Physical and Chemical Changes

Q19. What is crystallisation?

Ans. Large crystals of pure substances can be formed from their solutions. The process is called crystallisation.

Q20. Why a wet iron pan often gets rusted if left in that state for some time?

Ans. A wet iron pan (tawa) often gets rusted if left in that state for some time because iron rust in the presence of oxygen and water.

Q21. Why does rusting takes place faster during rainy season?

Ans. Rusting takes place faster during rainy season because the content of moisture in air is high in rainy season.

Q22. What are physical properties?

Ans. Properties such as shape, size, colour and state of a substance are called its physical properties.

Q23. What is a physical change?

Ans. 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?

Ans. A 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?

Ans. A slice of an apple acquires a brown colour if it is not consumed immediately due to the formation of new substances.

Q26. Write down the equation representing the process of rusting.

Ans. The process of rusting can be represented by the following equation:

Iron (Fe) + Oxygen (O₂, from the air) + water (H₂O) → rust (iron oxide Fe₂O₃)

Q27. Why tearing of paper into pieces is a physical change?

Ans. Tearing 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?

Ans. On dissolving the magnesium oxide in water it forms a new substance. This change can be written in the form of the following equation:

Magnesium oxide (MgO) + Water (H₂O) → Magnesium hydroxide [Mg(OH)₂]

Q29. Why is spoiling of food a chemical change?

Ans. Spoiling 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 change?

Ans. Setting of curd is a chemical change because

                          i.        Once curd is formed it cannot be reversed back into milk.

                         ii.        Curd and milk have different properties.

Q31. Why explosion of fireworks is a chemical change?

Ans. 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.

Ans. For rusting, the presence of both oxygen and water (or water vapour) is essential. Painting 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?

Ans. 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.