Changes Around us (By Dharmendra Sir)

 

VERY SHORT- ANSWER QUESTIONS: 

Q. 1. Name the cause that changes temperature of a body.

Ans. Heat

Q. 2. Mention one change caused by force/pressure.

Ans. Stretching of rubber band

Q. 3. Name one irreversible change.

Ans. Curd from milk

Q. 4. Is dissolution of sugar in water a physical or chemical change?

Ans. Physical change

Q. 5. Is the statement, “all physical changes are reversible” correct? If so, give an example to support your answer.

Ans. Melting of wax

SHORT ANSWER QUESTIONS:

Q. 1. In what ways does a candle change during burning?

Ans. During burning of candle its size, shape and weight are changed but its smell is not changed. Burning of a candle is a permanent change.

Q. 2. How do stirring and heating affect the dissolution of common salt in water?

Ans. Stirring and heating increase the kinetic energy of the molecules so that more and more salt molecules come in contact of water molecules and cause a faster dissolution of common salt in water.

Q. 3. A potter makes clay pots. These pots are dried and then baked. Identify the reversible and irreversible changes that take place in the process.

Ans. Potter uses wet clay to make pots of different size and shapes using potter's wheel. This is a reversible change because it can be converted into the original clay. The clay pots are dried & then baked to make them strong because baked clay cannot be turned to original clay, this is irreversible change.

Q. 4. Why is ripening of fruit considered a chemical change?

Ans. The ripening of fruit is considered a chemical change because its colour, taste and chemical composition are changed. It can’t be changed to its raw form again.

Q. 5. Mention two characteristics of a chemical change.

Ans.

1.  Compositions and chemical properties of the reacting substances get changed.
2.  One or more new substance is formed.

LONG ANSWER TYPE QUESTIONS:

Q. 1. Classify the following changes in as many ways possible:

1)     Growth of a tree

2)     Burning of a paper

3)     Weathering of rocks

4)     Making of ice-cream from milk

Ans.

1)     Growth of a tree is irreversible, permanent, slow and desirable change.

2)     Burning of Paper is irreversible, chemical, permanent, slow and undesirable change.

3)     Weathering of rocks permanent, irreversible, slow and desirable change.

4)     Making of ice-cream from milk is permanent, irreversible, fast and desirable change.

Q. 2. State two changes which are reversible.

Ans.

1)     Conversion of ice into water by heating and that of water into ice on cooling.

2)     Stretching of a rubber band

Q. 3. Mention one application of expansion on heating and contraction on cooling.

Ans. One application of expansion on heating and contraction on cooling is fixing iron rim on the bullock cart wheel. In this process the iron rim is made slightly smaller than diameter of wooden wheel. The iron rim is heated in dry-dung cake fire. Due to the rise in temperature, it expands. This hot rim is then put on the wheel, and cooled by pouring water over it. On cooling, the iron rim contracts and holds the wheel firmly.

Q. 4. Classify the following into physical and chemical change.

1)      Melting of wax

2)     Burning of agarbatti

3)     Tearing of cloth

4)     Cooking of food

Ans.

1	Melting of wax	Physical change
2	Burning of agarbatti	Chemical change
3	Tearing of cloth	Physical change
4	Cooking of food	Chemical change

Ans. Q. 5. Write points of difference between chemical and physical change.

	Physical change	Chemical change
1	Only the physical properties of the substance get changed.	The composition and chemical properties of the reacting substance get changed.
2	No new substance is formed.	One or more new substance is formed.
3	Example- Breaking of wooden stick	Example- Ripening of fruits

TICK THE ODD-ONE OUT GIVING REASON

ANSWER-

1. Formation of biogas from animal gung because it is an irreversible process and rest others are reversible change.

2. Permanent change because rest others are related to reversible change.

3. Evaporation of water because it is a physical change and rest others are chemical change.

4. Ignition of matchsticks because it is an irreversible change and rest others are reversible change.

DEFINE THE FOLLOWING TERMS

1.Reversible Change: The change which can be reversed by reversing the condition or removing the cause is called a reversible change. Example-Stretching of a rubber band.

2. Irreversible change: The change which cannot be reversed even by changing the conditions or by removing the cause is called an irreversible change.

Example- Change of milk into curd

3. Physical change: A change in which only the physical properties of a substance get changed and no new substance is formed is called a physical change. Example- Evaporation of water

4. Chemical change: The change in which the composition and chemical properties of the reacting substance get changed and which cannot be reversed by reversing the conditions is called a chemical change.

Example: Rusting of iron

HOTS (HIGHER ORDER THINKING SKILLS) 

Q. 1: Describe the photochemical reaction which takes place in plants resulting in the formation of food.

Ans. The photochemical reaction which takes place in plants resulting in the formation of food is photosynthesis. In this process plants utilize solar energy and convert carbon dioxide and water into sugar. The reaction is as follows:

The reaction for photosynthesis is as follows:

6CO2 + 6H2O + light → C6H12O6 + 6O2

Q. 2: Why do articles made of iron when placed in moist  air get covered with reddish-brown powdery substance? Is this process a physical or chemical change? Give reason to support your answer.

Ans. The articles made of iron when placed in moist air get covered with reddish-brown powdery substance because iron reacts with oxygen in presence of moisture and make rust ( iron oxide) that looks like reddish brown in color. This is a chemical change because in this process a new substance( iron oxide or rust) is formed and it is an irreversible chage.