Here in this post we provided NCERT Exemplar Problem Class 7 Science Physical and Chemical Changes Chapter 6 Full Explanation by group of Science Experts. We have provided here each questions answer with reason behind the answer.
Hope it help you!
Physical and Chemical Changes Class 7 Exemplar full Explanation:
-
Multiple Choice Questions
1) Which of the following is a physical change ?
a) Rusting of iron
b) Combustion of magnesium ribbon
c) Burning of candle
d) Melting of wax
Answer:- d) Melting of wax
Melting of wax is physical change other are chemical changes.
2) Which of the following is a chemical change ?
a) Twinkling of stars
b) Cooking of vegetables
c) Cutting of fruits
d) Boiling of water
Answer:- b) cooking of vegetables
Cooking of vegetables is a chemical change among the above.
3) A chemical change may involve –
a) Change in colour only
b) Change in temperature only
c) Evolution of gas only
d) Any or all of the above
Answer:- d) Any or all of the above
A chemical change may involve change in colour, change in temperature and evolution of gas also.
4) Which of the following is/are true when milk changes into curd ?
I) Its state is changed from liquid to semi solid .
II) It changes colour.
III) It changes taste.
IV) The change cannot be reversed.
Choose the correct option from below.
a) I and II are correct.
b) II and III are correct
c) I, III and IV are correct
d) I to IV are correct
Answer:- c) I, III and IV are correct
When milk changes into curd it’s state is changed from liquid to semi solid, it changes in taste, and the process is irreversible that change cannot be reversed.
5) A man painted his main gate made up of iron , to
i) Prevent it from rusting
ii) Protect it from sun.
iii) Make it look beautiful.
iv) Make it dust free.
Which of the above statement (s) is / are correct ?
a) I) and II)
b) II) and III )
c) Only II)
d) I) and III )
Answer:- d) I and III
Gate made up of iron if coated with paint it will prevent iron from rusting and make a look beautiful also.
6) Iron pillar near the Qutub minar in Delhi is famous for the following facts. Which of these facts is responsible for its long stability ?
a) It is more than 7 meters high.
b) It weighs about 6000 kg.
c) It was built more than 1600 years ago.
d) It has not rusted after such a long period.
Answer:- d) it has not rusted after such a long period.
This is the fact of Qutub minar that the pillars are not rusted after such a long period.
7) Galvanisation is a process used to prevent the rusting of which of the following ?
a) Iron
b) Zinc
c) Aluminium
d) Copper
Answer:- a) Iron
The deposition process in which a layer of zinc on iron done is called Galvanisation.
8) Paheli’s mother made a concentrated sugar syrup by Dissolving sugar in hot water. On cooling crystals of sugars got separated . This indicates a –
a) Physical change that can be reversed.
b) Chemical change that can be reversed.
c) Physical change that cannot be reversed.
d) Chemical change that cannot be reversed.
Answer:- a) Physical change that can be reversed.
Sugar crystals are reappeared this is the physical change that is reversible.
9) Which of the following statement is incorrect for a chemical reaction?
a) Heat may be given out but never absorbed.
b) Sound may be produced.
c) A colour change may take place.
d) A gas may be evolved.
Answer:- a) Heat may be given out but never absorbed.
Heat can be given out also can be absorbed.
10) Two drops of dilute sulphuric acid were added to 1 g of copper sulphate powder and then small amount of hot water was added to dissolve it ( step I) on cooling, beautiful blue coloured crystal got separated ( step II ) . Step I and step II are :
a) Physical and chemical changes respectively.
b) Chemical and physical changes respectively.
c) Both physical change.
d) Both chemical change.
Answer:- c) both physical change.
Step I and step II both are physical changes.
-
Very Short Answer Questions
11) State whether following statements are true or false.
a) When a candle burns, both physical and chemical changes take place.
Answer:- True.
b) Anaerobic bacteria digest animal waste and produce biogas.
Answer:- True.
c) Ships suffer a lot of damage through they are painted.
Answer:- True.
d) Stretching of rubber band is not a physical change.
Answer:- False.
12) Melting of wax is a change where a solid changes to liquid state. Give one more such change which you observe in your surroundings.
Answer:- melting of wax is a physical change where a solid changes to liquid state. One more such example in surrounding related to this is Melting of ice. Melting of ice is also a physical process which can be reversed, such as melting of wax. After melting both changes from solid to liquid.
13) What kind of change is shown by tearing of paper ?
Answer:- size , shape , colour and state of substance are the physical properties. Changes occur in the physical properties are known as physical changes. Tearing of paper is a physical change. This changes are generally reversible, but tearing of paper is a physical change which cannot be reversible.
-
Short Answer Questions
(14) Match the items of Column I with the items of column II.
Column I | Column II |
A)Large crystal | 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:-
A)Large crystal → IV) Sugar candy
B)Depositing a layer of zinc on iron → VI) Galvanisation
C)Souring of milk → V)chemical change
D) carbon dioxide. → I) turns lime water milky
E) iron oxide → III) rust
F) Dissolving common salt In water →II) physical change
(15) Fill in the blanks in the following statements using the words given in the box.
Rusted, colourful, substances, chemical, physical, reversible, iron oxide , object
- Making sugar solution is a ___________ change .
- A physical change is generally_____________ .
- Grinding of wheat grain changes its size . It is a ___________ change.
- Iron benches kept in lawns and gardens get __________. It is a ______________ change because a new __________ Is formed.
Answer:-
- Making sugar solution is a physical
- A physical change is generally reversible.
- Grinding of wheat grain changes its size. It is a physical
- Iron benches kept in lawn and gardens get rusted. It is a chemical change because a new substance is formed.
16) Classify the following processes into physical or chemical changes.
I) Beating of aluminium metal to make aluminium foil.
Answer:- Physical change
Reason: Beating of aluminium metal is a physical change . Changing shape is a physical property. Therefore change in physical properties is a physical change.
II) Digestion of food.
Answer:- Chemical change
Reason: Digestion of food needed many enzymes and acids (hydrochloric acid) . Thus it a chemical change.
III) cutting of a lot of wood into pieces.
Answer:- Physical change
Reason: Cutting is a change in physical property i.e change in shape therefore it is a physical property.
IV) Burning of crackers.
Answer:- Chemical change
Reason: Burning of crackers is a chemical change with sound. There is chemical reaction in the cracker when it burns thus it is a chemical change.
17) Write word equations for two chemical reactions with the help of materials given in the box.
Air, copper sulphate, iron, vinegar, iron oxide, carbon dioxide, iron sulphate, copper , lime water , water.
Answer:- the word equation for chemical reaction are –
A) Copper sulphate + Iron → Iron sulphate + copper
B) Iron+ air + water → Iron oxide
18) Explain the following :
A) Lime water turns milky on passing carbon dioxide gas into it.
Answer:-carbon dioxide + lime water → calcium carbonate + water
Co2+ [ Ca(OH)2] → (CaCO3)+ H2O
This is the reaction when carbon dioxide is passed through the lime water. Calcium carbonate and water are formed as the product of this reaction. There is formation of calcium carbonate which is white in colour and insoluble in water. A precipitate is formed of calcium carbonate which gives the look lime water a milky look.
B) Bubbles are produced when acetic acid is added to a solution of sodium hydrogen carbonate.
Answer:- when the chemical reaction occurs between the acetic acid and sodium hydrogen carbonate, the carbon dioxide gas is evolved . Due to this carbon dioxide gas bubbles are produced.
-
Long Answer Questions
19) Give two examples for each of the following cases.
A) Physical changes which are reversible.
Answer:- a) Melting of ice
b) Melting of wax
c) Folding of paper
B) Physical changes which are not reversible.
Answer:- a) Breaking of glass
b) Tearing of paper
C)Chemical changes.
Answer:- a) Rusting of iron
b) reaction of baking soda
20) Give an example of a chemical reaction for each of the following situations.
A) A change in colour is observed
Answer:-when an metal iron and copper sulphate solution reacted to each other the chemical reaction occurs and change in colour is observed.
B) A gas is evolved.
Answer:-when baking soda is reacted with the vinegar , In this reaction carbon dioxide gas is evolved.
C) Sound is produced.
Answer:-Burning of crackers is the chemical reaction in which sound is produced.
21) If you leave a piece of iron in the open for a few days, it acquires a film of brownish substances , called rust.
a) Do you think rust is different from iron?
Answer:- yes. Rust is different from the iron . As rusting is the result of chemical reaction occurred on the iron.
b) Can you change rust back into iron by some simple method?
Answer:- No. Rusting of iron is an chemical change which is irreversible. Therefore rust can not form again iron.
c) Do you think formation rust from iron is a chemical change ?
Answer:- Yes. Formation of iron is an chemical change which is irreversible.
d) Give two other examples of a similar type of change .
Answer:- two examples of the same type of chemical change as Rusting of iron are –
A) Formation of curd from milk.
B) Burning of any Paper.
22) A student took a solution of copper sulphate in a beaker and put a clean iron nail into it and left it for about an hour.
A) What changes do you expect?
Answer:- when the above reaction occur the colour of the solution changes from blue colour to green colour in beaker. And a brown colour deposition found on the surface of the iron nail.
B) Are these changes chemical in nature ?
Answer:- yes. The change after the reaction are chemical. As a result of the chemical change green coloured iron sulphate and brown coloured copper formed.
C) Write a word equation of the chemical change, if any .
Answer:-
The word equation for the given reaction is as follows:-
Copper sulphate + iron
(Blue)
↓
Iron Sulphate + copper
(Green) (Brown)