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

 

YADNYA e-Learning

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

7th Std Science 1 The Living World : Adaptations and Classification l Question and Answer

YADNYA e-Learning

Science Studies -  7th Standard

1. The Living World : Adaptations

and Classification

Question And Answer

1. Find my match !

2. Read the paragraph and answer the following questions.

I am a penguin. I live in polar region covered by snow. My abdomen is white. My skin is thick with a layer of fat underneath. My body is spindleshaped. My wings are small. My toes are webbed. We live in flocks.

(a) Why is my skin white and thick and why is there a thick

layer of fat underneath ?

Ans: Penguin skin is white and thick and there is a thick layer of fat underneath because it keep them warm under such extreme cold conditions.

(b) Why do we live in flocks sticking close to each other ?

Ans: In order to escape the extreme cold conditions, they nestle together to keep each other warm hence they live in flocks sticking close to each other.

(c) Which geographical region do I inhabit? Why?

Ans: Penguin inhabit in polar region, because we are aquatic flightless bird.

(d) Which adaptations should you have to enable you to live

permanently in the polar region ? Why ?

Ans: Black and white skin with thick layer of fat beneath it and presence of strong muscles in chest are characteristics that help penguin to live permanently in polar region.

3. Who is lying ?

(a) Cockroach – I have five legs.- Lying

(b) Hen – My toes are webbed.- Lying

(c) Cactus – My fleshy, green part is a leaf.- True

4. Read each of the following statements.Write a paragraph about adaptation with reference to each statement.

(a) There is extreme heat in deserts.

Ans: Deserts are characterized by severe scarcity of water. Hence, desert animals have a thick skin to prevent loss of water from the body. Their legs are long with flat and cushioned soles. The nostrils are protected by folds of skin. The eyelashes are long and thick. Rats, snakes, spiders, lizards in deserts live in deep burrows during daytime and are active at night.

(b) Grasslands are lush green.

Ans: Diverse types of bushes and grasses are found in the grasslands. Fibrous roots of grasses prevent soil erosion. Forests in the equatorial region are dense. Animals like tiger, elephants and deer can remain hidden in these grasses. However, grasses in cold regions are very short. Animals like the rabbit are found in such grasses. Vast meadows are found in hilly areas as well as plains.

(c) Insects are found in large numbers.

Ans: Insects are found in large numbers because they have developed mechanism which help them to survive even in harshest of environment.

(d) We hide.

Ans: There are certain species which are able to hide themselves by bending their colour with that of surrounding. Like lizards, butterfly, frog etc.

(e) We have long ears.

Ans: Animals with long ears are found in grassland, It is an adaptive mechanism which enables them to receive sound from long distance.

5. Answer the following.

(a) Why is the camel called the ‘Ship of the desert’ ?

Ans: Camel called the ship of desert because it is only the means of transport found in desert. The body of camel has undergone various modifications that help to survive in hot and dry conditions of desert.

(b) How can the plants like cactus and acacia live in deserts

with scarce water ?

Ans: Desert plants are either leafless or their leaves are like small needles or have been modified into thorns. As a result, they lose very little water by evaporation. The stem stores water and food and is therefore fleshy. The stems are green as they perform photosynthesis in the absence of leaves. Their roots penetrate deep into the soil and some roots spread away into the soil in search of water. There is a thick layer of a waxy Cactus substance on the stems of these plants, too.

(c) What is the inter-relationship between adaptations of organisms and their surroundings ?

Ans: Adaption is defined as the modification or change in the organism body or behaviour that help to survive in a particular environment. The environment in which an animal survives consist of many different things. These factors can be in form of climate the kinds of food plant that grow in it.

(d) How are organisms classified ?

Ans: The hierarchy of classification was developed by Carolyn Linnaeus. It refer to the organisation or classification of organisms in order to rank the importance. Include order, family, genus, and species in that order thus species is the basic unit of classification.