Selina Concise Class 8 Chemistry Solution Chapter No. 2- ‘Physical and Chemical changes’ For ICSE Board Students.
(1) Solution:
(a) A Physical change :
Answer : A change in physical properties or condition like shape , size , state , of substance without changing in chemical constituents are known as physical change.
(b) A chemical change :
Answer :. A change to form one or more new substance from their original substance and different chemical properties from original one , this type of change known as a chemical change.
(2) Solution:
a) Drying of wet clothes .
Answer : Drying of wet clothes are physical change .
(In drying of wet clothes water evaporate , or evaporation of water takes place which is physical change.)
b) Manufacture of salt from sea water.
Answer : Manufacture of salt from sea water is a physical change.
( salt is form or obtained by the evaporation of sea water by the method of crystallization , which is reversible and mit is physical change)
c) Butter getting rancid .
Answer : butter getting rancid is a chemical change.
(In the butter sugar and lactose are present , which is converted into lactic acid due to the bacteria that’s why we observed rancid which is chemical change)
d) Boiling of water :
Answer ; Boiling of water is a physical change.
(During boiling of water in converted into steam or water vapour , When it get cold environment it condense or come back to original position in water , which is reversible process so it is a physical change)
e) Burning of paper .
Answer : Burning of paper is a chemical change .
(When paper is burn new substance ash is produced , and this ash can not be reversed to make paper , so it is a chemical change)
f) Melting of wax .
Answer : Melting of wax is a physical change.
( When wax is melt only state of wax is change it means change solid to liquid which is reversible , new substance are not form that’s why it is physical change)
g) Burning of coal .
Answer : Burning of coal is a chemical change.
( When coal is burnt new substance that is carbon dioxide gas and ash is produced which is chemical change)
h) Formation of clouds.
Answer : Formation of clouds is a physical change.
( Clouds are due to the evaporation of water , when condensation is occur this come back to original position means water so it is physical change)
i) Making of a sugar solution .
Answer : Making of sugar solution is a physical change.
( Making of a sugar solution , sugar is dissolved in water only state of sugar is change solid to liquid . That’s why it is a physical change.)
j) Glowing of an electric bulb .
Answer : Glowing of an electric bulb is a physical change.
(When blub is on in the blub temporary rise in temperature occur and bulb give energy in the form of light if it will get off the temperature decrease light is off, that’s why Glowing of an electric bulb is physical change.
k) Curdling of milk .
Answer : Curdling of milk is a chemical change .
(when we add some acid like lemon juiceor vinegar in milk , milk contain caseins protein , lactose which react with acid and form new product which semi solid different properties from milk , hence it is said chemical change)
l) Rusting of iron .
Answer : rusting of iron is a chemical change.
(Rusting of iron due to the oxidation reaction occur between iron and moisture in air , , and rust form on the surface of iron hence it is chemical Change)
m) Roasting of potato .
Answer : Roasting of potato is a chemical change .
(When we roast or cooked potato we observed many type of changes occur in potato which is not reversible and new taste , properties also change that’s why roasting of potato is a chemical change)
n) Formation of alloys .
Answer: formation of alloys is chemical change.
(many types of different elements combine with each other and form alloy , hence from different elements new substance that is alloys are form so it is a chemical change.)
3) . Solution
a) The process of a liquid changing into a solid is called …………………..
Answer : reversible
Explanation: (In this process liquid converted into a solid due to lowering the temperature to the freezing point of liquid it get convert into a solid like water convert into a ice in a refrigerator at 0°C and again back to its original position ( water / liquid ) when it gain heat or energy hence this process called reversible )
b) A change which alert the composition of a substance is known as a ………..change.
Answer : chemical change
Explanation: (when a substance change and not reversible it means permanent , form a new substance , and properties are also different from their original one this change is a chemical change).
c) There is no change in the ………….. of the substance during a physical change.
Answer : mass
Explanation: (When the physical changes occur in a substance it change only their size and shape which is reversible ,and temporary , it will not changes in their mass , composition , chemical properties , as compared to original one ).
d) The reaction in which energy is evolved is called ………………
Answer : Chemical reaction
Explanation: (In the chemical reaction heat or energy is evolved for example when we burnt paper it produce carbon dioxide, some water vapour and during this reaction energy is produced in the form of light or heat.)
4) Solution:
a) Answer : Freezing of water to ice and evaporation of water are physical changes because it is reversible process when it happened only shape or state of water is change which is temporarily , there is no new substance are formed which have different chemical properties as compared to original one ( water).
b) Answer : Burning of candle is both physically and a chemical change because when it is burnt wax of the candle melted which reversible , melted wax can come again their original position when it get cooled , hence it’s physical change , but when candle is burnt there is carbon dioxide and water vapour are produced , which is new substance are formed , and also a energy is released in the form of light or heat , so this is chemical changes that’s why burning of candle are both physical and chemical change.
c) Answer : Burning of paper is a chemical change because when paper is burnt there is new substance are formed like carbon dioxide and water vapour and also energy are released in the form of light or heat.
d) Answer : Cutting of cloth piece is a physical change , thought it can not be reversed because when we cut the piece of cloth it only change in their shape and There is no new substance are formed ,the composition cloth pieces are same as original one , that’s why it is physical change.
5.) Solution:
Answer :
Sr.No | Physical change | Chemical change |
1. | In physical change there is no change in a chemical properties of substance . which are form by original one and no new substance are formed. | In a chemical change , chemical Properties are different from original one and one or more new substance are formed. |
2 | Physical change are temporary | Chemical change are permanent |
3 | In physical changes energy is not evolved | In chemical change energy is evolved |
4 | In physical change there is no change in mass of the matter which involved. | In chemical change there is change in mass of the matter which involved . |
5. | In physical change exchange of energy Not occur | In chemical change exchange of energy occur |
6 | In a physical change the substance which are involved in a change, back again their original position. | In a chemical change the substance which are involved in this change , not come back to their original position. |