Class 9 FRANK Solutions Chemistry Chapter 5: Physical and chemical changes

Physical and chemical changes Exercise 74

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.

Solution 2

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

Solution 3

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

Solution 4

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

Solution 5

Solution 6

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.

Solution 7

Solution 8

(a) Redox reaction

Solution 9

(a) Exothermic reaction
(b) Endothermic reaction

Solution 10

Solution 11

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.

Solution 12

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.

Solution 13

Solution 14

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.

Physical and chemical changes Exercise 75

Solution 15

Solution 16

Solution 17

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.

Solution 18

(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.

Solution 19

Solution 20

Solution 21

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.

Solution 22

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

Solution 23

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

Solution 24

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

Solution 25

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.

Solution 26

Solution 27

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.

Solution 28

Solution 29

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.

Solution 30

Solution 31

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.

Solution 32

(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

Solution 33

(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.

Solution 34

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.

Solution 35

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.

Physical and chemical changes Exercise 76

Solution 36

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.

Solution 37

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.

Solution 38

Solution 39

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.

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