Question 1

Which of the following is true?

Accepted Answer

Oxygen and water are the essential things required for rusting. Rusting will only take place in presence of water and air.

Question 2

Which of the following are chemical changes? (a)  Growth of a plant (b)  Rusting of Iron (c)  Mixing of iron fillings and sand (d)  Cooking of food (e)  Digestion of food (f)  Freezing of water (g)  Burning of a candle.

Accepted Answer

A chemical change is a change that brings a change in the chemical properties of matter and a new substance is formed.Examples of chemical changes from the given list of changes are:Rusting of iron, Cooking of food, Digestion of food, and Burning of a candle.

Question 3

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Accepted Answer

Burning of the candle is both physical and chemical change. Burning of the candle melts the wax and hence physical state of wax has changed from solid to liquid. Again the wax combines with the atmospheric oxygen and changes to carbon dioxide, heat and light. Thus both the changes are accompanied by the burning of the candle.

Question 4

Which of the following is a physical change?

Accepted Answer

A physical change is a process in which the physical state of the substance is changed. Dissolving sugar in water is physical change as the solid sugar is converted to a liquid state in water, so it changes its physical state.Hence option C is correct.

Question 5

Corrosion can be prevented by:

Accepted Answer

An alloy is a mixture of metals or a mixture of a metal and another element. Alloying the metal surface prevents rust from developing and a combination of metals may reduce the overall cost of the material. Tin is a soft, malleable, silver-color metal. It is generally used to coat other metals and to make alloys.Tin is not reactive to acidic foods, it is not allergenic, does not rust, and can prevent any type of corrosion.It is also used to coat steel used for cookie cutters, to help prevent rusting.Galvanization is the process of applying a protective zinc coating to steel or iron, to prevent rusting of iron, in other words, corrosionMentioned techniques can be used to prevent corrosion and therefore all options are correct.The correct answer is option  D .

Question 6

Photosynthesis in plants is a ________ change.

Accepted Answer

Photosynthesis is the process through which
plants convert light energy into chemical energy. Here is the chemical
reaction involved: water and carbon dioxide combine to form glucose and oxygen.
Since new chemical species are formed, photosynthesis is clearly a chemical
change.

Question 7

Which of the following change is not a chemical change?

Accepted Answer

The boiling of water is not a chemical change. If it boils for more time it changes to vapour state which is a physical change.Whereas boiling of egg, burning of paper and cloth involves both physical and chemical change. The burning of paper and cloth forms ash.

Question 8

Which of the following is a chemical change:

Accepted Answer

When iron comes in contact with air and moisture it reacts with it and forms a brownish layer on its surface due to the formation of its oxides. This is called rusting of iron. It is a chemical change since the new substance is formed.Melting of ice, boiling of water, and tearing of paper are all physical changes.Hence the option  C  is correct.

Question 9

Paheli's mother made concentrated sugar syrup by dissolving sugar in hot water. On cooling, crystals of sugar got separated. This indicates:

Accepted Answer

This is a physical change as there is not any new substance formed having different chemical composition and properties. Only the state of the substance is changed during the process and it can be reversed.

Question 10

Give two examples for the following casePhysical changes which are reversible.

Accepted Answer

A change in which
a substance undergoes a change in its
physical properties such as shape, size, state of a substance, colour, etc is called a physical
change. A physical change is generally
reversible. In such a change no new
substance is formed.Physical changes which are reversible: Melting of ice Folding of clothes