chemistry puzzles class 7
Answers
Page No 90:
Question A:
Solve the following crossword puzzle:
figure
Across
2. the process of coating a thin layer of tin on utensils
4. An alloy of iron
5. A reddish brown, soft, porous, flaky coating on iron articles
Down
1. The common name of acetic acid
3. The colour of crystals of copper sulphates
ANSWER:

Page No 91:
Question B.1:
Properties of substances usually remain same in a
(a) chemical changes
(b) physical change
(c) periodic change
(d) irreversible change
ANSWER:
(b) physical change
During a physical change, the properties of substances remain the same.
Page No 91:
Question B.2:
Melting of wax and heating of blade are the examples of
(a) physical change
(b) chemical change
(c) periodic change
(d) irreversible change
ANSWER:
(a) physical change
Melting of wax and heating of blade are the examples of physical change.
Page No 91:
Question B.3:
Eating away of iron due to rusting is called
(a) corrosion
(b) tinning
(c) alloying
(d) none of these
ANSWER:
(a) corrosion
Eating away of iron due to rusting is called corrosion.
Page No 91:
Question B.4:
Condition necessary for rusting is
(a) presence of air
(b) presence of water
(c) both (a) and (b)
(d) none of these
ANSWER:
(c) both (a) and (b)
Both the presence of air and water are necessary for rusting.
Page No 91:
Question B.5:
New substance is formed in this change
(a) physical
(b) chemical
(c) periodic
(d) irreversible
ANSWER:
(b) chemical
In a chemical change, a new substance is formed. This newly formed substance shows different properties from the original substance.
Page No 91:
Question B.6:
Is an alloy
(a) zinc
(b) tin
(c) stainless steel
(d) gold
ANSWER:
(c) stainless steel
Stainless steel is an alloy of iron.
Page No 91:
Question B.7:
The metal used for galvanisation is
(a) tin
(b) zinc
(c) lead
(d) copper
ANSWER:
(b) zinc
Galvanisation is a process of applying a thin coat of zinc on the surface of the iron sheets so as to prevent it from rusting.
Page No 91:
Question C:
Fill in the blanks:
1. ........................ is hydrated ferric oxide.
2. ........................ is used for galvanising iron.
3. We get ........................ by the process of crystallisation.
4. A chemical change is usually ........................ and permanent.
5. All chemical changes involve ........................ or ........................ of energy.
ANSWER:
1. Rust is hydrated ferric oxide.
2. Zinc is used for galvanising iron.
3. We get pure substances by the process of crystallisation.
4. A chemical change is usually irreversibleand permanent.
5. All chemical changes involve absorptionor liberation of energy.
Page No 91:
Question D:
Match the items in Column A with the items in Column B:
Column AColumn B1. Ripening of fruit(a) Stainless steel2. Cutting of wood(b) Breaking a brick3. Galvanisation(c) Chemical change4. Alloy(d) Zinc (e) Physical change
ANSWER:
Column AColumn B1. Ripening of fruit(c) Chemical change2. Cutting of wood(e) Physical change3. Galvanisation(d) Zinc4. Alloy(a) Stainless steel
Page No 91:
Question E:
Write True (T) or False (F) against the following statements in the given brackets:
1. Souring of milk is a physical change. ( )
2. Digestion of food is a physical change. ( )
3. Crystallisation is widely used to prevent rusting. ( )
4. Formation of steam from water on heating is a physical change. ( )
5. Glowing of the filament of a bulb is an example of a chemical change. ( )
ANSWER:
1. False
Souring of milk is a chemical change.
2. False
Digestion of food is a chemical change.
3. False
Crystallisation is widely used to obtain a compound in its purest form.
4. True
5. False
Glowing of the filament of a bulb is an example of a physical change.
Explanation:
1.Vinegar
2.Tinning
3.Blue
4.Stainless Steel
5.Rust
