Physical and Chemical Changes Chapter 2 (ICSE)
Class 7 - Concise Chemistry Selina
Physical Changes: The properties does not
change permanently.
Chemical Changes: The properties change
permanently and forma new substance.
Keywords:
Physical
change Chemical
change Vapors Sublimation
Dissolution Boiling Cooling States
Exercise:
Question 1
Define:
(a) a
physical change
(b) a
chemical change
(c) Give two
examples for each of the above two changes.
Solution:
(a) A Physical
change is a temporary change in substance. No new substance is formed and the
chemical composition and properties of the original substance remains the same,
even though some of its physical properties like state, color, shape, size,
appearance etc. may change.
(b) A Chemical change is a permanent change in substances. A new substances are formed whose chemical composition, physical and chemical properties are different from those of the original substances.
(c) Two examples
of physical and chemical changes:
Examples of
physical changes:
(1) Dissolution of salt in water.
(2) Water freezing into ice.
(1) Rusting of iron.
(2) Ripening of fruits.
What are
reversible and irreversible changes? Give one example for each.
Solution:
Reversible: When a change in a substance can
be reversed by changing the conditions, it is said to be a reversible change.
Example —
Vaporization of water.
Irreversible: If a substance cannot be brought back to its original state after a change, it is said to be an irreversible change.
Example —
Grinding of wheat into flour.
Mention a
change which is always
(a)
Desirable
(b)
Undesirable
(c) Periodic
(a)
Photosynthesis
(b) Rusting
of iron
(c) Change
of day and night
Is burning a
physical change or a chemical change? Why?
Solution:
Burning is a
chemical change because a substance reacts with air or oxygen to form an oxide
that is a new substance having new physical and chemical properties, and it
evaluates energy in the form of heat and light.
Question 5
A burning
candle shows both physical and chemical changes. Explain.
Solution:
When the
candle is lit, the wax undergoes oxidation, resulting in the production of
carbon dioxide and water vapors as new substances, accompanied by the release
of heat and light. This indicates a chemical change. Additionally, the candle
gradually decreases in size as it burns. As the wax melts and transitions into
a liquid state, it then solidifies upon reaching the floor, exemplifying a
physical change. Therefore, the burning of a candle involves both physical and
chemical changes.
State three
differences between evaporation and boiling.
Solution:
|
Evaporation |
Boiling |
|
·
Evaporation is a slow process. ·
It takes place from the surface of the liquid ·
It takes place at all temperatures below its boiling point. |
·
Boiling is a fast ·
Evaporation takes place from all parts of the
liquid during boiling process. ·
It takes place at a fixed temperature i.e. at
its boiling point.
|
State four
differences between physical and chemical changes.
|
Physical Changes |
Chemical Changes |
|
·
Change is temporary ·
No new substance is formed. There is change
only in physical properties. ·
Change can be reversed by simple methods. ·
Heat or light may or may not be given out or
consumed. |
·
Change is permanent ·
New substances are formed with entirely
different properties ·
Change cannot be reversed by simple methods ·
Heat or light or both are given out or
consumed. |
What do you
observe when:
(a) Water is
boiled
(b) a piece
of paper is burnt
(c) some ice
cubes are kept in a glass tumbler
(d) solid
ammonium chloride is heated
(e) an iron
nail is kept in tap water for few days
(f) a spoon
of sugar is heated in a pan
(g) a
lighted match stick is brought near the mouth of the test tube containing
hydrogen gas
(h) quick
lime is dissolved in water
(i) A little
amount of curd is added to a bowl containing warm milk and kept for five hours?
Solution:
(a) When
water is boiled it converts into vapours.
(b) When a
piece of paper is burnt, it changes into ash after giving light and heat.
(c) When
some ice cubes are kept in a glass tumbler, water droplets are seen on the outer
wall of the tumbler, and after some time ice cubes change into water.
(d) When
solid ammonium chloride is heated, it directly changes into vapors.
(e) When an
iron nail is kept in tap water for few days a reddish-brown coating is seen on
the nail.
(f) When a
spoon of sugar is heated in a pan it first melts and changes its color to
reddish-brown and finally it turns black and gets charred.
(g) When a
lighted match stick is brought near the mouth of the test tube containing
hydrogen gas a pop sound is heard and the flame of the match stick gets
extinguished.
(h) When
quick lime is dissolved in water a large amount of heat energy is evolved along
with a hissing sound is produced.
(i) The bowl
containing milk changes into curd.
Name a
chemical change that takes place in the presence of:
(a) Heat
(b) Light
(c)
Electricity
Solution:
(a) Cooking of food
(b)
Photosynthesis
(c)
Decomposition of water to hydrogen and oxygen
Question 1
Fill in the blanks
(a) A new
substance is always formed in a ............... change.
(b) Melting
of ice is a ............... change.
(c) When a
candle burns, wax melts. Melting of wax is a ............... change.
(d) Chemical
change occurs as a result of ............... between two substances.
(e) Burning
of a fuel is a ............... change.
(f)
Revolution of the earth around the sun is a ............... change.
(g) Growing
of a seedling into a plant is a ............... change.
Solution:
(a) chemical (b)
physical (c) physical (d) reaction
(e) chemical (f)
periodic (g) slow
Question 2
State
whether the following are physical or chemical changes.
(a) Glowing
of a bulb
(b) burning
of sugar
(c) heating
of water
(d) growing
of a piglet into a pig
(e) burning
of wood
(f) passing
electric current through a heater's rod
(g) water
cycle in nature
(h)
respiration in living beings
(i) shaping
a piece of glass
(j)
lightning
(k)
dissolving sugar in water
(l) heating
a mixture of iron fillings and sulphur
(m) mixing
oil with water
(n) cutting
wood into small pieces
(o)
photosynthesis
(p)
Digestion of food
(q) Melting
of wax
(r) Boiling
of an egg
(s) Slaking
of lime
Solution:
(a) Glowing of a bulb —
Physical change
(b) burning of sugar — Chemical change
(c) heating of water —
Physical change
(d) growing of a piglet into a pig — Chemical
change
(e) burning of wood —
Chemical change
(f) passing electric current through a heater's rod — Physical change
(g) water cycle in nature —
Physical change
(h) respiration in living beings —
Chemical change
(i) shaping a piece of glass —
Physical change
(j) lightning —
Chemical change
(k) dissolving sugar in water —
Physical change
(l) heating a mixture of iron fillings and sulphur — Chemical change
(m) mixing oil with water — Physical change
(n) cutting wood into small pieces — Physical
change
(o) photosynthesis —
Chemical change
(p) Digestion of food —
Chemical change
(q) Melting of wax —
Physical change
(r) Boiling of an egg —
Chemical change
(s) Slaking of lime —
Chemical change
Question 3
Match the following:
|
Column A |
Column B |
|
(a) Evaporation of water (b) Milk turning sour (c) Earthquake (d) Change of seasons (e) Glowing of bulb |
(i) Non-periodic change (ii) Periodic change (iii) Chemical change (iv) Physical change (v) Reversible change |
|
Column A |
Column B |
|
(a) Evaporation of water (b) Milk turning sour (c) Earthquake (d) Change of seasons (e) Glowing of bulb |
(iv) Physical change (iii) Chemical change (i) Non-periodic change (ii) Periodic change (v) Reversible change |
Question 4
Write True or False against the following statements:
(a) Cutting of paper into pieces is a chemical change.
(b) Rusting of iron is a chemical change.
(c) Earthquake is a desirable change.
(d) The melting of ice is a physical change.
(e) Burning of sugar is a temporary change.
Correct
Statement — Cutting of paper into pieces is a physical change not a chemical
changes.
(b) True
(c) False
Correct
Statement — Earthquakes is an undesirable and natural change.
(d) True
(e) False
Correct
Statement — Burning of sugar is a permanent change or chemical change.
Question 5
Select the correct alternative(MCQ):
(a) A substance which cannot sublime is:
(i) Iodine (ii)
camphor (iii) sugar (iv) dry ice
Solution:
(iii) sugar
Reason —
Sugar is not a sublime substance because it does not directly changes into
vapour on heating.
(b) When you
put some ice cubes in a glass droplets of water are formed on the outer wall of
the glass. This explains the phenomenon of:
(i) Melting (ii) freezing (iii) condensation (iv) evaporation
Solution:
(iii) condensation
Reason — Water vapour present in air when comes
in contact with the outer wall of the cold glass
condenses to form
water droplets.
(c)
Burning is a:
(i) slow process (ii) fast process (iii)
natural process (iv) none of the
above
Solution:
(ii) fast
process
Reason —
Burning is a fast process, because it shows immediate reaction.
(d)
Which one of
the following is volatile in nature?
common salt
(i) petrol (ii)
water (iii) milk (iv) curd
Solution:
(i) petrol
Reason —
Petrol is volatile. It is easily evaporated at normal temperature.
An example of both physical and chemical change is:
(i) burning of candle (ii) melting
of ice
(iii) cooking
of food (iv) glowing of bulb
(i) burning of a candle
Reason — When a candle is ignited, it creates
carbon dioxide and water vapors as new substances, accompanied by the release
of heat and light. Additionally, the candle gradually diminishes in size,
indicating a chemical change. As the candle burns, the wax liquefies and drips
onto the floor before solidifying again, representing a physical change.
(f)
The compound
rust is a hydrated oxide of:
(i) Copper (ii) Aluminum (iii) Iron (iv) Gold
(iii) Iron
Reason — The hydrated oxide of iron is called rust.
(g) When sugar is heated its color changes into:
(i) Red (ii) blue (iii) black (iv) grey
Solution:
(iii) black
Reason — When sugar is heated, it first melts
and changes its colour to reddish-brown and finally it
turns black and gets
charred.
A pop sound
is heard when a lighted match stick is brought near the mouth of a jar. This
indicates the release of:
(i) oxygen
gas (ii) hydrogen gas (iii) nitrogen gas (iv) water vapor
(ii) hydrogen gas
Reason — When a lighted match stick is brought
near the mouth of the jar containing hydrogen
gas a pop sound is
heard and the flame of the match stick gets extinguished.
(i)
When we add
water to the following substances, which one will show a chemical change?
(i) Salt (ii)
sugar (iii) oil (iv)
quick lime
(iv) quick lime
Reason — Quick lime reacts with water to form
calcium hydroxide and a large amount of heat is
produced which is a
chemical change.
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