Class 7 Science Chapter-18 Solutions

      CLASS-7 SCIENCE SOLUTION

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                  CHAPTER-18

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           WASTEWATER STORY

Exercise Questions

1. Fill in the blanks:

(a) Cleaning of water is a process of removing ____________.

(b) Wastewater released by houses is called ____________.

(c) Dried ____________ is used as manure.

(d) Drains get blocked by ____________ and ____________.

Solution:

(a) Cleaning of water is a process of removing pollutants.

(b) Wastewater released by houses is called sewage.

(c) Dried sludge is used as manure.

(d) Drains get blocked by cooking oil and fats.

2. What is sewage? Explain why it is harmful to discharge untreated sewage into rivers or seas.

Wastewater released by houses is called sewage. Untreated sewage should not be discharged into the sea because sewage consists of pollutants and harmful microorganisms which will contaminate water causing disease in people using contaminated water.

3. Why should oils and fats be not released in the drain? Explain.

Solution:

Oils and fats should not be released in the drain because drains get blocked by cooking oil and fats.

4. Describe the steps involved in getting clarified water from wastewater.

Solution:

Use an aerator from an aquarium to bubble air through the sample in the glass jar. Allow several hours for aeration; leave the aerator attached overnight. If you do not have an aerator, use a mechanical stirrer or a mixer. This reduces the foul smell of wastewater.

Then, the water is filtered through the layers of sand, fine gravel and medium gravel. Filtration makes the wastewater clean from various types of pollutants. The water is filtered continuously until it becomes clear.

Water is disinfected by using a chlorine tablet.

5. What is sludge? Explain how it is treated.

Solution:

Solids like faeces settle at the bottom while treating sewage, and this material is called sludge. Sludge can be treated as follows:

Sludge is removed using a skimmer and then transferred to a tank where it is decomposed by anaerobic bacteria to produce biogas.

6. Untreated human excreta is a health hazard. Explain

Solution:

Untreated human excreta is a health hazard because it consists of various disease-causing microorganisms and pollutants that will contaminate the soil and water resource from where people draw water for drinking and domestic purposes. When people use water contaminated with human excreta, they get diseases like Cholera, Typhoid, Dysentry and Hepatitis.

7. Name two chemicals used to disinfect water

Solution:

Chlorine and Ozone are used to disinfect water.

8. Explain the function of bar screens in a wastewater treatment plant.

Solution:

Bar screen removes large objects like rags, sticks, cans, plastic packets and napkins.

9. Explain the relationship between sanitation and disease.

Solution:

Sanitation and disease are interrelated as lack of sanitation leads to illness and good sanitation practices prevent the diseases.

10. Outline your role as an active citizen in relation to sanitation.

Solution:

Every citizen has a role to play in maintaining proper sanitisation. Following are the things we should follow to play an active role.
Ensure that our surroundings are kept clean.
The sewage system in the house should be properly managed
Report to the concerned authoritive immediately in case of leakages in sewage pipes.

11. Here is a crossword puzzle: Good luck!



Across

3. Liquid waste products

4. Solid waste extracted in sewage treatment

6. A word related to hygiene

8. Waste matter discharged from human body

Down

1. Used water

2. A pipe carrying sewage

5. Micro-organisms which causes cholera

7. A chemical to disinfect water

Solution:

Across

3. Sewage

4. Sludge

6. Sanitation

8. Excreta

Down

1. Wastewater

2. Sewer

5. Bacteria

7. Ozone


12. Study the following statements about ozone:

(a) It is essential for breathing of living organisms.

(b) It is used to disinfect water.

(c) It absorbs ultraviolet rays.

(d) Its proportion in air is about 3%. Which of these statements are correct?

(i) (a), (b) and (c)

(ii) (b) and (c)

(iii) (a) and (d)

(iv) All four

Solution:

The answer is (ii) (b) and (c)

Class 7 Science Chapter-17 Solutions

      CLASS-7 SCIENCE SOLUTION

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                  CHAPTER-17

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       FORESTS: OUR LIFELINE

Exercise Questions

1. Explain how animals dwelling in the forest help it grow and regenerate.

Solution:

Animals dwelling in the forest help it grow and regenerate in the following ways
Animals help in dispersing plant seeds.
Decaying animal dung provides nutrients for plants to grow
Microorganisms convert the dead plants and animals to humus

2. Explain how forests prevent floods.

Solution:

Plants in the forests will not allow the rainwater to fall directly on earth and these plants also hold water which helps in preventing floods.

3. What are decomposers? Name any two of them. What do they do in the forest?

Solution:

The micro-organisms which convert the dead plants and animals to humus are known as decomposers. Example: bacteria and fungi. They help in recycling of nutrients by decomposing dead plants and animals.

4. Explain the role of forest in maintaining the balance between oxygen and carbon dioxide in the atmosphere.

Solution:

Plants in the forests consume carbon dioxide and release oxygen by photosynthesis process. This helps in balancing oxygen and carbon dioxide in the atmosphere.

5. Explain why there is no waste in a forest.

Solution:

There is no waste in a forest because waste created is bio-degradable, which gets converted to humus by the action of microorganism.

6. List five products we get from forests.

Solution:

i) Medicines

ii) Spices

iii) Wood

iv) Paper

v) Gum

7. Fill in the blanks:

(a) The insects, butterflies, honeybees and birds help flowering plants in .

(b) A forest is a purifier of and .

(c) Herbs form the layer in the forest.

(d) The decaying leaves and animal droppings in a forest enrich the .

Solution:

(a) The insects, butterflies, honeybees and birds help flowering plants in pollination.

(b) A forest is a purifier of air and water .

(c) Herbs form the lowest layer in the forest.

(d) The decaying leaves and animal droppings in a forest enrich the soil.

8. Why should we worry about the conditions and issues related to forests far from us?

Solution:

We should worry about the forest for the following reasons.
A decrease in forest results in an increase in carbon dioxide content in the atmosphere. This results in global warming.
Depletion of forests results in soil erosion
Decrease in forest adversely affect the lives of animals living in the forest.
Absence of forests leads to flooding more often.
Due to the reduction of forest land will turn barren which is called desertification.

9. Explain why there is a need of variety of animals and plants in a forest.

Solution:

Variety of plants and animals in the forests helps plants to regenerate and grow. More excellent range of plants supports herbivores, which in turn serve as food for carnivores. Decomposers turn dead animals and plants into humus; thereby maintain nutrient recycling. This wide variety makes forest a dynamic living entity.

10. In Fig. 17.15, the artist has forgotten to put the labels and directions on the arrows. Mark the directions on the arrows and label the diagram using the following labels: clouds, rain, atmosphere, carbon dioxide, oxygen, plants, animals, soil, roots, water table.



Solution:



11. Which of the following is not a forest product?

(i) Gum

(ii) Plywood

(iii) Sealing wax

(iv) Kerosene

Solution:

The answer is (iv) Kerosene

12. Which of the following statements is not correct?

(i) Forests protect the soil from erosion.

(ii) Plants and animals in a forest are not dependent on one another.

(iii) Forests influence the climate and water cycle.

(iv) Soil helps forests to grow and regenerate.

Solution:

Statement (ii) Plants and animals in a forest are not dependent on one another – is incorrect

13. Micro-organisms act upon the dead plants to produce

(i) sand (ii) mushrooms (iii) humus (iv) wood

Solution:

Answer is (iii) humus

Class 7 Science Chapter-16 Solutions

      CLASS-7 SCIENCE SOLUTION

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                  CHAPTER-16

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   WATER: A PRECIOUS RESOURCE

Exercise Questions

1. Mark ‘T’ if the statement is true and ‘F’ if it is false:

(a) The freshwater stored in the ground is much more than that present in the rivers and lakes of the world. (T/F)

(b) Water shortage is a problem faced only by people living in rural areas. (T/F)

(c) Water from rivers is the only source for irrigation in the fields. (T/F)

(d) Rain is the ultimate source of water. (T/F)

Solution:

a) True

b) False

c) False

d) False

2. Explain how groundwater is recharged.

Solution:

The rainwater and water from other sources such as rivers and ponds seep through the soil and fills the empty spaces and cracks deep below the ground. The process of seeping of water into the ground is called infiltration. The groundwater thus gets recharged by this process.

3. There are ten tubewells in a lane of fifty houses. What could be the long term impact on the water table?

Solution:

If ten tube-wells in a lane of fifty houses are situated, the groundwater will get used for domestic purposes. This results in depletion of groundwater level and water table would go down.

4. You have been asked to maintain a garden. How will you minimise the use of water?

Solution:

We can reduce the usage of water for gardening by adopting drip irrigation method. In this method, water will reach the roots of the plant drop by drop, which will reduce the wastage of water.

5. Explain the factors responsible for the depletion of water table.

Solution:

Factors responsible for the depletion of the water table are as follows:

Increase in population; Increase in population leads to increased demand for water for domestic and drinking purpose. This reduces the water table.

Industrialisation: Industries need a lot of water for manufacturing purposes. This reduces the water table.

Agriculture: Agriculture requires a lot of water for growing crops; this results in depletion of water table.

6. Fill in the blanks with the appropriate answers:

(a) People obtain groundwater through________ and ________.

(b) Three forms of water are ________, ________ and ________.

(c) The water bearing layer of the earth is ________.

(d) The process of water seepage into the ground is called ________.

Solution:

(a) People obtain groundwater through tube wells and hand pumps.

(b) Three forms of water are ice, water and vapour.

(c) The water bearing layer of the earth is Hydrosphere.

(d) The process of water seepage into the ground is called infiltration.

7. Which one of the following is not responsible for water shortage?

(i) Rapid growth of industries

(ii) Increasing population

(iii) Heavy rainfall

(iv) Mismanagement of water resources

Solution:

The answer is (iii) Heavy rainfall

8. Choose the correct option. The total water

(i) in the lakes and rivers of the world remains constant.

(ii) under the ground remains constant.

(iii) in the seas and oceans of the world remains constant.

(iv) of the world remains constant.

Solution:

The answer is (iv) of the world remains constant.

9. Make a sketch showing groundwater and water table. Label it.

Solution:

Class 7 Science Chapter-15 Solutions

      CLASS-7 SCIENCE SOLUTION

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                  CHAPTER-15

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                       LIGHT

Exercise Questions

1. Fill in the blanks:

(a) An image that cannot be obtained on a screen is called ____________.

(b) Image formed by a convex __________ is always virtual and smaller in size.

(c) An image formed by a __________ mirror is always of the same size as that of the object.

(d) An image which can be obtained on a screen is called a _________ image.

(e) An image formed by a concave ___________ cannot be obtained on a screen.

Solution:

(a) An image that cannot be obtained on a screen is called virtual image.

(b) Image formed by a convex mirror is always virtual and smaller in size.

(c) An image formed by a plane mirror is always of the same size as that of the object.

(d) An image which can be obtained on a screen is called a real image.

(e) An image formed by a concave lens cannot be obtained on a screen.

2. Mark ‘T’ if the statement is true and ‘F’ if it is false:

(a) We can obtain an enlarged and erect image by a convex mirror. (T/F)

(b) A concave lens always form a virtual image. (T/F)

(c) We can obtain a real, enlarged and inverted image by a concave mirror. (T/F)

(d) A real image cannot be obtained on a screen. (T/F)

(e) A concave mirror always form a real image. (T/F)

Solution:

a) False

b) True

c) True

d) False

e) False

3. Match the items given in Column I with one or more items of Column II.

Column-I	Column-II
(a) A plane mirror	(i) Used as a magnifying glass
(b) A convex mirror	(ii) Can form image of objects spread over a large area.
(c) A convex lens	(iii) Used by dentists to see enlarged image of teeth.
(d) A concave mirror	(iv) The image is always inverted and magnified
(e) A concave lens	(v) The image is erect and of the same size as the object.
	(vi) The image is erect and smaller in size than the object.

Solution:

Column-I	Column-II
(a) A plane mirror	(v) The image is erect and of the same size as the object.
(b) A convex mirror	(ii) Can form an image of objects spread over a large area.
(c) A convex lens	(i) Used as a magnifying glass
(d) A concave mirror	(iii) Used by dentists to see an enlarged image of teeth.
(e) A concave lens	(vi) The image is erect and smaller in size than the object.

4. State the characteristics of the image formed by a plane mirror

Solution:

Characteristics of the image formed by a plane mirror are as follows:
Image distance and object distance are equal
Size of object and image are equal
The image formed is erect and virtual
Images are laterally inverted

5. Find out the letters of English alphabet or any other language known to you in which the image formed in a plane mirror appears exactly like the letter itself. Discuss your findings.

Solution:

A, H, I, M, O, T, U, V, W, X, Y alphabets form images in a plane mirror exactly like the letter itself because these alphabets are laterally symmetric.

6. What is a virtual image? Give one situation where a virtual image is formed.

Solution:

The image that cannot be obtained on a screen is called a virtual image. The image formed by a plane mirror is virtual.

7. State two differences between a convex and a concave lens.

Solution:

Convex Lens	Concave Lens
Thick in the middle and thin at the edge	Thin in the middle and thick at the edge
Image formed is real or virtual	Image formed is virtual

8. Give one use each of a concave and a convex mirror.

Solution:

Concave mirrors are used in the headlights of cars and scooters.

Convex mirrors are used as side-view mirrors in vehicles.

9. Which type of mirror can form a real image?

Solution:

The concave mirror can form a real image.

10. Which type of lens forms always a virtual image?

Solution:

Concave lens form a virtual image.

Choose the correct option in questions 11–13

11. A virtual image larger than the object can be produced by a

(i) concave lens (ii) concave mirror

(iii) convex mirror (iv) plane mirror

Solution:

The answer is (ii) concave mirror

12. David is observing his image in a plane mirror. The distance between the mirror and his image is 4 m. If he moves 1 m towards the mirror, then the distance between David and his image will be

(i) 3 m (ii) 5 m

(iii) 6 m (iv) 8 m

Solution:

The answer is (iii) 6 m

13. The rear view mirror of a car is a plane mirror. A driver is reversing his car at a speed of 2 m/s. The driver sees in his rear view mirror the image of a truck parked behind his car. The speed at which the image of the truck appears to approach the driver will be

(i) 1 m/s (ii) 2 m/s

(iii) 4 m/s (iv) 8 m/s

Solution:

The answer is (iii) 4 m/s