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Science 5 Marks Questions

Physical and Chemical Changes · CBSE STD 7 (CBSE - English Medium). 10 questions.

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10 questions · timed · auto-graded

Question 15 Marks
Classify the changes involved in the following processes as physical or chemical changes:

Photosynthesis

Dissolving sugar in water

Burning of coal

Melting of wax

Beating aluminium to make aluminium foil

Digestion of food

Answer
Photosynthesis → Chemical change

Dissolving sugar in water → Physical change

Burning of coal → Chemical change

Melting of wax → Physical change

Beating aluminium to make aluminium foil → Physical change

Digestion of food → Chemical change

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Question 25 Marks
Differentiate between physical and chemical reactions.
Answer
The difference between physical change and chemical change is as follows: Physical change
  1. No new substance is formed. A substance undergoes a change in its physical properties.
  2. Physical change is generally reversible.
Example- Melting of ice, lighting of bulb Chemical Change.
  1. One or more new substances are formed by chemical reaction.
  2. Chemical change is irreversible.
Example- burning of coal, photosynthesis. Physical change
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Question 35 Marks
Write the differences between physical and chemical changes.
Answer
Differences between physical and chemical changes are:
S.No.
Physical change
Chemical change
$i.$
No new substance is formed.
New substance is formed.
$ii.$
It is a temporary change.
It is a permanent change.
$iii.$
Physical change is easily reversible.
Chemical change is irreversible.
$iv$
Very little energy (heat, etc) is absorbed or given out in a physical change.
A lot of energy (in the form of heat, light, sound etc) is absorbed or given out in a chemical change.
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Question 45 Marks
Fill in the blanks in the following statements using the words given in the box.
rusted, colourful, substance, chemical, physical, reversible, iron oxide, object
  1. Making sugar solution is a ____________ change.
  2. A physical change is generally____________.
  3. Grinding of wheat grain changes its size. It is a ____________ change.
  4. Iron benches kept in lawns and gardens get____________. It is a _________ change because a new _________ is formed.
Answer
  1. Making sugar solution is a physical change.
  2. A physical change is generally reversible.
  3. Grinding of wheat grain changes its size. It is a physical change.
  4. Iron benches kept in lawns and gardens get rusted. It is a chemical change because a new substance is formed.
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Question 55 Marks
Describe two changes that are harmful. Explain why you consider them harmful? How can you prevent them?
Answer
Harmful changes are,
  • Rusting of iron: Rusting of iron is harmful because it slowly destroys iron articles and makes them useless. Since, iron is used in making large number of objects or articles such as bridges, grills, railings, gates and bodies of cars, buses, trucks and ships, etc. Rusting of iron causes a great loss over a period of time. Prevention Rusting can be prevented by oiling, greasing or painting. It can also be prevented by galvanisation.
  • Decaying of fruits: Decaying of fruits causes health hazards. Due to decaying of fruits, there is a lot of monetary loss in food industry. Prevention Fruits can be preserved by keeping them at low temperature and by using some specific preservatives.
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Question 65 Marks
Which type of change takes place in the following and state whether the energy is evolved or absorbed during the change? Burning of a candle, lightning of a bulb, preparation of food by green plants, volcanic eruption, evaporation of petrol, burning of $LPG.$
Answer
  1. Burning of a candle Chemical change as well as physical change and energy evolved.
  2. Lightning of a bulb Physical change, energy evolved.
  3. Preparation of food by green plants Chemical change, energy absorbed.
  4. Volcanic eruption Chemical change, energy evolved.
  5. Evaporation of petrol Physical change as no new chemical substance is formed, energy absorbed.
  6. Burning of LPG Chemical change because LPG on burning form $CO_2$ and $H_2O$, energy absorbed.
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Question 75 Marks
Describe how crystals of copper sulphate are prepared.
Answer
Crystals of copper sulphate are prepared by the method of crystallization. The process is as follows. A cupful of water is taken in a beaker. Few drops of dilute sulphuric acid are added to this. The water is then heated and when it starts boiling, copper sulphate powder is added with stirring. Copper sulphate powder should be added on till the solution becomes saturated. It is then filtered into a china dish and allowed to cool. The solution should be kept undisturbed. Slowly, the crystals of copper sulphate separate out.
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Question 85 Marks
Explain the physical and chemical processes along with examples.
Answer
A change in which a substance undergoes a change in its physical properties is called physical change, physical properties involves shape, size, colour and state of a substance. It is generally reversible; in physical change no new substance is formed. For example: evaporation, condensation etc. A change in which one or more new substances are formed is called chemical change. Chemical change is irreversible. It is also called chemical reaction; in this type of change a new substance is formed. Example- Burning of coal, photosynthesis. The difference between physical change and chemical change is as follows- Physical change.
  1. No new substance is formed. A substance undergoes a change in its physical properties.
  2. Physical change is generally reversible.
Example- Melting of ice, lighting of bulb Chemical Change.
  1. One or more new substances are formed by chemical reaction.
  2. Chemical change is irreversible.
Example- burning of coal, photosynthesis.
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Question 95 Marks
Explain the formation of crystal of copper sulphate.
Answer
Take a clean beaker $(250\ ml)$ and put the powdered impure sample of copper sulphate in it.
  • Add distilled water and stir the contents gently with the help of a glass rod.
  • In order to make the solution more clear add two or three drops of concentrated sulphuric acid in it.
  • Heat the solution in the beaker to $60-70^\circ $ c on a wire gauze.
  • Stir it continuously and add more impure copper sulphate until no more of it dissolves.
  • Filter the solution and collect the filtrate in a china dish.
  • Place the china dish over wire gauze kept over a tripod stand and heat it gently (do not boil).
  • As the solution gets heated, stir it with a glass rod. This helps in uniform evaporation and prevents the formation of a solid crust.
  • When the volume of the solution is reduced to one-half, take out a drop of the concentrated solution on one end of the glass rod and cool it by blowing air. Formation of thin crust indicates that crystallization point is reached.
  • Turn off the burner, cover the dish with a watch glass, and keep it undisturbed. As the solution cools down, crystals separate out. Slow cooling ensures better crystallization.
  • Decant the mother liquor and wash the crystals with a thin stream of cold water with the help of a wash bottle.
  • Dry the crystals by pressing them gently between sheets of filter paper.
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Question 105 Marks
Match the items of Column I with the items of Column II.
 
Column I
 
Column I
$(a)$
Large crystals
$(i)$
Turns lime water milky
$(b)$
Depositing a layer of zinc on iron
$(ii)$
Physical change
$(c)$
Souring of milk
$(iii)$
Rust
$(d)$
Carbon dioxide
$(iv)$
Sugar candy $(Mishri)$
$(e)$
Iron oxide
$(v)$
Chemical change
$(f)$
Dissolving common salt in water
$(vi)$
Galvanisation
Answer
 
Column I
 
Column I
$(a)$
Large crystals
$(iv)$
Sugar candy (Mishri)
$(b)$
Depositing a layer of zinc on iron
$(vi)$
Galvanisation
$(c)$
Souring of milk
$(v)$
Chemical change
$(d)$
Carbon dioxide
$(iv)$
Sugar candy (Mishri)
$(e)$
Iron oxide
$(iii)$
Rust
$(f)$
Dissolving common salt in water
$(ii)$
Physical change
Note:
  • Carbon dioxide form calcium carbonate with lime water $\left[\mathrm{Ca}(\mathrm{OH})_2\right]$ and turns it milky. It is a chemical change.
  • During rusting iron reacts with moist oxygen to form iron oxide, which is called rust.
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