∙ When the metal rim is heated, it expands and can easily be placed on
the wooden wheel.
∙ Cold water is then poured to cool the metal rim. Once cooled it contracts and fits tightly onto the wheel. This helps in the smooth movement of the wheel and less wear and tear of the wood.
∙ In making of a spade, the iron blade is heated so that it expands and fits onto the wooden handle. After cooling the iron blade contracts and remains firmly attached to the handle.
(c) Differentiate between:
i. Physical change and Chemical change
Ans:
ii. \Natural and Artificial changes
Ans:
6. Give reasons for the following:
(a) Gaps are left between railway tracks.
Ans: Heat causes metals to expand. Gaps are left between railway tracks to allow the expansion of the metal during summer. Otherwise, the tracks would bend and may cause accidents. Hence gaps are left between the railway tracks.
(b) Melting of ice is a reversible process.
Ans: Ice kept out of the refrigerator, begins to melt and changes into water. If the same water is put back into the freezer, it changes back to ice. Hence, melting of ice is a reversible process.
Picture based questions
7.
(a) What causes the change in the object as seen in the picture?
Ans: Pressure
(b) Describe the changes in the object at the molecular level.
Ans: ∙ When we apply pressure, the molecules of a substance undergo a change in terms of their position.
∙ In most cases, the substance regains its original state or shape as soon as the pressure is released.
Activity based question
8. Picture A Picture B Picture C
Why does the metal ball slip through the ring in picture A but not in picture C? Ans: On heating, the ball expands and increases in diameter. This restricts the ball from slipping through the ring in picture C. After cooling, the metal ball contracts and slips through the ring in picture A.
Fun based questions
9. Classify the changes given below into fast and slow changes.
HOTS
10.
(a) How will you separate two steel vessels that are stuck to each other? Ans: The two steel vessels can be separated from each other by heating since heating results in the expansion of metal vessel.
(b) Sonam mixes all the materials required to make a chocolate cake and then she bakes the mixture in the oven. List different types of changes taking place while making the cake.
Ans: chemical ,irreversible, slow and desirable
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