7th Std Science 13 Changes – Physical and Chemical l Question and Answer

 

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Science Studies -  7th Standard

13. Changes – Physical and Chemical

Question And Answer

 

1. Distinguish between the following.

(a) Physical change and chemical change

Ans:

Physical change	chemical change
The properties of the original substances remain the same in the physical change.	The properties of the original substances alter in the chemical change.
The physical changes are reversible.	The chemical changes are irreversible.
Their original composition remains unchanged.	Their original composition changes.

 

 

(b) Periodic change and non-periodic change

Ans:

Periodic change	non-periodic change
Changes occurring again and again after a definite interval of time are called periodic changes.	Changes not occurring again and again after a definite interval of time are called non-periodic changes.
There is constancy in periodic changes.	There is no constancy in non-periodic changes.
Eg. Movement of hands of clock	Eg. Breaking of storm

(c) Natural change and man-made change

Ans:

Natural change	man-made change
The change occurring by natural cause is called natural change.	The change induced by man are called man-made change.
Most of natural events are happened due to natural changes.	Man-made changes are due to man’s wish to satisfy his needs.

2. Under which different types can the following changes be classified?

(a) Conversion of milk into yoghurt.

Ans: Chemical, irreversible, slow

(b) Bursting of a cracker.

Ans: Chemical, irreversible, fast

 

(c) Occurrence of an earthquake.

Ans: Physical, Chemical, irreversible, natural

 

(d) Revolution of the earth around the sun.

Ans: Periodic, slow, natural

 

(e) Stretching of a spring.

Ans: Physical, reversible

 

3. Give reasons.

(a) While purchasing canned food, its expiry date should be checked.

Ans: Every canned item has a limited shelf life. This is the time limit during which the food item it safe to eat. This is mentioned on the can. But later, after the expiry date is over, the food may not remain in a good condition due to irreversible chemical changes in it. Consuming such items can harm our health. Therefore while purchasing canned food, its expiry date should be checked.

 

(b) An iron article should be given a coat of paint.

Ans: An iron article rusts naturally which is called corrosion. Corrosion take place due to oxygen, moisture, vapours of chemicals in the air. Corrosion makes articles weak and unusable. Therefore iron article should be given a coat of paint.

 

(c) A wooden article should be polished.

Ans: A wood is an organic substance that is affected by termites or fungus. This natural change can result into irreversible damage. If polish is applied to such wooden articles, the rate of natural changes slows down. Thus to prevent this harmful change, a wooden article should be polished.

 

(d) Copper and brass utensils should be tinned.

Ans: The metals like copper and brass gather a greenish layer due to slow chemical changes. This is corrosion which happens due to oxygen, moisture and vapours of chemical in the air. For the prevention of such corrosion, copper and brass utensils are tinned. Moreover, the tinned utensils are safer for cooking.

 

(e) A dry handkerchief gets wet at once on dipping in water, but it takes long for a wet handkerchief to dry.

Ans: A dry handkerchief absorbs the water, once it is dipped into the water. This is a fast change. But drying such moist handkerchief is a slow process which occurs due to evaporation. Evaporation is the physical change which depends upon atmospheric temperature and humidity. It may therefore take long for drying a wet handkerchief.       

 

4. What will you take into account while identifying the following?

(a) A physical change in a substance.

Ans: It will be seen that whether the substance has changed its properties and composition. If both of these have remained unchanged then it is a physical change.

 

(b) A chemical change in a substance.

Ans: It will be seen that whether, the substance is changed in its properties. Whether the composition of the substance is changed, whether the change is irreversible. If all these changes are taking place then it will be a chemical change.

 

5. Read the paragraph and note down the various types of changes it mentions.

It was nearing six o’clock in the evening. The sun was setting. A breeze was blowing. Leaves on the tree were shaking. Sahil was sitting in the courtyard, rolling balls of wet soil and shaping them into various toys. Then he felt hungry and went into the house. Mother made a dough from wheat flour and fried purees. While

eating hot purees, his attention was drawn outside the window. It had started raining. There was lightning, too. Sahil was enjoying his dinner in the dim light.

Ans:

1)  It was nearing six o’clock in the evening

Time- reversible, periodic

2)  The sun was setting

Sunset- periodic, reversible

3)  A breeze was blowing

Blowing of wind- physical, non-periodic

4)  Leaves on the tree were shaking

Movements due to wind- physical, periodic

5)  Sahil was sitting in the courtyard, rolling balls of wet soil and shaping them into various toys

Making toys of soil- physical

6)  Then he felt hungry and went into the house

Hunger- periodic

7)  Mother made a dough from wheat flour and fried purees

Frying- physical, irreversible

8)  While eating hot purees, his attention was drawn outside the window

Seeing randomly- Non-periodic

9)  It had started raining

Raining- physical, periodic if raining in specific season