Physical And Chemical Changes Class-8 Dalal ICSE New Simplified Chemistry Chapter-2 . We Provide Step by Step Solutions of Exercise/Lesson -2 Physical And Chemical Changes with Objective Type Questions, Fill in the blanks and Give reason of Dr Viraf J Dalal Middle School Chemistry Allied Publishers. Visit official Website for detail information about ICSE Board Class-8.

EXERCISE-2

## Question 1.

Change – is the law of nature which occurs in everyday life.
State when a substance [Le. matter] undergoes a kind of change.

A substance i.e. matter undergoes a kind of change when subjected to energy. When energy is applied to or removed from matter, it is said to be going under changes.

Example – sugar crystals caramelize when they are heated, i.e. heat energy is applied.

Question 2.
Give a reason why

(a) Freezing of water is a reversible change while burning of a candle – an irreversible change,
(b) Change of seasons is a periodic change while change of weather is a non-periodic change.

(a) Reversible changes are changes in a substance that can be reversed back to their original state. Irreversible changes are changes in substance that will have a permanent effect, and they cannot be reversed.

Freezing of water is a reversible change because frozen water can be melted on heating.

Burning of a candle is an irreversible change because once the candle melts, the wax cannot be molded again.

(b) Periodic changes are changes that occur at regular intervals. Non-periodic changes are changes that take place at irregular intervals.

Change of seasons is periodic because it changes in a particular number of months. Change of weather is sudden and not regular, that’s why it is a non-periodic change.

Question 3.
State which of the following pertain to – chemical changes.

(a) Substance retains its identity
(b) The change is permanent
(c) No new substance is formed
(d) Is easily reversible
(e) Produces one or more new substances by change in its composition
(f) The change is temporary
(g) The composition and properties of the original substance are not altered.

Chemical change :

(b) The change is permanent.
(e) Produces one or more new substances by change in its composition.
(g) The composition and properties of the original substance care not altered.

Question 4.
Give four reasons why burning of a magnesium ribbon in air is considered a chemical change.
Burning of magnesium ribbon in air is a chemical change because:

• It is an irreversible change, as after burning the ribbon cannot be unburnt.
• A new product is formed with new properties, i.e. MgO
• The composition of magnesium changes.
• Heat energy is required for this chemical change.

Question 5.
Compare the energy changes involved during a physical and a chemical change.

Physical Change: There is no addition or deduction of energy during the physical change, but the energy required for completion of change is released when the change is reversed.

Chemical Change: Energy like light, pressure, heat energy is required for chemical changes. Such energy is generally absorbed or released during the chemical change.

Question 6.
Is change of state of matter – a physical or a chemical change. Give reasons.
Change in state of matter is a physical change because of the physical condition and appearance changes but not the chemical composition.

Question 7.
State which of the following terms connected with change of state of matter ie.

(a) Melting
(b) Vaporisation
(c) Condensation
(d) Freezing

pertain to –

1. Conversion of a liquid into a vapour or gas
2. Conversion of a liquid into a solid.
3. Conversion of a solid into a liquid on heating.

1. Conversion of liquid into vapour or gas. – (b) Vaporization
2. Conversion of liquid into solid. – (d) Freezing
3. Conversion of solid into a liquid on heating. – (a) Melting

Question 8.

Potassium chloride is added to water and stirred. A salt solution is obtained which is then boiled leaving behind a residue.

(a) Is the above change physical or chemical
(b) Name the residue which remains behind after the salt solution is boiled.
(c) Is the change reversible or not reversible.
(d) Are the composition and properties of the original substance altered.
(e) Give a reason why the above experimentation would not be possible, if calcium carbonate is taken, in place of potassium chloride.

(a) Physical change.
(b) White residue of potassium chloride.
(c) Change is reversible.
(d) No, the composition and properties are not altered.
(e)  we take calcium carbonate the above experiment cannot be possible as no new substance with new properties is produced, since CaCo3 is insoluble in water. Boiling will also not work as CaCo3 is in solid form in water.

Question 9.
Give a reason why –

(a) Heating a platinum wire is a reversible change but, heating a magnesium wire is an irreversible change.
(b) Addition of zinc nitrate to water is a physical change but, addition of zinc to dilute nitric acid is a chemical change.
(c) Rusting of iron is a chemical change but, magnetization of iron is a physical change.

(a) When Platinum gets heated, it changes color. When cooled it retains its earlier color and it does not react easily with oxygen in the air. Magnesium reacts with Oxygen in the air and forms magnesium oxide which cannot be undone.

(b) Zinc nitrate dissolves in water and hence is a physical change. The reaction between zinc and nitric acid produces H2 gas which gets oxidized causing a chemical change.

(c) Iron rust has a different composition from iron and has new properties, which cannot be reversed. Magnetization of iron is a temporary change, which does not change the composition of iron but only physical properties.

Question 10.
Select the chemical changes from the following list of changes

(a) Vaporisation of water into water vapour.
(b) Boiling of milk.
(c) Respiration in mammals.
(d) Rotting of eggs.
(e) Drying of a fruit
(f) The carbon cycle.
(g) Occurrence of lightening.
(h) Breaking of glass.
(i) Butter turning rancid.
(j) Glowing of an electric bulb.
(k) Crystallisation of a salt from its solution.
(l) Change of seasons.
(m) Preparation of carbon dioxide from calcium carbonate & dilute hydrochloric acid.

The following changes are chemical changes:

(c) Respiration in mammals – Inhalation of oxygen and exhaling carbon dioxide, which is irreversible and composition of oxygen changes.

(d) Rotting of eggs – Change in composition of the content of eggs, and it is irreversible.

(g) Occurrence of lightning – Energy is produced in this change.

(i) Butter turning rancid – The composition of butter changes in this change.

(m) Preparation of carbon dioxide from calcium carbonate & dilute hydrochloric acid – The change is permanent and irreversible, and a new substance is formed.

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