FRANK Solutions for Class 9 Chemistry Chapter 5 - Physical and chemical changes

Chapter 5 - Physical and chemical changes Exercise 74

Question 1

What is a physical change?

Solution 1

A physical change is a temporary change in which no new substance is formed and the composition or identity of the substance is not altered although certain specific physical properties may be changed.

Question 2

What are the major distinctions between physical and chemical changes?

Solution 2

Question 3

What is photochemical reaction?

Solution 3

The chemical reactions which occur with the absorption of light energy are called photochemical reactions.
Examples-
Decomposition of silver nitrate takes place in the presence of light.

Question 4

How can you prove experimentally that magnesium gains weight on burning in air?

Solution 4

As the burning substance combines with oxygen ,the total mass of the products should be greater than that of the burning substance. For example,when,magnesium is burnt,a new substance magnesium oxide is formed,whose weight is greater than that of the original magnesium.
Experiment - A crucible is weighed containing about 0.5 gm of magnesium.Now crucible is heated.When magnesium begins to burn,the lid is put back on the crucible and the lid is occasionally raised to allow air to enter and burn the magnesium such that no product is lost..When,all the magnesium has been burnt up, the crucible is allowed to cool and then on weighing it  we observe that there is gain in weight.

Question 5

Give three differences between respiration and burning?

Solution 5

Question 6

Name three conditions necessary for burning?

Solution 6

Three conditions necessary for burning are-
(i) The substance to be burnt must be combustible.
(ii) A supporter of combustion such as air or oxygen must be present.
(iii) A combustible substance must be heated to its ignition temperature.

Question 7

What is a   chemical change?

Solution 7

A chemical change is a permanent change in which the original substance gives rise to one or more substances with different properties.

Question 8

What are the exothermic and endothermic reactions?

Solution 8

The reactions in which heat is evolved are called exothermic reactions while the reactions in which heat is absorbed are called endothermic reactions.

Question 9

Indicate which of the following statements are true and which are false.
(a) A change, whether physical or chemical is always permanent.
(b) Photosynthesis in plants is a physical change.
(c) The mass of a substance does not change on burning.
(d) Carbohydrate burn in air to give carbon and water.
(e) A chemical change is always accompanied by the evolution or absorption of heat.

Solution 9

(a) False
(b) False
(c) False
(d) False
(e) True

Question 10

Why do we call the burning of paper as chemical change?

Solution 10

Question 11

What are the possible conditions for a chemical change?

Solution 11

Possible conditions for a chemical change are-
(i) One or more new substance is formed during reaction.
(ii) The change occurring during the reaction is permanent.
(iii) The mass of the substance undergoing a chemical change is generally altered.
(iv) Chemical change involves  making and breaking of bond.

Question 12

What is double decomposition ? Give two exam

Solution 12

Question 13

State what energy change accompanies the following chemical reaction?
(a) Air is passed over white hot coke.
(b) Steam is passed over white hot coke.

Solution 13

(a) Exothermic reaction
(b) Endothermic reaction

Chapter - Exercise

Solution 1

Chapter 5 - Physical and chemical changes Exercise 75

Question 1

Give balanced equations where possible or where it is not possible ,explain by means of examples.
(a) A reaction which gives out heat.
(b) A reversible reaction.
(c) A reaction with a solid and a gas which produces heat and light.
(d) A reaction which takes place with the help of sunlight.
(e) A reaction which is brought about by an electric current.

Solution 1

Question 2

Give balanced equation for the action of heat on the following:
Silver nitrate, Copper (II)nitrate, Aluminium hydroxide, Silver carbonate and Potassium nitrate

Solution 2

Question 3

Oxidation and reduction go hand in hand. Explain.

Solution 3

When oxidation occurs there is a loss of electrons but simultaneously there is a gain of electrons by other species which is called reduction. These both process occur simultaneously so we can say that both oxidation and reduction go hand in hand and such reactions are known as redox reaction.

Question 4

Name the substances which are oxidized and which are reduced in the following reactions.

Solution 4

(a) Copper is oxidized to copper sulphate while sulphur in sulphuric acid is reduced to sulphur dioxide.
(b) Silver in silver oxide is reduced to silver while oxygen in hydrogen peroxide is oxidised to molecular oxygen.

Question 5

Name and define the four important types of chemical reactions.

Solution 5

Question 6

Give a possible explanation for the following observations:
(a) Gold is found free(uncombined )in nature.
(b) Compounds of sodium do not decompose on heating.
(c) Silver articles turn black on prolonged exposure to air.
(d) When ammonium nitrate is dissolved in water contained in a beaker, the beaker becomes cold.
(e) When magnesium is dissolved in hydrochloric acid contained in a beaker ,the beaker becomes hot.

Solution 6

Question 7

As a candle burns, first the physical change then chemical change takes place. Explain.

Solution 7

A candle is a stick of paraffin wax with cotton wick.As a candle burns wax melts and trickles down. It gets solidified shortly. This is physical change. Paraffin wax is a mixture of hydrocarbons. When wick catches fire, paraffin wax melts, evaporates and burns in air like any hydrocarbon to give carbon dioxide and water. This is a chemical change.

Question 8

Fill in the blanks:
(a) Dissolving common salt in water is an example of ______ change.
(b) Dissolving zinc in dil. H2SO4 is an example of ______ change.
(c) A ______ change is one in which one or more different substances are formed.
(d) A ______ change can easily be reversed.

Solution 8

(a) physical
(b) chemical
(c) chemical
(d) physical

Question 9

Give two examples for each of an oxidation  and reduction type of reactions, which take place on a large scale.

Solution 9

Two examples are:-
(i) Burning of wood-carbon get oxidized and oxygen gets reduced.
(ii) Rusting-In it iron is oxidized.

Question 10

Give an example for each of the following:
(a) An oxidizing agent not containing oxygen.
(b) A substance which acts as both oxidizing and reducing agent.
(c) A carbonate stable towards heat.
(d) A compound decomposed by light.
(e) A positive catalyst.

Solution 10

(a) Chromium(VI) .
(b) Hydrogen peroxide
(c) Barium carbonate.
(d) Silver nitrate.
(e) Manganese dioxide.

Question 11

What is meant by the term ignition temperature? How is it related to burning?

Solution 11

Ignition temperature - Ignition temperature is the lowest temperature up to which temperature of a substance must be raised so that it catches fire.
A combustible substance must be heated to its ignition temperature for burning.

Question 12

Give three differences between respiration and combustion of fuels?

Solution 12

Question 13

Show that a physical change can easily be reversed.

Solution 13

On heating few crystals of iodine in a test tube, the grey crystals sublimes and dense violet fumes are seen. On cooling, the vapours again form the crystals. So, a physical change can be reversed.

Question 14

When hydrogen burns in air, the change is chemical. Give two reasons to support this assertion.

Solution 14

Question 15

Freezing of water to ice and evaporation of water are both physical change. Explain.

Solution 15

When water is freezed and evaporated, these both are physical changes because-
(i) The change is temporary and reversible.
(ii) No new substance is formed and the chemical composition of the original substance remains the same.
(iii) Mass of the substance remains unchanged
(iv) The amount of energy required to bring about a physical change is generally equal to the amount of energy required to reverse the change. Hence,there is no net energy change involved.

Question 16

What is synthesis? What kind of chemical reaction is synthesis? Support your answer by an example?

Solution 16

Question 17

Air is necessary for burning, comment.

Solution 17

Air is necessary for burning. Incorrect amount of air in fuel combustion accounts for the largest losses in combustion system. If the fuel does not get enough air for combustion it will generate smoke and a potentially unhealthy mixture of gas products.

Question 18

(a) What are combustible and non-combustible substances?
(b) Name two substances other than oxygen that support combustion.

Solution 18

(a) Combustible substances -The substances that catch fire and burn easily. Ex-Wood, Charcoal, petrol, kerosene etc.
 Non-combustible substances-Substance which cannot burn in air or oxygen are called as non- combustible substances. Ex-Nitrogen gas, carbon dioxide etc.
(b) Two substances other than oxygen that support combustion are-
(i) Hydrogen
(ii) Nitrogen

Question 19

State two important processes, which:
(a) release CO2 into the atmosphere.
(b) remove CO2 from the atmosphere.

Solution 19

(a) (i) Burning of coal in air releases CO₂ in air.
(ii) Respiration releases carbon dioxide and water vapours.
(b) (i) Photosynthesis removes CO₂ from the atmosphere. Plants take carbon dioxide from the atmosphere in the presence of sunlight and use it to synthesise glucose with the liberation of oxygen.
(ii) Some man made chemical activities such as setting of mortar also use atmospheric carbon dioxide and helps in removing carbon dioxide.

Question 20

What would be the effect on burning if the proportions of nitrogen and oxygen in the air were reversed?

Solution 20

Nitrogen is inert in nature and does not support combustion while oxygen supports combustion.If proportions of nitrogen and oxygen in the air were reversed then the rate of combustion of substances will increase.

Question 21

Give an experiment to show a physical change?

Solution 21

Heating of sulphur-
If some powdered sulphur is heated gently in a glass test tube, it melts to a pale yellow liquid. Flame is removed to stop heating, it is quickly changed back to solid sulphur.

Chapter 5 - Physical and chemical changes Exercise 76

Question 1

What is activity series? What is the role it plays in displacement reactions?

Solution 1

Activity series - The arrangement of the metals in the decreasing order of their chemical reactivity is called the activity series.
   In displacement reactions, a more reactive element (metal or non-metal) displaces a lesser reactive element from its compound. With the help of the activity series, it is possible to predict which metals will displace other metals from their solutions.

Question 2

How the balance of oxygen and carbon dioxide is maintained in nature?

Solution 2

Balance of oxygen and carbon dioxide is maintained in nature because there is a natural oxygen cycle and a natural carbon cycle operating all the time by which the desired proportions of the two gases in the air are maintained. This is also known as carbon dioxide-oxygen cycle.

Question 3

Solution 3

Question 4

Write short note on carbon dioxide cycle.

Solution 4

Carbon dioxide from the atmosphere enters the plant through photosynthesis, where carbohydrates are produced. From green plants, the carbon in the form of carbohydrates, etc. enter the animal and human bodies. The atmospheric carbon dioxide gets dissolved in oceans by diffusion. Marine algae and photosynthetic bacteria obtain carbon dioxide from water.
Carbon dioxide returns to the atmosphere by respiration, combustion of fossil fuels like coal, wood, petroleum etc., weathering of rocks, volcanic eruptions etc.

Question 5

Write about oxygen cycle.

Solution 5