1) Classify the changes
involved in the following processes as physical or chemical changes:
involved in the following processes as physical or chemical changes:
(a) Photosynthesis
(b) Dissolving sugar in water
(c) Burning of coal
(d) Melting of wax
(e) Beating aluminium to make aluminium
foil
foil
(f) Digestion of food
ANSWER: – (a)
Photosynthesis → Chemical change
Photosynthesis → Chemical change
(b) Dissolving sugar in water → Physical change
(c) Burning of coal → Chemical change
(d) Melting of wax → Physical change
(e) Beating aluminium to make
aluminium foil → Physical change
aluminium foil → Physical change
(f) Digestion of food → Chemical change
2) State whether the following statements
are true or false. In case a statement is false, write the corrected statement
in your notebook.
are true or false. In case a statement is false, write the corrected statement
in your notebook.
(a) Cutting a log of wood into
pieces is a chemical change. (True/False)
pieces is a chemical change. (True/False)
(b) Formation of manure from
leaves is a physical change. (True/ False)
leaves is a physical change. (True/ False)
(c) Iron pipes coated with zinc
do not get rusted easily. (True/False)
do not get rusted easily. (True/False)
(d) Iron and rust are the same
substances. (True/False)
substances. (True/False)
(e) Condensation of steam is
not a chemical change. (True/False)
not a chemical change. (True/False)
ANSWER :-
(a)
False
False
Cutting a log of wood
into pieces is a physical change.
into pieces is a physical change.
(b) False
Formation of manure from
leaves is a chemical change.
leaves is a chemical change.
(c) True
(d) False
Iron and rust are
different substances. Rust is iron oxide Fe2 O3
different substances. Rust is iron oxide Fe2 O3
(e) True
3) Fill in the blanks in the
following statements:
following statements:
ANSWER: –
(a)
When carbon dioxide is passed through limewater, it turns milky due to the formation of calcium
carbonate (CaCO3).
When carbon dioxide is passed through limewater, it turns milky due to the formation of calcium
carbonate (CaCO3).
(b) The chemical name of baking
soda is sodium hydrogen carbonate.
soda is sodium hydrogen carbonate.
(c) Two methods by which rusting
of iron can be prevented are galvanization and painting.
of iron can be prevented are galvanization and painting.
(d) Changes in which only
physical properties of a substance change are called physical changes.
physical properties of a substance change are called physical changes.
(e) Changes in which new
substances are formed are called chemical changes.
substances are formed are called chemical changes.
4) When baking soda is mixed
with lemon
juice, bubbles are formed with the evolution of a gas. What type of change is
it? Explain.
with lemon
juice, bubbles are formed with the evolution of a gas. What type of change is
it? Explain.
ANSWER:-When
baking soda (Sodium hydrogen carbonate) is mixed with lemon juice (citric
acid), bubbles are formed. The
bubbles are formed due to the evolution of carbon dioxide gas. This is a chemical change.
baking soda (Sodium hydrogen carbonate) is mixed with lemon juice (citric
acid), bubbles are formed. The
bubbles are formed due to the evolution of carbon dioxide gas. This is a chemical change.
5) When a candle burns, both
physical and chemical changes take place. Identify these changes. Give another
example of a familiar process in which both the chemical and physical changes
take place.
physical and chemical changes take place. Identify these changes. Give another
example of a familiar process in which both the chemical and physical changes
take place.
ANSWER:-When
a candle burns, both physical and chemical changes take place.
a candle burns, both physical and chemical changes take place.
Physical change → Melting of wax
Chemical change → Burning of wax
6) How would you show thatsetting of curd is a chemical change?
ANSWER:-Once
the curd is formed, milk cannot be re-obtained from it. Also, both milk and
curd have different properties. Since these are the properties of a chemical change, setting of curd is a chemical
change.
the curd is formed, milk cannot be re-obtained from it. Also, both milk and
curd have different properties. Since these are the properties of a chemical change, setting of curd is a chemical
change.
7) Explain why burning of wood
and cutting it into small piece are considered as two different types of
changes.
and cutting it into small piece are considered as two different types of
changes.
ANSWER: –
When we burn wood, a new substance, coal, is formed. Therefore, it is a
chemical change. However, when we cut wood, only the shape and size of the wood are changed. No new substance
is formed. Therefore, it is a physical change.
When we burn wood, a new substance, coal, is formed. Therefore, it is a
chemical change. However, when we cut wood, only the shape and size of the wood are changed. No new substance
is formed. Therefore, it is a physical change.
8) Describe how crystals of
copper sulphate
are prepared.
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.
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.
9) Explain how painting of an
iron gate prevents it from rusting.
iron gate prevents it from rusting.
ANSWER:-Rusting
is aided by both moisture (water) and air (oxygen). By painting an iron gate,
we prevent its contact from the air and moisture present in the atmosphere.
Hence, rusting is prevented.
is aided by both moisture (water) and air (oxygen). By painting an iron gate,
we prevent its contact from the air and moisture present in the atmosphere.
Hence, rusting is prevented.
10) Explain why rusting of iron
objects is faster in coastal areas than in deserts.
objects is faster in coastal areas than in deserts.
ANSWER:-Both air and moisture are
required for rusting to take place. In coastal areas, the quantity of moisture
present in air is more than that in deserts. In desert areas, the amount
of moisture in air is even lower.
Therefore, rusting of iron objects is faster in coastal areas.
required for rusting to take place. In coastal areas, the quantity of moisture
present in air is more than that in deserts. In desert areas, the amount
of moisture in air is even lower.
Therefore, rusting of iron objects is faster in coastal areas.
11)The gas we use in the
kitchen is called liquefied petroleum gas (LPG). In the cylinder it exists as a
liquid. When it comes out from the cylinder it becomes a
gas (Change −A) then it burns (Change − B). The following statements
pertain to these changes. Choose the correct one.
kitchen is called liquefied petroleum gas (LPG). In the cylinder it exists as a
liquid. When it comes out from the cylinder it becomes a
gas (Change −A) then it burns (Change − B). The following statements
pertain to these changes. Choose the correct one.
(i) Process − A is a chemical change.
(ii) Process − B is a chemical change.
(iii) Both processes A and B
are chemical changes.
are chemical changes.
(iv) None of these processes is
a chemical change.
a chemical change.
ANSWER: – (ii)
Process − B is a chemical change.
Process − B is a chemical change.
12) Anaerobic bacteria digest
animal waste and produce biogas (Change −
A). The biogas is then burnt as fuel (Change −
B). The following statements pertain to these changes.
animal waste and produce biogas (Change −
A). The biogas is then burnt as fuel (Change −
B). The following statements pertain to these changes.
Choose the correct one.
(i) Process − A is a chemical change.
(ii) Process − B is a chemical change.
(iii) Both processes A and B
are chemical changes.
are chemical changes.
(iv) None of these processes is
a chemical change.
a chemical change.
ANSWER:-iii)
Both processes A and B are chemical changes
Both processes A and B are chemical changes