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VII SCIENCE CHANGES AROUND US EVALUATION QUESTION ANSWERS
CHANGES AROUND US
Evaluation
I. Choose the best answer
1. When a woolen 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
Ans: a. physical change
2. ___________ of the following are endothermic changes.
a. Condensation and melting
b. Condensation and freezing
c. Evaporation and melting
d. Evaporation and freezing
Ans: a. Condensation and melting
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.
Ans: c. cow dung to bio-gas.
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
Ans: a. Earthquake
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
Ans: d. Melting of polar ice caps
II. Fill in the blanks
1. Filling up a balloon with hot air is a _____ change.
Ans: 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 _____ change.
Ans: liquid, gas, physical
4. Spoiling of food is a ___________ change.
Ans:chemical
5. Respiration is a _____________ change.
Ans: chemical
III. True or False. If false, give the correct answer.
1. Cutting of cloth is an example of a periodic change.
Ans: False
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
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
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
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. Tarnishing of silver is an example of endothermic change.
Ans: False
Tarnishing of silver is an example of chemical change.
IV. Match the following
Ans:
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 paperclip, 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
A rough piece of wood is sanded and polished resulting in change in texture
Painting the grill
Bending a paperclip
Pounding silver into thin plate
Rolling the chappathi dough into thin wire
Occurrence of day and night
dosa from the batter
blinking of eyelids
rotation of the earth
formation of eclipses
Chemical changes
Rusting of a iron nail
eruption of volcano
burning of matchstick
occurrence of a thunderstorm
VI. Analogy
1. Physical Change: Boiling::Chemical Change:________.
Ans: Burning
2. Wood to saw dust:_________ :: Wood to Ash: Chemical change
Ans: Physical change
3. Forest fire: ______ change::Change in period in a school: periodic change
Ans: Non-periodic change
VII. Very short answer type question
1. State two examples of periodic changes.
Rotation and revolution of earth
Heart beat
2. Mention any two exothermic reactions.
Burning of wood
Reaction of calcium oxide (slaked lime) with water
3. Cold milk is heated and it becomes hot. Which type of change it is?
Only the temperature increased from normal to higher temperature, so it is a reversible change.
4. What type of change is artificial ripening of fruit?
Irreversible change.
5. What type of change is colouring of a paper?
Physical change
6. Growing of nails is a periodic change. Why?
Because it ourrs periodically at regular intervals.
7. What type of energy changes is associated when ice melts?
Ice → water = solid → liquid = physical change
Ice → water = Ice absorbs heat from the atmosphere then melts, so it is an endothermic change.
VIII. Short answer type question
1. Distinguish physical and chemical changes.
2. How can a change occur in a substance?
Changes occur when substances are melted, frozen, boiled, condensed, sublimated, deposited,compressed,brokened or when substance undergoes rotting, burning, cooking etc.
3. Can you suggest a method to collect water from sea water?
✱ Evaporation.
✱ Evaporation is the technique used to separate dissolved solids from a solid-liquid mixture.
4. Is solar eclipse a periodic change? Give your reason.
✱ Yes, solar eclipse is a periodic change.
✱ It occurs repeatedly after a definite interval of time.
5. What is the difference between dissolution of sugar and burning of sugar?
Dissolution of sugar
When sugar is dissolved in water, it dissolves completely and becomes invisible.
It is a physical change
It is a temporary change
Burning of sugar
Burning of sugar activates a reaction between sugar and oxygen and complete combustion gives carbon dioxide and water.
It is a chemical change
It is a permanent change
IX. Long answer type question
1. Explain the following statement: Digestion is a chemical change.
✱ When we eat, our mouth physically breaks down food into small pieces.
✱ When food is physically changed into another form, mechanical digestion occurs.
✱ Mechanical digestion occurs in the mouth, stomach, and small intestine.
✱ Food is broken into smaller parts and mixes continually with enzymes and other gastric juices.
✱ Chemical digestion begins in the mouth when enzymes in saliva begin to break down carbohydrates.
✱ Most chemical changes of digestion occur in the small intestine, by hydrochloric acid.
2. How the iron blade is fixed into a wooden handle in tools used to dig the soil?
✱ The tool consists of a wooden handle and an Iron blade.
✱ Iron blade of the tools has a ring in which the wooden handle is to be fixed.
✱ Normally the ring is slightly smaller in size than a wooden handle.
✱ To fix the iron blade, the ring is to be heated so that it expands.
✱ When the ring is heated,it expands and now it is easy to fix the ring into the handle.
✱ Once it is fitted well, bring it down to normal temperature.
✱ When the ring cools down it contracts and fits into the handle firmly.
✱ Now the blade is fixed firmly to the wooden handle by this process.
X. Higher order Thinking questions
1. Peeled and unpeeled banana does not look the same. Does that mean peeling banana is a chemical change?
✱Yes, it is a chemical change.
✱As soon as we peel the banana the inner portion gets exposed to atmospheric oxygen and it is due to the presence of an enzyme called polyphenol oxidase, which is oxygen dependent.
✱This results in immediate oxidation and forms a brown polymer, which blackens with time.
✱ Not only bananas, fruits like grapes,apples and vegetables like potatoes, raw bananas also turn brown, black on peeling due to the presence of polyphenoloxidases.
2. A very hot glass on putting in cold water cracks. What does this change indicate?
✱Glass has low thermal conductivity.
✱Normally glass expands when it heats up and shrinks when it cools.
✱When a hot glass sinked in cold water, the surface in contact with cold water cools instantly.
✱The middle surface takes time to cool and takes time to conduct it to the hot surface.
✱So,on the other end, the hot surface still remains in hot condition.
✱Due to this uneven thermal behaviour glass cracks.
3. Boiling of water is a physical change; but boiling of egg is a chemical change. Why?
✱Boiling of water is a physical change.
✱Only change in the state of matter, i.e from liquid to gas
✱It is a reversible change because by condensation water vapor can turn back to liquid form.
But, boiling eggs is a chemical change .
Because;
✱Boiling an egg brings a new substance.
✱After boiling, the composition of egg changes.
✱The change can not be reversed so, it is a irreversible change.
XI. Assertion – Reason type question
1. Assertion: The explosion of fire cracker is a physical change.
Reason: A physical change is a reversible change.
a. Both A and R are true and R is the correct explanation of A.
b. Both A and R are 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.
Ans: 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 are true and R is the correct explanation of A.
b. Both A and R are 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.
Ans: 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 are true and R is the correct explanation of A.
b. Both A and R are 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.
Ans: Both A and R are 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 are true and R is the correct
explanation of A.
b. Both A and R are 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.
Ans: a. Both A and R are true and R is the correct
explanation of A.
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 are true and R is the correct explanation of A.
b. Both A and R are 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.
Ans: a. Both A and R are true and R is the correct
explanation of A.
