Changes Around Us Test

Result

Changes Around Us Test - 4

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Weekly Quiz Competition

Question 1
1 / -0

On lighting a candle, Paheli observed the following changes:

(i) Wax was melting.
(ii) Candle was burning.
(iii) The size of the candle was reducing.
(iv) Molten wax was getting solidified.

Among the above changes, those which can be reversed are

A
(i) and (ii)

B
(ii) and (iii)

C
(iii) and (iv)

D
(i) and (iv)

Solution

Solid wax can be melted on heating and molten wax can be re-solidified by cooling.
Thus, both these processes are reversible.

Question 2

1 / -0

Study the dichotomous key and identify the type of change represented by W, X, Y, Z and U.

I. (i) Molten wax is poured into a round mould.
(ii) The candle formed is lighted up. Go to II
II. (i) Melting of wax
(ii) Burning of wax
III. (i) Solidification of the wax on cooling
(ii) Formation of flame

	W	X	Y	Z	U
(1)	Physical	Physical	Chemical	Physical	Chemical
(2)	Physical	Chemical	Physical	Physical	Physical
(3)	Physical	Chemical	Chemical	Physical	Physical
(4)	Physical	Physical	Physical	Physical	Physical

(1)

(2)

(3)

(4)

Solution

Melting of wax by heating and solidification of wax on cooling are physical changes and are reversible.
Howerver, burning of wax is a chemical change during which the paraffin wax combines with oxygen to produce a flame and carbon dioxide and water are formed as products.

Question 3

1 / -0

Consider the following statements regarding the physical and chemical changes, and choose the correct option from the table.

A. A physical change is always brought about by heating a substance.
B. A chemical change always involves a change in the state.
C. A chemical change may be reversed under suitable conditions.
D. A physical change is not always reversible.
E. A chemical change invariably results in the formation of a new substance.

	A	B	C	D	E
(1)	x	x			x
(2)		x	x		x
(3)	x	x
(4)		x			x

(1)

(2)

(3)

(4)

Solution

Statement A is incorrect because a physical change may be brought about by cooling (e.g. freezing of water) or by changing pressure (e.g. liquefaction of LPG).

Statement B is incorrect because there are chemical changes where the state of the reactants and products is the same.

The statements C, D and E are correct.

Question 4
1 / -0

Which of the following statements are true when milk changes into curd?

(i) Its state is changed from liquid to semi-solid.
(ii) It changes colour.
(iii) It changes taste.
(iv) The change cannot be reversed.

A
(i) and (ii)

B
(ii) and (iii)

C
(i), (iii) and (iv)

D
(i) to (iv)

Solution

All the changes specified in (i), (iii) and (iv) are observed.
However, both milk and curd are white in colour, so no change in colour is involved.
Question 5
1 / -0

Analyse the two statements given below and select the correct option.

Statement I: 2CO + O₂ 2CO₂ represents a balanced chemical equation.
Statement II: The number of atoms of each type on both the sides of the equation is equal.

A
Both statements I and II are correct and statement II is the correct reason for I.

B
Both statements I and II are correct, but statement II is not the correct reason for I.

C
Statement I is correct, but statement II is incorrect.

D
Both statements I and II are incorrect.

Solution

In a balanced chemical equation, the number of atoms of each element on both the sides of the equation is equal.
Hence, both statements I and II are correct and statement II is the correct reason for I.
Question 6
1 / -0

Select the decomposition reactions from among the following:

(i)
(ii)
(iii)
(iv)

A
(i) and (ii)

B
(iii) and (iv)

C
(ii) and (iii)

D
(i) and (iv)

Solution

(iii) and (iv) are decomposition reactions because one reactant breaks down to form more than one product.
(i) is a neutralisation reaction, which is a type of double displacement reaction.
(ii) is the example of a combination reaction.
Question 7
1 / -0

Rolling of chapati and baking of chapati are the changes that

A
can be reversed

B
cannot be reversed

C
can be reversed and cannot be reversed, respectively

D
cannot be reversed and can be reversed, respectively

Solution

Rolling out a chapati from dough balls is a reversible, physical change.
But on baking, the dough undergoes a chemical change and the process cannot be reversed.
Question 8
1 / -0

A chemical change may involve

A
change in colour only

B
change in temperature only

C
evolution of gas only

D
Any or all of the above

Solution

A chemical change may involve any one or all of the mentioned changes.
Question 9
1 / -0

Paheli's mother made a concentrated sugar syrup by dissolving sugar in hot water. On cooling, crystals of sugar got separated.

This indicates a

A
chemical change that can be reversed

B
physical change that can be reversed

C
physical change that cannot be reversed

D
chemical change that cannot be reversed

Solution

Dissolution of sugar in water is a physical change.
On cooling a supersaturated solution, the excess sugar that had dissolved on heating precipitates out.
Hence, the observation indicates that it is a reversible physical change.
Question 10
1 / -0

Which of the following reactions correctly represents the balanced equation for the combustion of magnesium?

A
Mg + O₂ 2MgO

B
Mg + 2O₂ 2MgO

C
2Mg + O₂ 2MgO

D
Mg + O₂ 2Mg₂O₂

Solution

2Mg + O₂ 2MgO

The given reaction represents the combustion of magnesium in the presence of oxygen.
Question 11
1 / -0

Which of the given changes can be reversed?

A
Only A and B

B
Only A and C

C
Only C and D

D
Only B and D

Solution

Ice can be melted to form water again.
Also, the paper boat can be unfolded to get back the sheet of paper.
Hence, the changes represented in 'C' and 'D' are reversible.
Question 12
1 / -0

What is 'Z' in the given process?

Woollen yarn Z

A
Hand-knitted sweater

B
Sweater coat

C
Cardigan

D
Shawl

Solution

Hand-knitted woollen fabric can easily be undone to obtain the woollen yarn.
Question 13
1 / -0

If the sign '' is used in a chemical reaction, what does it indicate?

A
Temperature of the reactants is equal to the temperatures of the products.

B
Mass of all the reactant molecules is equal to the mass of all the product molecules.

C
All the reactants have converted to products.

D
Product molecules recombine to form reactant molecules and the reaction is reversible.

Solution

The sign indicates a reversible process in which the product molecules recombine to form reactant molecules and the reaction is reversible.
Question 14
1 / -0

Which of the following changes can be reversed?

A
Milk Curd

B
Animal waste Manure

C
Mango Mango pickle

D
Steam Ice

Solution

Milk converted to curd, mango converted to mango pickle, and animal waste converted to manure are examples of the changes which cannot be reversed.
Inter-conversion of states of water, e.g., steam to ice, can easily be reversed by changing the temperature and pressure conditions.
Question 15
1 / -0

Which of the following reactions involves a physical change?

A
Photosynthesis

B
Respiration

C
Sublimation

D
Rusting

Solution

Sublimation is a process by which a solid on heating converts to vapour without going into the liquid state.
This is a physical change.
Question 16
1 / -0

In a chemical reaction, _______ are produced as a result of a chemical change.

A
reactants

B
products

C
catalysts

D
heat and light

Solution

A chemical reaction involves the formation of a new substance having properties different from those of the reacting species.
The new substances formed are referred to as products.
Question 17
1 / -0

Which of the following is not a physical change?

A
Evaporation

B
Freezing

C
Mixing of sugar in coffee

D
Oxidation

Solution

Oxidation of a substance is a chemical change.
During oxidation, the substance reacts with oxygen or another electronegative element to form a new substance, having properties different from those of the reacting substance.
Question 18
1 / -0

Which of the following reactions is always exothermic?

A
Combination

B
Decomposition

C
Displacement

D
Double displacement

Solution

Combination reactions are always exothermic.
Question 19
1 / -0

Which of the following changes cannot be reversed?

A
Hardening of cement

B
Freezing of ice cream

C
Opening of a door

D
Melting of chocolate

Solution

Concrete is initially fluid, but with time it crystallises and sets into a hard mass of dense material. This is an irreversible process because once set, the concrete cannot be made fluid again.
Question 20
1 / -0

Salt can be separated from its solution (salt dissolved in water) because

A
mixing of salt in water is a change that can be reversed by cooling the solution

B
mixing of salt in water is a change that cannot be reversed

C
mixing of salt in water is a permanent change

D
mixing of salt in water is a change that can be reversed by evaporation

Solution

Mixing of salt in water is a reversible change because the components of the mixture can be separated by evaporation. Water vaporises and salt is left behind.
Thus, dissolution of salt in water is a reversible change.