and Chemical Changes
Q19. What is crystallisation?
crystals of pure substances can be formed from their solutions. The process is
Q20. Why a wet iron pan often gets rusted if left in that state
for some time?
wet iron pan (tawa) often gets rusted if left in that state for some time
iron rust in the presence of oxygen and water.
Q21. Why does rusting takes place faster during rainy season?
Rusting takes place faster during rainy season because the content of moisture
in air is high in rainy season.
Q22. What are physical properties?
such as shape, size, colour and state of a substance are called its physical
Q23. What is a physical change?
A change in which a substance undergoes a change in its physical properties is
called a physical change.
Q24. What is called a chemical reaction?
change in which one or more new substances are formed is called a chemical
change. A chemical change is also called a chemical reaction.
Q25. Why a slice of an apple acquires a brown colour if it is
not consumed immediately?
slice of an apple acquires a brown colour if it is not consumed immediately
to the formation of new substances.
Q26. Write down the equation representing the process of
The process of rusting can be represented by the following equation:
(Fe) + Oxygen (O2, from the air) + water (H2O) →
rust (iron oxide Fe2O3)
Q27. Why tearing of paper into pieces is a physical change?
of a paper is a physical change because when the paper is torn only the shape
and size of the paper is changed, no new substance is formed.
Q28. What happens when magnesium oxide is dissolved in water?
dissolving the magnesium oxide in water it forms a new substance. This change
can be written in the form of the following equation:
oxide (MgO) + Water (H2O) →
Magnesium hydroxide [Mg(OH)2]
Q29. Why is spoiling of food a chemical change?
of food is an unwanted quality change in a foodstuff, such as staling,
discoloration, the development of off-flavours and odours. It is a chemical
change as it cannot be brought back to its original form.
Q30. How would you show that setting of curd is a chemical
Setting of curd is a chemical change because
Once curd is formed it cannot be reversed
back into milk.
Curd and milk have different properties.
Q31. Why explosion of fireworks is a chemical change?
Explosion of a firework is a chemical change because explosion produces heat,
light, sound and unpleasant gases and once the crackers are burnt, it cannot be
brought back to its original form.
Q32. Explain how painting of an iron gate prevents it from
rusting, the presence of both oxygen and water (or water vapour) is essential.
of an iron gate prevents it from coming in contact with oxygen, or water, or
both and thus prevents it from rusting.
Q33. Why is hacksaw blade’s color change on heating considered
as a physical change?
Changing of hacksaw blade’s color on heating is considered as a physical change
because in this process only colour changes, no new substance is formed.
Q34. Why do ships suffer a lot of damage from rusting in spite
of being painted?
Ans. Ships suffer a lot of damage
from rusting in spite of being painted because the water of the sea contains
many salts which makes the process of rust formation faster.