Selina Concise Chemistry Class 7 ICSE Solutions – Physical and Chemical Changes

Selina Concise Chemistry Class 7 ICSE Solutions – Physical and Chemical Changes

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Selina Concise ICSE Solutions for Class 7 Chemistry Chapter 2 Physical and Chemical Changes

Points to Remember :

1. All changes are classified into two types
   (i) Physical change
   (ii) Chemical change.
2. Physical Change— A physical change is a temporary change in which no new substance is formed and chemical composition remains same. e.g. Melting of ice.
3. Chemical Change— A chemical change is a permanent change, in which a new substances are formed whose chemical composition and physical properties are different, e.g. Burning of a candle.
4. Chemical Reaction— Any chemical change in matter involving its transformation into one or more new substances is called a chemical reaction.
5. Chemical Equations— A chemical equation is the symbolic representation of a chemical reaction using the symbols and the formulae of the substances involved in the reaction.
6. The substances that react with one another are called reactants, and the new substances thus formed are called products.
7. A balanced chemical reaction is one in which the number of the atoms of each element on the
8. reactant side is equal to the number of atoms of that element on the product side.
9. The law of conservation of mass states that mass can neither be created nor be destroyed.
10. A chemical equation gives both qualitative and quantitative information about reactants aftd products.
11. The type of chemical reaction in which two substances combine to form a new substance is known as combination reaction.
12. The type of chemical reaction in which a substance breaks up on heating to form two or more simpler substances,
    which can be either elements or compounds, known as decomposition reaction.

Exercise

Question 1.
(a) Define:
(i) a physical change
(ii) a chemical change
(b) Give two examples for each of the above two changes.
Answer:
(a) (i) Physical chagne : A temporary change in which no new substance is formed, the composition of substance remains the same, though its state, shape and size may change.
(ii) Chemical change : A permanent change in which new substances are formed whose composition and properties are completely different from those of the original substances.
(b) Physical change:
(i) Change of water into its vapours.
(ii) Heating of iron rod.
Chemical change:
(i) Burning of wood.
(ii) Breathing.

Question 2.
What are reversible and irreversible changes ? Give one example for each.
Answer:

1. Reversible change : When a change in a substance can be reversed by changing the conditions, it is said to be a reversible change.
   Example : Melting of ghee or wax.
2. Irreversible change : When a substance can not be brought back to its original state after a change, it is said to be an irreversible change.
   Example : Souring of milk.

Question 3.
Mention a change which is always
Answer:

1. Desirable : Changes that are useful to man are desirable changes, e.g. change of milk into curd.
2. Undesirable : Change that brings about destruction is an undesirable change, e.g. floods and epidemics are undesirable changes.
3. Periodic : Changes that are repeated at regular intervals of time are called periodic changes, e.g. change of day and night.

Question 4.
Is burning a physical change or a chemical change? Why?
Answer:
Burning is a chemical change as new substance is formed with new properties and it cannot be reversed.

Question 5.
A burning candle shows both physical and chemical changes. Explain ?
Answer:
A burning candle produces wax vapours which solidify and form wax again is physical change. At the same time wax on burning produces water vapours and carbondioxide which escape into the atmosphere which are new substances with new properties. It is a chemical change.
Hence burning of candle shows both physical and chemcial changes.

Question 6.
State three differences between evaporation and boiling
Answer:

Evaporation	Boiling
Evaporation is a slow process. Evaporation takes place from the surface of the liquid. Evaporation takes place at all temperature.	Boiling is a fast process. Bgiling takes place from all parts of the liquid. Boiling takes place at a fixed temperature on heating.

Question 7.
State four differences between physical and chemical changes.
Answer:
Four differences are :

Physical change	Chemical change
No new substances with new properties are formed. It can be reversed by simple physical methods. It is temporary change. Energy is neither needed nor produced.	New substances with new properties are formed. Change cannot be reversed by simple physical methods. It is permanent change. Energy is either needed or evolved.

Question 8.
What do you observe when :

1. water is boiled
   On boiling water changes into steam (gas) physical change.
2. a piece of paper is burnt
   On burning piece of paper produces carbon dioxide and ash is left behind. Is a chemical change.
3. some ice cubes are kept in a glass tumbler
   Ice cubes (solid) turn into water (liquid) only state changes (physical change).
4. solid ammonium chloride is heated
   Solid ammonium chloride on heating changes into vapours (change of state) is physical change.
5. an iron nail is kept in tap water for few days
   We observe reddish brown coating on the nail called rust (entirely new substance) is chemical change.
6. a spoon of sugar is heated in a pan
   When a spoon of sugar is heated in a pan, black (charred sugar) (carbon) is seen. Is a chemical change.
7. lighted match stick is brought near the mouth of the test tube containing hydrogen gas.
   We observe that hydrogen bums at the mouth of test tube with blue flame and pop sound is heard. It is chemical change.
8. quick lime is dissolved in water.
   The following two observations will be observed
   (i) A hissing sound is observed.
   (ii) The mixture starts boiling and lime water is obtained.
9.  liittle ammount of curd is added to a bowl containing warm milk and kept for five hours.
   When a little amount curd is added to a bowl containing warm milk and kept for five hours, a permanent change occured.
   The milk will change to curd. On boiling water changes into steam (gas) physical change.

Question 9.
Name a chemical change which takes place in presence of:
Answer:
Heat:

1. Burning of paper.
2. Cooking of food need heat.

Light :

1. Formation of food by plants i.e. photosynthesis need light.
2. Hydrogen and chlorine react in presence of light

Electricity:

1. Water breaks into hydrogen and oxygen on passing electricity.
2. Sodium chloride solution breaks up into its components sodium and Chlorine on passing electric current through sodium chloride solution.

Objective Type Questions

Question 1.
Fill in the blanks.

1. A new substance is always formed in a chemical change.
2. Melting of ice is a physical change.
3. When a candle burns, wax melts. Melting of wax is a physical change.
4. Chemical change occurs as a result of rearrangement of atoms between two substances.
5. Burning of a fuel is a chemical change.
6. Revolution of the earth around the sun is a natural periodic change.
7. Growing of a seedling into a plant is chemical change.

Question 2.
State whether the following are physical or chemical changes.

1. glowing of a bulb — physical change,
2. burning of sugar — chemical change,
3. heating of water — physical change,
4. growing of a piglet into a pig — chemical change,
5. burning of wood — chemical change,
6. passing electric current through a heater rod — physical change.
7. water cycle in nature — physical change,
8. respiration in living beings — chemical change,
9. shaping a piece of glass — physical change,
10. lightning — chemical change,
11. (energy produced) dissolving sugar in water — physical change,
12. heating a mixture of iron filings and sulphur—chemical change.
13. mixing oil with water — physical change.
14. cutting wood into small pieces — physical change,
15. photosynthesis — chemical change.
16. Digestion of food — chemical change.
17. Melting of wax — physical change.
18. Boiling of an egg — chemical change.
19. Slaking of lime — chemical change.

Question 3.
Match the following:



Answer:

Question 4.
Write true or false against the following statements :

1. Cutting of paper into pieces is a chemical change.
   False
2. Rusting of iron is a chemical change.
   True
3. Earthquake is a desirable change.
   False
4. Melting of ice is a physical change.
   True
5. Burning of sugar is a temporary change.
   False

Multiple Choice Questions

Tick (√) the correct alternative from the choice given for the following statements :
Question 1.
A substance which can not sublime is
Answer:

1. iodine
2. camphor
3. sugar
4. dry ice

Question 2.
When you put some ice cubes in a glass, droplets of water are formed on the outerwall of the glass. This explains the phenomenon of
Answer:

1. melting
2. freezing
3. condensation
4. evaporation

Question 3.
Burning is a
Answer:

1. slow process
2. fast process
3. natural process
4. none of the above

Question 4.
Which one of the following is volatile in nature ?
Answer:

1. common salt
2. petrol
3. water
4. milk

Question 5.
An example of both physical and chemical change is
Answer:

1. burning of candle
2. melting of ice
3. cooking of food
4. blowing of bulb

Question 6.
The compound rust is a hydrated oxide of
Answer:

1. copper
2. aluminium
3. iron
4. gold

Question 7.
When sugar is heated, its colour changes into
Answer:

1. red
2. brown
3. black
4. grey

Question 8.
A pop sound is heard when a lighted match stick is brought
near the mouth of a jar. This indicates the release of gas.
Answer:

1. oxygen
2. hydrogen
3. nitrogen
4. water-vapour

Question 9.
When we add water to the following substances, which one will show a chemical change ?
Answer:

1. salt
2. sugar
3. oild
4. quick lime