Physical and Chemical Changes
Solutions for Chemistry, Class 7, ICSE
Exercise
9 questionsQuestion 1
Define:
(a) a physical change
(b) a chemical change
(c) Give two examples for each of the above two changes.
Answer:
(a) A physical change is a temporary change in which no new substance is formed and the chemical composition 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 which new substances are formed whose chemical composition and physical and chemical properties are different from those of the original substance.
(c) Examples of physical changes:
Dissolution of salt in water.
Water freezing into ice.
Examples of chemical changes:
Rusting of iron.
Ripening of fruits.
Question 2
What are reversible and irreversible changes? Give one example for each.
Answer:
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.
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.
Question 3
Mention a change which is always
(a) desirable
(b) undesirable
(c) periodic
Answer:
(a) Photosynthesis
(b) Rusting of iron
(c) Change of day and night
Question 4
Is burning a physical change or a chemical change? Why?
Answer:
Burning is a chemical change because a substance reacts with air or oxygen to form a new substance i.e. oxide, with evolution of energy in form of heat and light.
Question 5
A burning candle shows both physical and chemical changes. Explain.
Answer:
When a candle is lighted it, the carbon present in wax oxidises and produces carbon dioxide and water vapours as the new substances and along with it there is evolution of heat and light. This shows a chemical change. The candle also becomes smaller and smaller. When a candle burns wax melts and turns into liquid state that falls on floor and again it solidifies which shows a physical change. So burning of a candle is both a physical and a chemical change.
Question 6
State three differences between evaporation and boiling.
Answer:
| Evaporation | Boiling |
|---|---|
| Evaporation is a slow process. | Boiling is a fast process. |
| It takes place from the surface of the liquid. | Evaporation takes place from all parts of the liquid during boiling. |
| It takes place at all temperatures below its boiling point. | It takes place at a fixed temperature i.e. at its boiling point. |
Question 7
State four differences between physical and chemical changes.
Answer:
| Physical changes | Chemical changes |
|---|---|
| Change is temporary. | Change is permanent. |
| No new substance is formed. There is change only in physical properties. | New substances are formed with entirely different properties. |
| Change can be reversed by simple methods. | Change cannot be reversed by simple methods. |
| Heat or light may or may not be given out or consumed. | Heat or light or both are given out or consumed. |
Question 8
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) little amount of curd is added to a bowl containing warm milk and kept for five hours?
Answer:
(a) When water is boiled it converts into vapours.
(b) When a piece of paper is burnt, it changes into ash.
(c) When some ice cubes are kept in a glass tumbler, water droplets are seen on the outer wall of the tumbler and after sometime ice cubes changes into water.
(d) When solid ammonium chloride is heated, it directly changes into vapours.
(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 colour 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.
Question 9
Name a chemical change which takes place in presence of:
(a) Heat
(b) Light
(c) Electricity
Answer:
(a) Cooking of food
(b) Photosynthesis
(c) Decomposition of water to hydrogen and oxygen
Objective Type Questions
13 questionsQuestion 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.
Answer:
(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
Answer:
(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 | (i) Non-periodic change |
| (b) Milk turning sour | (ii) Periodic change |
| (c) Earthquake | (iii) Chemical change |
| (d) Change of seasons | (iv) Physical change |
| (e) Glowing of bulb | (v) Reversible change |
Answer:
| Column A | Column B |
|---|---|
| (a) Evaporation of water | (iv) Physical change |
| (b) Milk turning sour | (iii) Chemical change |
| (c) Earthquake | (i) Non-periodic change |
| (d) Change of seasons | (ii) Periodic change |
| (e) Glowing of bulb | (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) Melting of ice is a physical change.
(e) Burning of sugar is a temporary change.
Answer:
(a) False
Correct Statement — Cutting of paper into pieces is a physical change.
(b) True
(c) False
Correct Statement — Earthquake is an undesirable change.
(d) True
(e) False
Correct Statement — Burning of sugar is a permanent change.
Question 5a
Select the correct alternative:
A substance which cannot sublime is:
- iodine
- camphor
- sugar
- dry ice
Answer:
sugar
Reason — Sugar is not a sublime substance because it does not directly changes into vapour on heating.
Question 5b
Select the correct alternative:
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:
- melting
- freezing
- condensation
- evaporation
Answer:
condensation
Reason — Water vapour present in air when comes in contact with the outer wall of the cold glass condenses to form water droplets.
Question 5c
Select the correct alternative:
Burning is a:
- slow process
- fast process
- natural process
- none of the above
Answer:
fast process
Reason — Burning is a fast process.
Question 5d
Select the correct alternative:
Which one of the following is volatile in nature?
- common salt
- petrol
- water
- milk
Answer:
petrol
Reason — Petrol is volatile i.e. it easily gets evaporated at normal temperature.
Question 5e
Select the correct alternative:
An example of both physical and chemical change is:
- burning of candle
- melting of ice
- cooking of food
- glowing of bulb
Answer:
burning of candle
Reason — When a candle is lighted it produces carbon dioxide and water vapours as the new substances and along with it there is evolution of heat and light. The candle also becomes smaller and smaller. This shows a chemical change. When a candle burns wax melts and turns into liquid state that falls on floor and again it solidifies which shows a physical change.
Question 5f
Select the correct alternative:
The compound rust is a hydrated oxide of:
- copper
- aluminium
- iron
- gold
Answer:
iron
Reason — Hydrated oxide of iron is called rust.
Question 5g
When sugar is heated its colour changes into:
- red
- blue
- black
- grey
Answer:
black
Reason — When sugar is heated, it first melts and changes its colour to reddish-brown and finally it turns black and gets charred.
Question 5h
A pop sound is heard when a lighted match stick is brought near the mouth of a jar. This indicates the release of:
- oxygen gas
- hydrogen gas
- nitrogen gas
- water vapour
Answer:
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.
Question 5i
When we add water to the following substances, which one will show a chemical change?
- salt
- sugar
- oil
- quick lime
Answer:
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.