CLASS 7

SCIENCE – CHAPTER – SOIL

Q1. Tick the most suitable answer in question 1 and 2.

In addition to the rock particles, the soil contains
(i) Air and water
(ii) Water and plants
(iii) Minerals, organic matter, air and water
(iv) Water, air and plants
Answer:
In addition to the rock particles, the soil contains
(iii) Minerals, organic matter, air and water.

Q3. Match the items in Column I with those in Column II:

Answer:

Column I Column II

(i) A home for living organisms (b) All kinds of soil

(ii) Upper layer of the soil (c) Dark in colour

(iii) Sandy soil (a) Large particles

(iv) Middle layer of the soil (e) Lesser amount of humus

(v) Clayey soil (d) Small particles and packed tight

Q4. Explain how soil is formed.

Answer:
Soil is formed through the process of weathering. Weathering is a process of
physical breakdown and chemical decomposition of rocks and minerals near or
at the surface of the earth. This physical and chemical decomposition is
primarily done by wind, water, and climate. As a result of these processes,
large rock pieces are converted into smaller pieces and eventually to soil.

Q5. How is clayey soil useful for crops?

Answer:
Following are the properties of clayey soil:

1. It has very good water holding capacity.

2. It is rich in organic matter.

For growing crops such as wheat, gram, and paddy, the soil that is good at
retaining water and rich in organic matter is suitable. Therefore, clayey soils
having these characteristics are useful for such kind of crops.

Q6. List the differences between clayey soil and sandy soil.
Answer:

Clayey Soil Loamy Soil

(i) It has much smaller particles. (i) It has much larger particles.

(ii) It can hold good amount of water. (ii) It cannot hold water.

(iii) It is fertile. (iii) It is not fertile.

(iv) Air content is low. (iv) Air get trapped between the particles.

(iv) Particles are tightly packed (iv) Particles are loosely packed

(iv) Good for growing various crops. (iv) Not suitable for growing crops.
Q8. Razia conducted an experiment in the field related to the rate of
percolation. She observed that it took 40 min for 200 mL of water to percolate
through the soil sample. Calculate the rate of percolation.
Answer:

9. Explain how soil pollution and soil erosion could be prevented.

Answer:
Prevention of soil pollution:
The persistent build-up of toxic compounds in the soil is defined as soil
pollution. To prevent soil pollution, its causes must be controlled.

1. Reduce the use of plastics: Plastics and polythene bags destroy the
fertility of soil. Hence, these should be disposed off properly and if
possible, their use should be avoided.
2. Industrial pollutants: Some waste products from industries and homes
pollute soil. These pollutants should be treated chemically to make them
harmless before they are disposed off.
3. Insecticides: Other pollutants of soil include pesticides and insecticides.
Therefore, excessive use of these substances should be avoided.

Prevention of soil erosion:

Removal of the upper fertile layer of the soil (top soil) by strong wind and
flowing water is known as soil erosion. Following steps can be taken to reduce
soil erosion:

1. Mass awareness to reduce deforestation for industrial purposes.

2. Helping local people to regenerate degrading forest.
3. Planting trees.

EXTRA QUESTIONS

Question 1.
In how many groups the soil has been categorised? Name them.
Answer:
The soil has been categorised into four major groups, i.e.

 clay
 silt
 sand
 gravel

Which soil is considered to be the most fertile and used for growing crops?
ANS- Loamy soil is the most fertile soil and used for growing crops.

Which type of soil absorbs more water but allows less wateMo percolate?
Answer: Clayey soil absorbs more water but allows less water to percolate.

When we add some dry soil to the water in a glass and stir it, bubbles are
observed from the soil. What causes these bubbles to raise
Answer:When dry soil is added in the water in a glass and stir it, bubbles
appear. These bubbles are caused due to the air present in between the soil
particles.

What is weathering?
Answer:Weathering is a natural process by which huge rock is broken down
into tiny particles but at a very slow rate.

There is shimmering of air just above the soil during hot summer day. What
does the term shimmering indicate? [HOTS]
Answer:The term shimmering means that the air shines with a slightly shaking
light.

Name two soil types used for cultivation.

Answer: Claley soil and loamy soil are rich in humus and are suitable for
growing plants like wheat and gram.

Soil has particles of different sizes. Arrange the words given below in
decreasing order of their particle size. Rock, clay, sand, gravel, silt [NCERT
Exemplar]
Answer: The soil particles with the decreasing order of their particle sizes can
be shown as
Rock > gravel > sand > silt > clay
There are two components of soil one that comes from living thing and other
from non-living thing. What are these two components?
Answer: two major components of soil are weathered rock particles, i.e. non-
living components and humus that consists of living organisms like fungi,
bacteria, worm, insects, etc.

Question 12.
It is necessary to ban the use of polythene bags and plastics. Do you agree? If
yes, why? [HOTS]
Answer:
Polythene bags and plastics pollute the soil as they kill the organisms living in
the soil.
That’s why, it is necessary to ban the use of polythene bags and plastics.

Question 13.
List the substances which pollute the soil.
Answer:
The substances which pollute the soil are polythene bags, plastics, waste
products, chemicals, pesticides, etc.

Question 15.
Explain the term humus that is generally found in the uppermost layer of soil.
Answer:
The rotting dead matter in the soil is called humus.

Question 16.
Can we make toys with the soil obtained from a field? Explain.
Answer:
No, soil from a field cannot be used to make toys. In order to make toys, the
soil should be clayey.

Question 17.
The percolation rate varies from soil to other soil. Which soil would have the
highest and least percolation rate?
Answer:
Percolation rate of water is highest in the sandy soil, while the lowest in the
clayey soil.

Question 18.
How can a farmer convert acidic soil into neutral soil? [NCERT Exemplar]
Answer:
The farmer can convert acidic soil into neutral soil by adding a small quantity
of quicklime or slaked lime solution to the soil.

Question 19.
Do all types of soil absorb water to the same extent? Give reason for your
answer.
Answer:
No, it is because different types of soil has different absorbing capacity of
water.

Question 20.
For planting rice, which kind of soil would be most suitable?
Answer:
For planting paddy (rice), clayey soil that is rich in organic matter and have a
good water retaining capacity is ideal.

Question 1.
The soil is said to be inseparable part of our life. Why?
Answer:
Soil is one of the most important natural resources. It is the uppermost layer of
earth’s crust which is blackish-brown in colour. Major functions of the soil are:

 It supports plant growth by holding the roots firmly and supplying water
and nutrients to the plants.
 It acts as a natural habitat for many organisms like earthworm, fungi,
bacteria, ants, etc.
 It is also essential for agriculture which provides us food, clothing and
shelter for all.
 It supplies water and nutrients to plants.

Therefore, we can say that the soil is inseparable part of our life.

Question 4.
A student visits two types of field and observes that soil from field A has high
percolation rate of water, while soil from field B has low percolation rate. In
field B, paddy crop was grown while in field A, there was no standing crop.
Name two types of soil present in two fields. [HOTS]
Answer:
Soil in field A is sandy soil while in field B soil is clayey soil which has low
percolation rate and water-logging is seen. Since, paddy requires lot of
standing water, therefore it is grown in field B.

Question 5.
Describe how clayey soil is useful for crops?
Answer:
Clayey soil is useful for the crops because

 it has good water retaining capacity.

 it is rich in humus.
 it is fertile in nature.

Crops like wheat and rice can be easily grown in clayey soil.
 CHAPTERS -ELECTRIC CURRENTS AND ITS EFFECTS

Q.5. Name any two effects of electric current.

Ans. Electric current has the following effect :
(i) Electric current can give rise to heating and lighting.
(ii) Electric current can convert a straight conductor into a temporary magnet.

Q.6.When the current is switched on through a wire, a compass needle kept

nearby gets deflected from its north-south position. Explain.
Ans. When current is passed through the wire, it deflects the compass near it
from its north-south position like a magnet. This is called magnetic effect of
the current. As we know that needle of the compass is made up of a thin
magnet. When this needle comes in contact with another magnet then the like
poles of the magnet repel each other and opposite poles attract each other. So
the deflection is seen in the needle. In this case the wire behaves like a
magnet and causes deflection in needle of the compass.

Q.8.Fill in the blanks:

(a) Longer line in the symbol for a cell represents its ___________terminal
(b) The combination of two or more cells is called a ____________.
(c) When current is switched ‘on’ in a room heater, it ___________.
(d) The safety device based on the heating effect of electric current is called a
_____________.

Ans.(a) positive (b) battery (c) becomes red hot and emits heat (d) fuse.

Q.9.Mark ‘T’ if the statement is true and ‘F’ it is a false:

(a) To make a battery of two cells, the negative terminal of one cell is
connected to the negative terminal of the other cell. [T/F]
(b) When the electric current through the fuse exceeds a certain limit , the fuse
wire melts and breaks. [T/F]
(c) An electromagnet does not aatract a piece of iron. [T/F]
(d) An electric bell has an electromagnet. [T/F]
Ans. (a) F (b) T (c) F (d) T

Q.10. Do you think an electromagnet can be used for separating plastic bags
from a garbage heap? Explain.
Ans. No, the plastic bags do not get attracted by the magnet, so they cannot
be separated by an electromagnet. Plastic bags are not magnetic materials,
only magnetic materials like iron can be attracted by the magnet.

Q.ll. An electrician is carrying out some repairs in your house. He wants to

replace a fuse by a piece of wire. Would you agree? Give reasons for your
response.
Ans. No, we would not agree to allow to replace the fuse by a wire. Wires in
the fuses are of specific melting points. So wd should always use ISI marked
fuses in our houses to prevent short circuits

Mention the name of the two devices that work on the basis of magnetic
effects of current.
Answer:
The devices that work on the basis of magnetic effects of current are
loudspeaker and electric bell.

Question 2.
Name the device used these days in place of eiectric fuses in electrical
circuits.
Answer:
The device used in these days in place of electric fuse is MCB (Miniature
Circuit Breaker).

Question 3.
State the property of a conducting wire is utilised in making electric fuse.
INCERT Exemplar]
Answer:
Electric fuse wire is made up of special material which has low melting point.
As if high amount of current is passed, it melts to disconnect the electric
circuit and prevent us from causing any damage.

Question 4.
Explain why are CFLs (Compact Fluorescent Lamps) preferred over electric
bulbs. [NCERT Exemplar; HOTS]
Answer:
Compact fluorescent lamps are preferred over electric bulbs because electric
bulbs use more power of electricity and it also losses electrical energy in the
form of heat but it is not so in compact fluorescent lamps.

Question 5.
Name the type of mark for which we should look at an electrical appliance
before buying, [HOTS]
Answer:
ISI mark is a mark for which we should look at an electrical appliance before
buying.

Question 6.
Briefly mention which part of the symbol of battery shows positive and
negative terminals?
Answer:
The positive terminal is represented by longer vertical line while negative
terminal is represented by shorter vertical line.

Question 7.
Briefly state the effects of electricity.
Answer:
The three effects of electricity are

 Chemical effect
 Heating effect
 Magnetic effect

Question 8.
When does a circuit is said to be overloaded?
Answer:
It is overloaded by connecting too many devices to it. When too many devices
get connected, then a circuit is said to be overloaded.

Question 9.
Why is an electric fuse required in all electrical appliance? [NCERT Exemplar]
Answer:
Electric fuse is required in all electrical appliances to prevent damage from
excessive current flow and during short circuit.

Question 10.
Give the name any two electrical appliances in which electromagnets are
used.
Answer:
Electric fan and electrical motor are the electrical appliances in which
electromagnets are used.

Question 11.
Paheli does not have a night lamp in her room. She covered the bulb of her
room with a towel in the night to get dim light. Has she taken the right step?
Give one reason to justify your answer. [NCERT Exemplar; HOTS]
Answer:
No, she has not taken the right step. Because due to excessive heat of bulb,
the towel may burn and it also results in the wastage of electrical energy.

Question 12.
The nails attract the pins. Comment.
Answer:
When the current is passed through the wire wound on the nail, it behave as
electromagnet. Due to this reason, the nails attract the pins.
Question 13.
Name the scientist who discovered the magnetic effect of electric current.
Answer:
The scientist who discovered the magnetic effect of electric current is Hans
Christian Oersted.

Question 14.
If the filament of the bulb is broken, would the circuit be complete? Would the
bulb still glow?
Answer:
If the filament of the bulb is broken, the circuit will not be complete.
So, the bulb will not glow.

Question 15.
Name some electric appliances where the heating effect of the electric current
is used.
Answer:
Some of the electric appliances where the heating effect of the electric
current is used are electric heater, geyser, micro-oven, room heater, boiler,
etc.

Question 16.
If the current through the coil stops flowing. Will the coil remain an
electromagnet?
Answer:
When the current through the coil stops flowing, the coil does not remain an
electromagnet.

Question 17.
State whether the bulb glows when the circuit is opened.
Answer:
No, the bulb does not glow when the circuit is opened. The bulb glows only
when the circuit is closed.

Question 18.
What happens to the current in a circuit at the time of short circuit?
Answer:
The value of the current in a circuit increases heavily at the time of short
circuit.

Question 19.
We should not touch the lighted bulb. Explain, why.
Answer:
The lighted bulb connected to main supply should not be touched by our hand
directly, as a lighted bulb can be very hot and can burn our hand as well.
Electric Current and Its Effects Class 7 Science
Extra Questions Short Answer Type Questions
Question 1.
Name two electric devices for each where
(a) heating effect of current is used and
(b) magnetic effect of current is used. [NCERT Exemplar]
Answer:
(a) Heating effect of current is used in electric heater and geyser.
(b) Magnetic effect of current is used in electric bell and cranes to lift heavy
magnetic materials from one place to other.

Question 2.
Why do we cover plug pinholes which are within the reach of children with
cellotape or a plastic cover when not in use? [NCERT Exemplar; HOTS]
Answer:
We do cover plug pinholes which are within the reach of children with
cellotape or plastic cover to avoid electric shocks. If unconsciously, a child
puts his finger in the electric socket, the shock may be fatal.

Question 3.
Can we use the same fuse in a geyser and a television set? Explain. [NCERT
Exemplar; HOTS]
Answer:
No, we cannot use same fuse in a geyser and in a television set because the
fuse used in every appliances has some limit to withstand the current flows
through it. So, different appliances have different fuses.

Question 4.
If cells are placed side by side. Then, how are the terminals of the cells
connected?
Answer:
If cells are placed side by side, then with the help of some connecting wires,
the positive terminal of one cell is connected to the negative terminal of other
to produce a combined power of all cells which can be called a battery.

Question 5.
Explain how a battery can be constructed.
Answer:
As we know that a battery is a combination of two or more cells and it can be
constructed by placing cells property on cell holder in such a way that the
positive terminal of one cell is connected to the negative terminal of other.
A piece of wire is connected to each of the two metal clips on the cells holder.

Question 6.
Distinguish between an open circuit and a closed circuit.
Answer:
Difference between an open circuit and a closed circuit

Open (Electric) circuit Closed (Electric) circuit

An open electric circuit is or electric A closed circuit is an electric path which

path which begins from the positive begins from the positive terminal of a cell

terminal of a battery or cell, gets broken or battery and terminates at its negative

at some point. terminal without any break.

Question 7.
Boojho made an electromagnet by winding 50 turns of wire over an iron screw.
Paheli also made an electromagnet by winding 100 turns over a similar iron
screw. Which electromagnet will attract more pins? Give reason. [NCERT
Exemplar; HOTS]
Answer:
Since the magnetic effect directly depends on the number of turns of the coil.
As, Paheli’s coil has more number of turns than Boojho. So, her electromagnet
is stronger than Boojho.
So, electromagnet of Paheli attracts more pins as compared to Boojho.

Question 8.
Does the electric current have other effect except heating? Name it.
Answer:
Yes, electric current have other effect except heating, i.e. magnetic effect of
current.
When electric current is passed through a coil, there is a magnetic field
developed around the coil or wire, if magnetic compass is placed near by, it
deflects the magnetic needle.

Question 9.
Explain the following.
(a) Copper and aluminium wires are usually employed for electricity
transmission. Explain the reason.
(b) Explain how does the resistance of a wire vary with its length.
(c) The tungsten is used almost exclusively for filament of electric lamp.
Comment.
Answer:
(a) Due to the low resistance and strength of aluminium and coppers, both
these metals are usually employed for the transmission of electricity.
(b) There is always an increase in the length of the wire with the increase in its
resistance.
(c) Since tungsten has high melting point and high resistance. So, that is why,
the tungsten is used exclusively for filament of electric lamp.

Question 10.
If we connect more cells in the circuit, then what will happen?
Answer:
If we connect more of cells in the circuit, then the nail will attract more pins. It
is due to the reason that the current flowing through the wire wound on the
nail will get increase which in turn will increase the strength of the
electromagnet.

Question 12.
If the current flows through wire, does the wire behave like a magnet?
Answer:
When the current flows through any wire, a magnetic field is developed around
that wire or coil and it behaves like magnet. It can be analysed by placing a
magnetic compass around the wire, it will show deflection of the needle.

Question 13.
If current is passed through a coil, does the pins cling to the coil?
Answer:
When an electric current is passed through a coil, it gets magnetised due to
phenomena of magnetic effect of current. When magnetic materials such as
pins are placed near to it. It gets attracted by the coil or we can say that pins
cling to the coil.

Question 14.
Batteries used in tractors, trucks and inverters are also made from cells. Then
why it is called a battery? [HOTS]
Answer:
The cell is the unit of battery when more than one cell are combined together,
it forms a battery. In trucks, tractors and inverters, cells are internally
arranged and we need not to connect it externally, so we called it as batteries

CHAPTER – 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 untreated 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
as 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:

Sanitisation and disease are interrelated as lack of sanitisation lead to illness and good
sanitisation 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.

EXTRA QUESTIONS

Question 6.
Name two things that can be made from sludge extracted during treatment of
sewage.
Answer:
Biogas and manure.

Question 7.
The Eucalyptus trees are planted along sewage ponds. Give reason. [HOTS]
Answer:
The Eucalyptus trees are planted along sewage ponds because these trees
absorb all surplus wastewater rapidly and release pure water vapour into the
atmosphere.

Question 8.
One can avoid the addition of the load in WWTP. Justify.
Answer:
By following proper sanitation and house-keeping practices, by creating less
waste at an individual level, we can avoid the addition of the load in
wastewater treatment plant.

Question 9.
Name two inorganic impurities present in sewage. [NCERT Exemplar]
Answer:
Inorganic impurities present in sewage are nitrates and phosphates.

Question 10.
Mention the use of dried sludge.
Answer:
It is used as manure, thus returning organic matter and nutrients to the soil.

Question 11.
In which type of toilets, human excreta is treated by earthworm?
Answer:
Vermi-processing toilets.

Question 12.
Give some examples of onsite sewage disposal systems.
Answer:
Chemical toilets, septic tanks, composting pits, etc.

Question 13.
Sewage contains some organic impurities in it. Name them.
Answer:
Human faeces, animal wastes, oil, urea pesticides, herbicides, fruits and
vegetables are the organic impurities present in sewage.

Question 14.
Mention the common process of cleaning of wastewater.
Answer:
Sewage treatment.

Question 15.
World Water Day is celebrated on
Answer:
22 March.

Question 16.
Mention the period proclaimed as the International Decade for action on Water
for Life.
Answer:
2005-2015 is the period proclaimed as the International Decade for action on
Water for Life.

Question 17.
Name the three processes involved in treatment of wastewater.
Answer:
Treatment of wastewater involves physical, chemical and biological processes
for removing different types of contaminants.
Question 18.
Explain sewers and sewage.
Answer:
Sewers are the pipes acting as a transport system that carries sewage from
point of origin to the point of disposal. The wastewater released from homes,
industries, hospitals, offices and other uses is called sewage.

Question 19.
The used water should not be wasted. Do you agree? Justify.
Answer:
The used water should not be wasted but it should be cleaned.

Question 20.
Bleaching powder is mixed in water, why? Think and give appropriate reason
for it. [HOTS]
Answer:
Bleaching powder is mixed in water to make it safe for drinking because it kills
the harmful germs present in water.

Question 21.
Natural water is the purest form of water. Explain why or why not.
Answer:
Natural water is not pure always. It is because all types of water except
rainwater contain salts dissolved in it. These salts make the water impure.

Question 22.
Mention the role of effluent treatment plants in cities.
Answer:
The effluent treatment plants filter out undissolved materials form water.

Question 23.
Protozoa is causative organism of which waterborne disease?
Answer:
Dysentry.

Question 24.
In the chemical process of water treatment, water is treated with some
chemicals. Describe chlorination in light of it. [HOTS]
Answer:
Chlorination is the process of adding chlorine (chemical) in water to make it
safe for drinking.

Question 25.
Sanitation can be improved. Explain, what step should be taken to improve it.
Answer:
Low cost onsite sewage disposal system must be encouraged to improve
sanitation.

Question 26.
Give the percentage of water being activated in sludge.
Answer:
About 97% of water is being activated in sludge.

Question 27.
Animal waste, oil and urea are some of the organic impurities present in
sewage. Name two more organic impurities present in sewage. [NCERT
Exemplar]
Answer:
Fruits and vegetable wastes, pesticides and herbicides are organic impurities
present in sewage other than animal wastes, oil and urea