Changes Around Us Book Back Questions 7th Science Lesson 10
7th Science Lesson 10
10] Changes Around Us
Book Back Questions with Answer and Do You Know Box Content
Do You Know?
Another way of preventing rusting is to deposit a layer of a metal like chromium or zinc on iron. This is called galvanization and you will learn about this detail in higher classes.
The Iron Pillar at Delhi: Amazingly there is an iron that did not rust! There is an iron pillar at the Qutub complex in Delhi which is more than 1600years age. Even after such a long period, the iron pillar kept in open spaces has not rusted at all. This shows that Indian scientists made great advances in metal making technology even at 16th century which enabled them to make this iron pillar having the quality of great rust resistance.
Louis Pasteur (1822 – 1895), a French chemist and microbiologist was the first person to describe the process of fermentation. He described that fermentation occurs in the absence of air and in the presence of micro organisms such as yeast. He discovered the cure for rabies.
Catalysts are substances that speed up the process of a chemical change and it will not undergo any change during the course of the reaction. For example, yeast acts as the catalyst in the fermentation of sugar. You will learn more about catalyst in your higher classes.
Choose the best answers:
1. When a woollen yarn is knitted to get a sweater, the change can be classified as ________________
(a) Physical change
(b) Chemical change
(c) Endothermic change
(d) Exothermic change
2. _______________ of the following are endothermic changes.
(a) Condensation and melting
(b) Condensation and freezing
(c) Evaporation and melting
(d) Evaporation and freezing
3. The chemical change is ________________
(a) Water to clouds
(b) Growth of a tree
(c) Cow dung to bio-gas
(d) Ice-cream to molten ice-cream
4. ________________ is an example of a periodic change.
(a) Earthquake
(b) Formation of rainbow in sky
(c) Occurrence of tides in seas
(d) Showering of rain
5. ________________ is not a chemical change.
(a) Dissolution of ammonia in water
(b) Dissolution of carbon-di-oxide in water
(c) Dissolution of oxygen in water
(d) Melting of polar ice caps
Fill in the blanks:
1. Filling up a balloon with hot air is a ___________________ change.
2. Stretching gold coin into a ring is a _____________ change.
3. Opening a gas cylinder knob converts _______________ fuel into _______________ fuel. This is an example of __________ change.
4. Spoiling of food is a _____________ change.
5. Respiration is a ________________ change.
State true or false. If false, correct the statement:
1. Cutting of cloth is an example of a periodic change.
2. Taking a glass of water and freezing it by placing it in the freezer is a chemical change.
3. A beam plant collecting sunlight and turning it into bean seeds is an example of physical and non-periodic change.
4. If the chemical properties of a substance remain unchanged and the appearance or shape of a substance changes it is called a periodic change.
5. Tarnishing of silver is an example of endothermic change.
Match the following:
| A | B | C |
| Melting | Change of state from liquid to solid | Ticking of clock |
| Condensation | Change of state from liquid to gas | Formation of ice cube |
| Evaporation | Change of state from solid to liquid | Collecting flowers |
| Freezing | Change of state from gas to liquid | Ice cube to water |
| Periodic change | Occurs at irregular time intervals | Water to steam |
| Non-periodic change | Occurs at regular time intervals | Steam to water drops |
Complete the analogy:
1. Physical Change: Boiling :: Chemical change: ______________
2. Wood to saw dust: ________________ :: Wood to Ash: Chemical change.
3. Forest fire: _________________ change :: Change in period in a school: period change.
Consider the statements given below and choose the correct option:
1. Assertion: The explosion of fire cracker is a physical change.
Reason: A physical change is a reversible change.
(a) Both A and R is true and R is the correct explanation of A
(b) Both A and R are true but R is N not the correct explanation of A
(c) A is true but R is false
(d) A is false but R is true
2. Assertion: The process of conversion of liquid water to its vapours by heating the liquid is called boiling.
Reason: The process of conversion of water vapours to liquid by cooling the vapours is called condensation.
(a) Both A and R is true and R is the correct explanation of A
(b) Both A and R is true but R is not the correct explanation of A
(c) A is true but R is false
(d) A is false but R is true
3. Assertion: Burning of wood log to charcoal is a physical change.
Reason: The products formed of burning a piece of wood can be easily converted back to wood log.
(a) Both A and R is true and R is the correct explanation of A
(b) Both A and R are true but R is N not the correct explanation of A
(c) A is true but R is false
(d) Both A and R is false
4. Assertion: The formation of iron oxide from iron is a chemical change.
Reason: For the rust to form from iron, it must be exposed to air and water.
(a) Both A and R is true and R is the correct explanation of A
(b) Both A and R is true but R is not the correct explanation of A
(c) A is true but R is false
(d) A is false but R is true
5. Assertion: A drop of petrol when touched with finger gives a chill feeling.
Reason: The above phenomenon is an endothermic one.
(a) Both A and R is true and R is the correct explanation of A
(b) Both A and R is true but R is not the correct explanation of A
(c) A is true but R is false
(d) A is false but R is true
Answers:
Choose the correct answers:
1. Physical change 2. Condensation and melting 3. Cow dung to bio-gas 4. Occurrence of tides in seas
5. Dissolution of carbon-di-oxide in water
Fill in the blanks:
1. Physical 2. Physical 3. Liquid, Gaseous, Chemical 4. Chemical 5. Exothermic chemical
State True or False:
1. False
Correct Statement: Cutting of cloth is an example of a physical change
2. False
Correct Statement: Taking a glass of water and freezing it by placing it in the freezer is a physical change
3. False
Correct Statement: A bean plant collecting sunlight and turning it into bean seeds is an example of chemical and non – periodic change.
4. False
Correct Statement: If the chemical properties of a substance remain unchanged and the appearance or shape of a substance changes it is called a physical change.
5. False
Correct Statement: Tarnishing silver is an example of chemical change.
Match the following:
1.Melting: Change of state from solid to liquid: Ice cube to water
2.Condensation: Change of state from gas to liquid: Steam to water drops
3.Evaporation: Change of state from liquid to gas: Water to steam
4.Freezing: Change of state from liquid to solid: Formation of ice cube
5.Periodic change: Occurs at regular time intervals: Ticking of clock
6.Non-periodic change: Occurs at irregular time intervals: Collecting flowers
Complete the analogy:
1.Burning 2. Physical change 3. Non-periodic
Consider the statements given below and choose the correct option:
1.A is true but R is false
2.Both A and R is true but R is not the correct explanation of A
3.Both A and R is false.
4.Both A and R is true and R is the correct explanation of A
5.Both A and R is true and R is the correct explanation of A