PSEB Punjab Board Class 7 Science Textbook Solution Chapter 6 Physical and Chemical Changes Exercise Questions and Answers
PSEB Punjab Board Class 7 Science Textbook Solution Chapter 6 Physical and Chemical Changes Exercise Questions and Answers
Chapter 6 Physical and Chemical Change
Q.1 Fill in the blanks:
1. A change in which only physical properties of a substance are changed, is a…………. change.
Answer:- Physical change.
2. Changes that lead to formation of new substances are called…………..changes.
Answer:- Chemical change.
3. Fossil fuels produce…………. gas on burning.
Answer:- Nitrogen oxide.
4. When carbon dioxide is passed through lime water, it turns…………..
Answer:- Milky.
5. ………. is the method for prevention of rusting of iron object’s.
Answer:- Greasing and oiling.
Q.2 Match the column:
Answers :-
1. Physical change – iv) Reversible change.
2. Chemical change – ii) Formation of new substances.
3. Prevention from rusting – i) Galvanization.
4. Evolution of carbon dioxide – iii) Mixing of vinegar and baking soda.
Q.3 Choose the correct option
1.) The example of chemical change is :
(a) eruption of volcano □
(b) burning of candle □
(c) cooking of food □
(d) all of the above □
Answer:- d. All of the above
2. When the acidic acid is mixed with baking soda, the gas evolved is :
(a) hydrogen
(b) oxygen
(c) carbon dioxide
(d) carbon monoxide
Answer:- c. Carbon dioxide.
3. For rusting of iron objects, the essential requirement is :
(a) air (oxygen)
(b) moisture (water)
(c) open surface of object
(d) all of the above
Answer:- d. All of the above.
4. For prevention of rust, we use :
(a) a coat of oil and greece
(b) a coat of paint
(c) galvanization
(d) all of the above
Answer:- d. All of the above.
5. The chemical formula of Rust is :
(a) Fe₂O₃
(b) FeCO3
(c) Fe2O3.xH2O
(d) FeCO3.xH2O
Answer:- c. Fe2O3.xH2O
Q.4 True or false:
1. Cutting a log of wood is a chemical change.(T/F)
Answer:- False.
2. Formation of manure from leaves is a physical change.(T/F)
Answer:- False.
3. Iron pipes coated with zinc do not get rusted easily.(T/F)
Answer:- True.
4. Iron and rust of the same substance.(T/F)
Answer:- True.
5. Condensation of steam is not a chemical change.(T/F)
Answer:- True.
Q.5 Very short answer type questions
1. What is rust? Write its chemical formula.
Answer:- rest is an reddish brown oxide formed by the reaction of iron and oxygen in the
presence of water and moisture.
Chemical formula :- Fe2O3.xH2O
2. Write the conditions necessary for rusting of iron.
Answer:- Conditions necessary for rusting of iron is open surface of the iron object,
presence of air, and presence of moisture.
3. Why are iron object’s painted frequently?
Answer:- Iron object’s painted frequently to prevent rusting.
4. What is galvanization?
Answer:- a layer of aluminium or zinc deposited over surface of iron object this process is called as galvanization
5. Name two metals which are deposited on the surface of iron objects during the process of galvanization.
Answer :- Aluminium or Zinc are two metals which are deposited on the surface of iron
objects during the process of galvanisation.
6. Burning of candle is an example of which type change physical change or chemical change? Give reasons.
Answer:- Burning of candle is an example of chemical change because when candle burns the oxygen in the air reacts and forms carbon dioxide.
7. Why burning of firework is harmful?
Answer:- explosion of firework produces heat light sound and unpleasant gases that
pollute the atmosphere that’s why burning of firework is harmful.
8. What is crystallisation?
Answer:- large crystals of pure substances can be formed from their solutions is called as crystallization.
Q.6 Short answer type questions:
1. Mixing of baking soda and vinegar is a chemical change or a physical change. Discuss?
Answer:- Mixing of baking soda and vinegar is a chemical change because after there
mixing new chemical substance is formed.
2. Explain how cutting and burning of wood are different type of change?
Answer:- The burning of wood leads to the formation of new substances like ash(carbon), carbon dioxide gas, water vapour, heat and light. This change is irreversible and hence a chemical change.While cutting wood into smaller pieces is a physical change as there is no change in the original composition of wood. Also, no new substances are formed.
3. What will happen when carbon dioxide is passed through lime water?
Answer:- When carbon dioxide is passes through lime water it turns milky.
4. Why does the colour of copper sulphate( CuSO) solution change, when an iron nail is dipped into it? Write chemical equation also.
Answer:- The colour of solution of test tube turns green due to the formation of new
substance that is Iron sulphate.
Chemical equation:-
CuSO4 + Fe → FeSO4 + Cu.
5. Magnesium ribbon is burnt and the ash formed is mixed with water. Now answer the following:
(a) Write equation for the burning of magnesium.
(b) What will happen when the mixture of ash and water is added to (a) blue litmus solution and (b) red litmus solution.
(c) Name the substance formed by mixing ash and water. Is it acidic or basic?
Answer:- a. Magnesium (Mg) + Oxygen (O2) → Magnesium oxide (MgO).
b. (a) There is no change in colour of blue litmus paper when we put drop of mixture.
(b) The colour of red litmus paper changes to blue.
c. The solution of magnesium hydroxide above is basic in nature.
Q.7 Long answer type questions:
1. What are physical and chemical changes? Write differences between physical and chemical changes?
Answer:-
a) Physical change are a change in which only physical properties of substances is
change.
b) Result in the formation of new substances is called chemical change.
- Physical change:-
1. There is the change in the physical state properties of substances.
2. No new substance is formed.
3. It is mostly reversible.
● Chemical change:-
1. Chemical properties and chemical composition of substance is changed.
2. New substance is formed.
3. It is mostly irreversible.
2. What do you understand by rusting of iron ? Write the necessary conditions required for rusting of iron objects. How rusting of iron and iron objects can be prevented?
Answer:- This chemical change occurs in iron and articles made of iron when exposed to the atmosphere. A reddish brown or orange coloured layer is formed on the surface of the iron objects due to their reaction with oxygen and water present in the atmosphere. It is called rust and this process is called rusting of iron.
Conditions necessary for rusting of iron
- Open surface of the iron object
- Presence of air (oxygen)
- Presence of moisture or humidity (water)
Prevention of rusting:
We can control rusting by preventing iron articles coming in contact with oxygen and water.
The following methods may be used to prevent rusting:
- By greasing or oiling:- By coating the surface of iron objects with a thin film of oil or grease.
In fact these coats should be applied regularly to prevent rusting. - By painting :- By coating the surface with paint evenly and frequently. . By the process of
- galvanization :- A layer of non reactive metal such as aluminium or zinc is deposited over the surface of iron objects. This process of depositing a layer of non reactive metal over the surface of iron is called galvanization. The layer of non-reactive metal prevents the iron articles from coming in contact with moisture and oxygen directly. Thus it prevents rusting.
3. Give detail of process of crystallisation of copper sulphate (CuSO4).
Answer:- Salt can be obtained by the process of evaporation of seawater. The salt
obtained in this manner is not pure and its crystals are small. The shape of the crystals
cannot be seen clearly. However, large crystals of pure substances can be formed from their solutions. The process is called crystallisation. It is an example of a physical change.
Material Required:- Beaker, water, dil H₂SO, burner, copper sulphate, funnel, filter paper, etc.
Method:- Take 200ml water in a beaker and add a few drops of dilute sulphuric acid. Heat the water. When it starts boiling, add copper sulphate powder slowly while stirring
continuously. Continue adding copper sulphate powder till no more powder can be dissolved. Filter the solution with the help of a funnel and filter paper. Allow it to cool. Do not disturb the solution while it is cooling. Look at the solution after some time.
Observation:- After some time you will get clear and pure crystals of the copper sulphate.
Conclusion:- The process of crystallization is physical change i.e. a change from liquid state to solid state (crystal form),
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