7th Science Guide Term 2 Lesson 3 Changes Around Us

7th Standard Science Term 2 Solution Guide in English Medium | Lesson.3 Changes Around Us

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7th Science Guide Term 2 Lesson 3 Changes Around Us

I . Choose the correct answer.

1. When a woolen yarn is knitted to get a sweater, the change can be classified as _______.

1. physical change
2. chemical change
3. endothermic change
4. exothermic change

Ans : physical change

2.  ___________ of the following are endothermic changes.

1. Condensation and melting
2. Condensation and freezing
3. Evaporation and melting
4. Evaporation and freezing

Ans : Evaporation and melting

3. The chemical change is ____________.

1. water to clouds
2. growth of a tree
3. cow dung to bio-gas.
4. ice-cream to molten ice-cream

Ans : growth of a tree

4. ________is an example of a periodic change.

1.  
2. Formation of rainbow in sky
3. Occurrence of tides in seas.
4. Showering of rain

Ans : Occurrence of tides in seas.

5. _________ is not a chemical change.

1. Dissolution of ammonia in water
2. Dissolution of carbon-di-oxide in water
3. Dissolution of oxygen in water
4. Melting of polar ice caps

Ans : Melting of polar ice caps

II. Fill in the blanks:

1. Filling up a balloon with hot air is a _____ changeAns : physical
2. Stretching gold coin into a ring is a ________ change.Ans : physical
3. Opening a gas cylinder knob converts _____ fuel into ________ fuel. This is an example of _____ changeAns : Liquid, gaseous, chemical
4. Spoiling of food is a ___________ change.Ans : Chemical
5. Respiration is a _____________ change.Ans : Exothermic chemical

III. Match the following:

1. Melting	Change of state from liquid to solid	Ticking of clock
2. Condensation	Change of state from liquid to gas	Formation of ice cube
3. Evaporation	Change of state from solid to liquid	Collecting flowers
4. Freezing	Change of state from gas to liquid	Ice cube to water
5. Periodic change	Occurs at irregular time intervals	Water to steam
6. Non-periodic change	Occurs at regular time intervals	Steam to water drops
Ans : 1 – C – D, 2 – D – F, 3 – B – E, 4 – A – B, 5 – E – A, 6 – F – C

IV. True or False – If false give the correct answer:

1. Cutting of cloth is an example of a periodic changeAns : False.

Cu. An : Cutting of cloth is an example of a physical change.

2. Taking a glass of water and freezing it by placing it in the freezer is a chemical change.Ans : False.

Cu. An. : Taking a glass of water and freezing it by placing it in the freezer is a physical change

3. A bean plant collecting sunlight and turning it into bean seeds is an example of physical and non-periodic change.Ans : False.

Cu. An. :  A bean plant collecting sunlight and turning it into bean seeds is an example of chemical 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.Ans : False.

Cu. An. : if the chemical properties of a substance remain unchanged and the appearance or shape of a substance changes it is called a fossil change.

5. Tarnishing of silver is an example of endothermic change.Ans : False

Cu. An. : Tarnishing of silver is an example of change

V. Classify the following changes as physical and chemical changes

• A rough piece of wood is sanded and polished resulting in change in texture, Rusting of a iron nail, Painting the grill, Bending a paper clip, Pounding silver into thin plate, Rolling the chappathi dough into thin wire, Occurrence of day and night, eruption of volcano, burning of matchstick, dosa from the batter, blinking of eyelids, occurrence of a thunderstorm, rotation of the earth, formation of eclipses

Physical changes

• Bending a paper clip.
• Pounding silver into thin plate.
• Rolling a chappathi dough into thin wire.
• Occurrence of a day and night.
• Blinking of eyelids.
• Rotation of the earth.
• Formation of eclipses.
• A rough piece of wood is sanded and polished resulting in change in texture.

Chemical changes

• Rusting of a iron
• Eruption of volcano
• Burning of matchstick.
• Painting the grill
• Dosa from the batter.
• Occurrence of a thunderstorm.

VI. Analogy:

1. Physical Change : Boiling :: Chemical Change : ________.Ans: Burning
2. Wood to saw dust :_________ :: Wood to Ash : Chemical changeAns : Physical change
3. Forest fire: ______ change::Change in period in a school: periodic changeAns : Non – periodic

VII. Assertion and Reason:

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true

1. Assertion (A) : The explosion of fire cracker is a physical change.

Reason (R) : A physical change is a reversible change.

Ans : A is false but R is true

2. Assertion (A) : The process of conversion of liquid water to its vapours by heating the liquid is called boiling.

Reason (R) : The process of conversion of water vapours to liquid by cooling the vapours is called condensation.

Ans : Both A and R are true but R is not the correct explanation of A.

3. Assertion (A) : Burning of wood log to charcoal is a physical change.

Reason (R) : The products formed of burning a piece of wood can be easily converted back to wood log.

Ans : Both A and R are false

4. Assertion (A) : The formation of iron oxide from iron is a chemical change.

Reason (R) : For the rust to form from iron, it must be exposed to air and water.

Ans : Both A and R are true and R is the correct explanation of A.

5. Assertion (A) : A drop of petrol when touched with finger gives a chill feeling.

Reason (R) : The above phenomenon is an endothermic one.

Ans : Both A and R are true and R is the correct explanation of A.

VIII. Very short answer :

1. State two examples of periodic changes.

• Seasonal changes, motion of hands of a clock.

2. Mention any two exothermic reactions.

• Burning of magnesium ribbon and burning of a candle.

3. Cold milk is heated and it becomes hot. Which type of change it is?

• Reversible.

4. What type of change is artificial ripening of fruit?

• Irreversible chemical change.

5. What type of change is coloring of a paper?

• Physical change.

6. Growing of nails is a periodic change. Why?

• Growing of nail is a periodic change, because it occurs periodically at regular intervals.

7. What type of energy changes is associated when ice melts?

Melting

Ice  —————>  Water

• Physical change
• Endothermic (heat energy is absorbed)

IX. Short Answer :

1. Distinguish physical and chemical changes

Physical changes	Chemical changes
1. No new substance is formed	New substance is formed
2. Reversible	Irreversible
3. Change in physical properties like size, shape, state	Change in properties of reactants and products
4. Melting of ice, tearing of paper freezing, evaporation vaporization	Burning of paper, photosynthesis, digestion of food, rusting of iron

2. How can a change occur in a substance?

• A change can occurs in a substance by an alteration in the properties such as colour, texture and the state of the substance since there is formation of a new substance.

3. Can you suggest a method to collect water from sea water?

• Evaporation.

4. Is solar eclipse a periodic change? Give your reason.:

• Yes, solar eclipse is a periodic change as it occurs after a definite interval of time.

5. What is the difference between dissolution of sugar and burning of sugar?

Dissolution of sugar	Burning of sugar
1. When sugar is dissolved in water it disappears. It we taste the solution, the sugar is still present in dissolved form.	Fire activates a chemical reaction between sugar and oxygen. The oxygen in the air reacts with the sugar as the chemical bonds broke.
2. If water is evaporated we get back the sugar.	Energy is released in the form of smoke.
3. So it is a physical and reversible change.	So, burning a sugar is a chemical change.

X. Long answer type question :

1. Explain the following statement: Digestion is a chemical change.

• When we eat, our mouth physically break down food into small pieces.
• Mechanical digestion occurs in the mouth, stomach and small intestine.
• Food is chemically changed in digestion when new, smaller substances are formed.
• Moreover, we will never be able to get back the raw material in the same form as it was before.
• Digestion of food is a permanent change which is irreversible.

2. How the iron blade is fixed into a wooden handle in tools used to dig the soil?

• First the ring in the iron blade of spade is heated.
• Heating of the blade leads to its expansion and thus the ring gets bigger.
• This happens because metals always expand on heating.
• The metal blade is then fitted easily into the wooden handle.
• After this, cold water is usually poured on the iron blade which leads to the contraction of the expanded iron blade.
• The spade can also be left to cool down, undisturbed so that it is firmly fixed, (vu) The blade is attached firmly to the wooden handle of a spade in this process.

XI. Picture based Questions:

1. Observe the picture and list down the changes that are accompanied in the picture.

 

• Physical change
• Chemical change
• Exothermic

2. Observe the picture containing a kettle and note that it has salt water in it and answer the following questions.

 (a) What is name of the process that is done to the kettle?

Boiling

(b) What will happen to the content of the kettle?

Water in the kettle gets converted into vapour.

(c) What kind of change is occurring on the cold surface of the metal plate?

Endothermic

(d) What can you say about the quality of water that is obtained in the beaker?

Water vapour converted into liquid (condensation)