Electrical Circuits
Q
IV

Answer the following
What is an electrical circuit? What are its basics components.
Ans. Electrical circuit is a closed conducting path of electric current through an electric appliance, a
switch and a source of electricity. The basic components are switch, cell and bulb.

Ans.

Ans.

Ans.
Explain the importance of a switch in a circuit.
A switch is used to make or break the path of electric current in the circuit.
Draw a neat labelled diagram of a closed simple circuit.
For diagram refer page no 37 figure no 3.2
What are the advantages of a parallel circuit as compared to a series circuit ?
Advantages of a parallel circuit

Appliances can work to their full capacity because each appliance gets designed voltage.

When the cells are connected in parallel bulb and cells last longer.

Electrical appliances in a series circuit can not work to their full capacity why?
Ans. Electrical appliances in series circuit are interdependent and do not work to their full capacity
due to sharing of voltage.

State the difference between series and parallel circuits.
Ans
Parallel circuit

Series circuit
The electrical appliances are connected
appliances are connected
The electrical
in a chain with a positive terminal of an
have a common appliance connected to a negative terminal
a common negative terminal.
in such way that they
positive and
of the other.

Appliances do not work to their full capacity
their full capacity
as there is sharing of voltage.
Appliances can work to
because
each appliance gets designed
voltage.

How do you connect cells of each 1.5 volt to form a battery of 12 volt? Find the number of cells
required.
Ans.
Connect the cells to form a battery so that negative terminal of one cell is in contact
with positive terminal of the next cell. If each cell of 1.5 volt , to form battery of 12 volt 8 cells
are required.

Draw a neat labelled circuit diagram of a domestic wiring system .
Ans.

For diagram refer page no 46 fig 3.16
What is earthing ? Why it is essential ?
Ans.
The connection of earth wire to all electrical fitting in the house is called earthing .
Earthing is essential because , leakage current in the circuit flows to the ground through the
earth wire and protect the user from electrical shocks.
Electrolysis
Q
I
choose the correct answer

It is an example for electrolyte ---- hydrochloric acid

The substance obtained at anode during the electrolysis of water is ------ oxygen

Helium is filled in weather balloons, because it is ---- non- inflammable

The gas used in advertising sign lamps is ---- neon

Argon is filled in the electrical bulb because it is ---- less reactive even at high
temperature.
Q II

Answer the following
Make a list of the difference between electronic conductors and electrolytic conductors.
Ans.
electronic conductors-----a)Conduct electric current by the movement of
electrons. b)
They do not go any chemical when electric current is passed through
them.
Electrolytic conductors-----a) Conduct electric current by the movement of ions.
b) They under go chemical change when electric current is passed through them.
2. Define electrolytes and non electrolytes .Give two example for each.
Ans. Electrolytes----The substance which conduct electricity either in molten state or in the
aqueous solution are called electrolytes. Ex Hydrochloric acid , Sodium chloride
Non electrolytes---- The substance which do not conduct electricity either in molten
state or in the aqueous solution are called non electrolytes. Ex. Sugar, glucose
3.
What is elecrolysis ?
Ans. The phenominon of decomposition an electrolyte by passing electricity through it is
called electrolysis.
4. Explain the process of electrolysis with the help of an example.
Ans. Take the apparatus electrolytic cell, Fill the container with cupric chromate solution and
pass the electric current. After a short period migaration of ions takes place. Copper ions which
are blue move towards the cathod and chromate ions which are yellow move towards the anode.

Ans
What is an electrolytic cell?
The apparatus in which electrolysis is carried out is called electrolytic cell.

With the help of a neat labelled diagram describe the process of electrolysis of water.
Ans.
For diagram refer page no 147
fig 9.1
Take pure water in aHoffman’s voltameter ,add dilute dilute acid to it. Pass electric
current. After some time it is found that, the volume of gas collected in the test tube ‘A’
which is connected to negetive terminal of the battery(cathode) is double the volume of the
gas collected in the test tube ‘B’ which is connected to the positive terminal of the battery
(anode) When a glowing splinter is brought near the gas released at cathode a pop sound is
heard. It is hydrogen . When a glowing splinter is brought near the gas released at anode, it
burns brightly. It is oxygen

Name the noble gases. Write any one use of each.
Ans. The noble gases are helium , neon, argon, xeon, krypton, and radon.
Helium -----1) Used as coolant in nuclear reactors. 2) Used in helium- neon lasers
Neon----1) Used in advertising sign lamps 2) Used as refrigerant
Argon---1) Used to fill in electric bulb
Krypton---Used in the airport runway light 2) Used in light house lamps
Xenon----Used in photographic flash units.
Radon----Used to kill the cancerous tissues.
Pollution
I choose correct answer
 A reason for pollution which may not be always natural is ……… mining
 The common name for the unavoidable end product of human activity that
spoils the quality of biosphere is ………………DDT
 One of the following is a biodegradable pollutant ……………….. kitchen waste
 One of the following is not a pollutant discharged from industries
…..sewage
II Fill in the blanks
 Human and animal waste ( faeces ) is an example of biodegradable
pollutant.
 The fuel which emits less pollutants is kerosene or LPG
 Green house effect causes an increase in temperature.
 Organic farming is suggested to avoid soil pollution.
III Match the following
A
B
1.
plastic
non-biodegradable
2.
kitchen waste
biodegradable
3.
sulphur dioxide
air pollutant
4.
pollen dust
biological air pollutant
5.
bio gas
alternative fuel
IV Answer
the
following
 Define pollution and mention the three areas of pollution you have studied.
 Any undesirable change in the air, water, or soil which disturbs the delicate
balance of the natural environment is called pollution.
 Air pollution
 Soil pollution
 Water pollution’


What are biodegradable pollutants ? Give two examples.
Any pollutant when converted into its harmless state through the activity
of organisms ( chiefly bacteria ) is called a biodegradable pollutant.
Example – kitchen waste, domestic sewage.
 Define non-biodegradable pollutants? Give two examples.

Any pollutant which is not converted into its harmless state by organisms
and remains unchanged for a long time is called a non-biodegradable
pollutant.
Example – plastic, heavy metals
 Differentiate biodegradable pollutants from non biodegradable pollutants.
Give an example for each.
Biodegradable pollutants example kitchen waste has undergoes
decomposition by the activity of bacteria. Whereas non biodegradable
pollutants eg Plastic cannot be converted into its harmless state by
organisms and remain unchanged for a long time.
 Make a list of any four causes of air pollution.
 Causes of air pollution
 Fuels used in industries, vehicles and thermal power plants emit carbon
monoxide, sulphur dioxide, nitrogen dioxide and other gases as well as
small particles.
 Chemical pesticides and insecticides sprayed on crops.
 Carbon dioxide and methane released into air during the decay of
organic matter such as fallen leaves and found waste.
 Dust particles produced during construction activities such as roads,
buildings and factories.
 Carbon dioxide, carbon monoxide and small particles released by
burning wood, agricultural waste and dung cakes.
 Menton any four causes of water pollution.
 Causes of water pollution
 Industrial wastes containing harmful chemicals
 Industrial discharge of hot water into water bodies.
 Domestic wastes and sewage containing harmful microorganisms and
detergents.
 Seepage of water containing fertilizers, pesticides and weedicides from
agricultural fields and entry of agricultural wastes.
 Discharge of mining wastes.
 Oil spillage.
 Explain how soil is polluted.
 When components harmful to human beings and other organism get into
the soil, the natural quality of soil gets changed or soil gets polluted.
 State the effects of air pollution.
 Effects of air pollution
 Affects health of human beings seriously. Affects the respiratory organs
( lungs ). It causes diseases such as asthma, bronchitis, carbon monoxide
blood poisoning and even cancer.
 Harms agricultural crops and other plants and reduces their yield.
 Long term air pollution causes acid rain.
 What are the effects of water pollution ?
 Effects of water pollution
 Polluted water is unstable for drinking and other domestic activities,
agricultural practices and even for aquatic organisms.
 Harmful micro organisms present in water may cause several diseases
such as diarrhea, jaundice, typhoid and others.
 Hazardous chemicals may cause nervous disorders, affect vital organs
such as liver and kidney to bring about permanent damage to the body.
 Harmful chemicals present in water even kill plants and animals.
 Pollution of water results in reduction of dissolved oxygen. This affects
survival of aquatic organisms.

Mention any two effects of soil pollution.

Effects of soil pollution
 Soil pollution affects useful organisms such as earthworms, ants and
others.
 The polluted soil becomes unusable.

Suggest any five measures to reduce air pollution

Measures to reduce air pollution
 Decreasing emissions from industries and motor vehicles using clean
technologies and regular maintenance of engines.
 Using fuel which emit less pollutants. For example kerosene, Liquid
Petroleum Gas (LPG) or sulphur free petrol.
 Using public transport like buses and trains and minimizing the use of
private vehicles like cars, motorcycles etc. Car pool may be good idea.
 Using renewable sources of energy such as solar energy, wind energy
and bio gas.
 Avoiding bursting of crackers.
 Growing more trees which act as screens or insulators against dust and
other particles and even absorb pollutants like oxides of sulphur.
 Strict enforcement of anti pollution laws.

What measures do you suggest to reduce water pollution?

Water pollution can be reduced by
 Releasing only treated sewage into natural water bodies.
 Cooling the over-heated waste water from industries before letting into
water bodies.
 Preventing the flow of water from agricultural fields into the natural
water bodies.
 Strict enforcement of anti-water pollution laws.

Suggest any two control measures to reduce soil pollution.
 Soil pollution can be reduced by
 Separating the wastes into biodegradable, non-biodegradable and
hazardous wastes, and disposing them using scientific method.
 Decreasing the over use of fertilizers and pesticides like neem cake.
 Disposing the industrial wastes properly.
 Growing and maintaining plants that absorb some of the pollutants.
 Bring out the relationship between growing trees and reducing air
pollution.


Trees act as a insulators against dust and other particles and even absorb
pollutants like oxides of sulphur.
How do harmful chemicals in the soil effect us ?
Ans. Harmful chemicals in the soil get into the plants and their products
such as fruits ,seeds , vegetables and grains . When human beings
consume these, their health is affected.

Why do you think stagnant water bodies like ponds and tanks appear
green?
Ans. If the stagnant water bodies like ponds and tanks appear will think
that water is no more clean.
17 .
Why is sewage treatment is necessary .?
Ans
Sewage treatment is necessary because sewage containing
harmful micro organisms and detergents.

Organic farming is preferred today why ?
Ans.
Organic farming is preferred today because no chemical fertilizers
and pesticides are used in this type of farming, the produce is less contaminated.
Reproduction
Q
I
Choose the correct answer

The reproductive part of a higher plant is the

The process of fussion of male and female gametes is called
flower.
fertilization.

Mature ovary forms the

The reproductive part of Bryophyllum

Potato is a root.

In ginger ,food is stored in its stem.

Reproduction by budding is found in yeast .

Pollen grains are formed in anther.

Reproduction by fragmentation is common in spirogyra.

Fruits develop from ovary.

Q II
Q
fruit.
is leaf bud.
Reproduction by spores is seen in bread mold.
Fill in the blanks

The fussion of male and female gametes is termed fertilization.

Ovules contain female gametes.

Pollen grains contain male gametes.

After fertilization , an ovule becomes seeds.

Plants produce seeds as a result of sexual reproduction.
III

Answer the following
Define reproduction. Mention its importance .
Ans
Reproduction is the capacity of all organisms to produce young ones of their own kind. It helps
in the continuity of a species.

Draw he structure of the flower you have studied and name its parts.

Name the two methods of reproduction in higher plants.
Ans The two methods of reproduction in higher plants are Sexual reproduction and
Asexual reproduction.

Define pollination.
Ans
The process of transfer of pollen grains from the stamen (anther) to the stigma of a
flower is called pollination.

What are pollinators ? Name different pollinators.
Ans. The agents that pollinate flowers are called pollinators. Pollinators are birds, insects,
bats, even air and water.

Explain each of the following with an example.

Fission Fission means division . It generally occurs in singled celled organisms. The
parent cell divides into two daughter cells it is termed binary fission, if more than two
daughter cell it is termed as multiple fission .

Fragmentation In this process, the parent body breaks into two or more pieces and each
pieces develops into a complete new organism.

Regeneration When some animals lose their body parts, they re-grow the lost parts of
their body and develop into complete animals.

Ans.


Spore formation Ferns, mushrooms and molds reproduce from tiny structure called
spores, especially during unfavorable conditions.

Budding A bud is developed on parent body as an out growth The bud grows into a
complete organism. Fully formed bud separates from the parent body.

Vegetative Reproduction
as stem , root and leaves.
It involves production of young individuals from plant parts such
Name two plants which usually reproduce without seeds.
Potato and ginger plants
reproduce without seeds.
Distinguish between
Androecium
Gynoecium
Androecium is male reproductive
reproductive
Gynoecium is the female
Part.
Part.
2.
Self-pollination
Cross-pollination
The transfer of pollen grains
grains from anther of
from anther of flower plant to
another flower of
to stigma of same flower or other
species.
The transfer of pollen
flower plant to stigma of
a different plant of the same
any other flower of same plant.
3
Sexual reproduction
Asexual reproduction

It involves two parent.
parent.
It involves a single

Gametes are produced.
produced.

Young ones produced show
identical to
Gametes are not
Young ones produced are
characters of both the parents
their parents.
or they do not resemble their
parents completely.
Q
IV
Match the following
A
B
Std

Budding

Vegetative reproduction

Fragmentation

Spore formation
fungi

Regeneration
sea star
Yeast
Bryophyllum
Spirogyra
VII
Light
Q I
Choose the correct answer

In a pin hole camera the image formed is small and inverted.

The centres of two imaginary spheres of which the lens is a part is called centre of curvature.

The distance between the optic centre and the principal focus is called focal length.

The nature of the image formed after refraction in a convex lens when the object is placed at
C1 is
real , inverted , same size
Q II
Fill in the blanks

Objects that do not emit light of their own is called non-luminous bodies.

We can see partially through translucent objects.

Q
Regular reflection occurs only on smooth surface.
III
Underline the word which does not belongs to the group

Stone , cardboard box , glass , aluminium sheet

Sun , moon , stars , bulb
Q IV
Give two examples for each of the following

Non-luminous body

Transparent objects

Source of light
Q
V

Ex
Ex
Ex
wall , table
glass, clear water
stars , candle
Give reason
Light has to fall on objects for it to visible
Ans. The objects reflect the light falling on them. When this reflected light reaches the eyes
,the objects are seen. So Light has to fall on objects for it to visible

The moon is called a non-luminous body.
Ans.

The moon is called a non-luminous body.
Because moon reflects the sun light.
When a light falls on a tree a shadow is formed.
Ans.
As light travels in a straight line, opaque object obstructs the path of light from falling on a
surface and shadow is formed.

Infinite number of images are formed when an object is placed between two parallel mirrors
facing each other.
Ans. Infinite number of images are formed when an object is placed between two parallel
mirrors facing each other. This is because the image formed in one mirror acts as an image for
the other plane mirror.

A concave lens is called a diverging lens.
Ans.

A concave lens diverge the light rays , So concave lens is called diverging lens.
In terrestrial telescope, the object is placed at the centre of curvature of the lens.
Ans.
To get real and same size of the object, In terrestrial telescope, the object is placed at
the centre of curvature of the lens.
Q
VI

Answer the following
How are we able to see things around us ?
Ans.
We able to see things around us because light falls on them and reaches our eyes.
Signals are then passed to brain. The brain helps us to understand the appearance , location and the
movement of the objects viewed.

What is the difference between luminous and non-luminous bodies? Give one example each.
Ans.
Non-lumonous
Luminous
Bodies that emit light of their own
do not emit light of their own
Bodies that
are called luminous bodies
are called non-luminous bodies
Ex Sun, stars
Ex planets , stones

Explain the rectilinear propagation of light with the of an experiment.
Ans.
Aim . light travels along a straight line
Materials :
Cardboards and
candle
Procedure: Cut three cardboards A, B and C. Pierce a hole in them so that the holes
are in the same straight line. Place a lighted candle so that its flame is at the same height as the
holes in the card board. When the card board is shifted slightly, the flame cannot be seen through C.
This is because light cannot bend and pass through B. This shows the light travels along the straight
line. This phenomenon is called rectilinear propagation of light. For figure refer page no 77 fig
5.5

How translucent bodies are different from opaque bodies?
Ans. The light passes partially through translucent substance where as opaque substances do not
allow the light to pass through them at all.

What is reflection of light?
Ans. The bouncing back of light from any surface is called reflection.

Differentiate between the reflection and refraction of light.
Ans. Reflection : The bouncing back of light from any surface is called reflection.
Refraction : When light travels obliquely from one medium to another, it deviates from its original
path. This property of light is called refraction.

Make a list of the different types
lens
Ans.
Types of Convex lens
a) convex lens
1) Double convex lens
b) concave
2) Plano-convex lens
3) concavo-convex
lens
Concave lens
1) Double concave lens
2)
Plano-concave lens
3) Convexo-concave lens
8.
Ans.
What is a spherical lens ?
Spherical lens is a piece of transparent material having one or two smooth ,curved surfaces.
9.
Draw the ray diagram for the following
When an object is placed beyond C 1 of a convex lens Write down the
nature and size of the image
Ans.
Refer page no 91 fig 5.34
Nature of the image --- Real. Inverted and diminished
10.
What is the nature and size of the image formed by convex lens when object is placed
between
C 1 and F1
Ans. Nature and size of the image formed by convex lens when object is placed between
C1 and F1 real , inverted and enlarged
11.
Where should the object be placed if the nature of the image formed in a convex lens is virtual,
erect and enlarged.
Ans. When the object is between optic centre(O) and principle focus (F) the image formed in a
convex lens is virtual, erect and enlarged.
12.
What is dispersion of light?
Ans.
The splitting of white light into several colours is called dispersion.
Std
VII
Science
Q I

Constellations
And Galaxies
Name the following
The branch of science that deals with the study of celestial objects.
_________Astronomy

A group of stars with the name assigned to it.
_____-Constellation .

The distance travelled by light , in vacuum in a year.
______Light year
Q II

Answer the following
What is the universe ?
Ans. The planets , stars, moon, satellites, all heavenly bodies, everything on the earth and the
emptiness in between forms the universe .

What are galaxies?
Ans.

Galaxies are a collection of stars ,gases and dust bound together by gravity.
Name the galaxy we are on. What type of galaxy is it?
Ans.

We are on milky way galaxy. The milky way is a spiral galaxy.
Define the following

Elliptical galaxy-_____Elliptical galaxies are mostly spherical in shape formed by the collision
of smaller spiral galaxies.

Spiral galaxies-------- Spiral galaxies consists of a flat central disk with spiral arms.

Irregular galaxies -----An irregular galaxy is a galaxy that does not have a distinct regular
shape.

What is an orbit? What is its shape .
Ans.
The path traced by a heavenly body around another is called an orbit. Orbit in the shape
of an ellipse.

Std
QI
Explain the following

Ursa major-----This constellation is also called the Great Bear. It is visible through out the
year but best seen in the month of April. Its Indian name is Saptharishi.

Ursa Minor ____Ursa minor means smaller bear. It is seen in the northern sky. The pole
star is one of the star in this constellation.

Orion ______This constellation is visible during winter. It consists of a group of seven bright
stars and several faint stars. The stars Betelgueuse and Rigel are two of its bright stars.Orion
is also called the hunter. Its Indian name is Mriga
VII
Allotropy of Carbons
Choose the correct answer
 The structure of diamond is tetrahedral.
 Diamond is used as a gem stone , because it is a giant crystal.
 Graphite is used as electrodes in dry cells, because it is a good conductor of
electricity.
 Coke is a good fuel , because it burns without smoke.
 The allotrope of carbon consisting of discrete molecules is fullerene.
 Substances obtained when coal is subjected to destructive distillation are
coal tar and coke.
 Charcoal floats on water , because it is porous.
 Allotropy is caused due to the difference in arrangement of atoms.
 The gas which turns lime water milky is carbon dioxide.

The compound of carbon which is used in the manufacture of cement is
calcium carbonate.
 The chemical name of baking soda is sodium bicarbonate.
Q
II
Match the following
A
B
 Bone charcoal
decolourization of sugar
a)
 Gas carbon
formed on the roofs of the
b)
the deposites
retorts during destructive distillation of coal
 Diamond
instrument
c)
glass cutting
 Fullerene
ball
d)
yellow spherical
 Coal
by slow decomposition of plants.
Q
III
e)
formed
State true or false. If false give reason
 Coke reacts with metal oxides to give metals.
Yes.
 Fullerene cannot be used as lubricants.
False. Fullerenes are bucky (spherical) balls, the balls can roll
between the surfaces. so fullerene is wonderful lubricant .
 Coal gas is obtained when coal is subjected to destructive distillation.
True.
 Sugar charcoal consists of a lot of carbon compounds as impurities.
False. Because sugar charcoal is the purest form of amorphous carbon.
 Gas carbon is a bad conductor of electricity.
False . It is a good conductor of electricity , so it is used in making
electrodes.
Q
IV
Answer the following
 What is allotropy? Name the allotropic forms of carbon . Mention one use of
each.
An s. An element can exist in more than one form. This phenomenon is called
allotropy.
The allotropic forms of carbon are diamond , graphite, fullerene, charcoal,
coke, coal, gas carbon, and lampblack.
Diamond—Used in the tools of cutting glasses and drilling rocks
Graphite---Graphite is mixed
lead.
with little amount of clay and used as pencil
Fullerene---Used as lubricants in electronic devices.
Charcoal---Charcoal is used as a fuel.
Coal ---Coal is used as fuel in thermal power plants and in steam engines
Coke---coke is used in the extraction of iron from
its ore.
Gas carbon—It is used in making electrodes.
Lampblack---It is used in the manufacture of printing ink, black paint, Indian
ink and shoe polish.
 Explain the preparation of wood charcoal, with the help of a neat labelled
diagram.
Ans.
Take a hard glass test tube. Put some wood shavings into it. Close the
mouth with one holed rubber stopper. Then connect it to another test tube as
shown in the figure.
For figure refer page no. 164
fig 10.10
Heat the hard glass test tube filled with wood shavings. We get charcoal as
residue in it. Wood tar and a mixture of organic compounds are obtained in
another test tube.
 What are the uses of charcoal.?
Ans. Uses of charcoal
1) It is used as a fuel.
2) It is used in the manufacture
of gun power. 3) Activated charcoal is used in the gas masks
of water
and in purification
 What are the uses of coal ?
Ans. Uses of coal
1) Coal is used as fuel in thermal power plants and in
steam engines. 2)It is used in the manufacture of coal gas , coal tar, and coke
3) It is used in the manufacture of fuel gases like water gas and producer gas
 Which allotropy of carbon is used in the manufacture of black paint ?
Ans. Lampblack is used in the manufacture of black paint.
 How coke is obtained ?
Ans. When coal is subjected to destructive distillation, coke is obtained as a
residue.

State any two uses of fullerene ?
Ans. Uses of fullerene
picture tubes
1) It is used as lubricants. 2) Fullerene are used in T.V
 How is calcium carbonate
prepared in the laboratory ?
Ans. Calcium carbonate is formed by the reaction of carbonic acid with calcium
hydroxide. Carbonic acid is formed when carbon dioxide dissolve in water.
Carbon dioxide
+ water →Carbonic acid
Carbonic acid + Calcium hydroxide
→ Calcium carbonate +water
 Write any one use of the following
Calcium carbonate ,
Sodium carbonate,
Sodium bicarbonate
Ans. Calcium carbonate___It is used in the manufacture of cement and glass.
Sodium carbonate____It is used in softening hard water.
Sodium bicarbonate____It is used in the manufacture of baking powder.
Allotropy of Oxygen
QI
Answer the following
 Name the allotropic forms of oxygen.
Ans.
Allotropic forms of oxygen are diatomic molecule and tri-atomic
molecule.
 What are the properties of oxygen ?
Ans. Properties of oxygen 1) Oxygen reacts with other elements to give the
respective oxides. 2)It react with some compounds and decomposes them.
 What is ozone ? state its properties.
Ans. Ozone is a light bluish gas found in the upper layer of the atmosphere.
Properties of ozone are 1) ozone is a poisonous gas with fishy smell.
2) it causes respiratory problems, if inhaled.
3) it damages plants and trees, if comes in contact with them.
4) it causes damages to automobile tyres and asphalt, if reacts with them.
4.
How does ozone layer protect the living organisms on the earth?
Ans. Ozone layer prevents ultraviolet rays coming from the sun and thus
prevents them to reach the earth. Thus it protects the life on the earth from
harmful effects of U.V rays.
5
What is depletion of ozone layer?
Ans. The decrease in the quantity of ozone in the upper layer of the atmosphere
is called depletion of ozone layer.
 What are the causes for the depletion of ozone layer?
Ans.
Pollutants like oxides of nitrogen and chlorofluoro carbon are the
causes for the depletion of ozone layer.
 Suggest some measures to prevent the depletion of ozone layer.
Ans.
Std VII
Q I
Control of air pollution will save the ozone layer.
Science
Chemical Bonding
Choose the correct answer

Single shell atoms try to acquire this configuration duplet


c) triplet
d) none of these
argon
b) helium
c) neon
d)
krypton
It is a noble gas radon.


b) duplet
On combing with other element chlorine acquires the configuration of argon.


Octet
chlorine
b) calcium
c) radon
d)
oxygen
Noble gases are non reactive because of duplet or octet configuration

duplet configuration
configuration
b)
octet configuration
c) duplet and octet
d) duplet or octet configuration
Q II
Q
Fill in the blanks

The electronic configuration of sodium is

The chemical bond of potassium chloride molecule is due to transfer of electron

Ionic compounds are poor conductors of electricity in the molten state.

Covalent compound are also called covalent molecule.

Ionic bonds are also called electrovalent bond
III
2,8,1
Name the following

The noble gas that has only two electrons. ----

The noble gas that has an atomic number 36 ---- krypton

A molecule that has double covalent bond .-------oxygen

The type of bond usually present in compounds that dissolve in water ----Ionic bond or
covalent bond

Two types of chemical bond – Electrovalent bond and covalent bond
Q
IV
Helium
Under the word which does not belong to the group

helium, xenon , argon , radon

Sodium chloride , potassium chloride , hydrogen chloride, magnesium chloride

Q
hydrogen, chlorine ,hydrogen chloride, nitrogen
V

Ans.

Give reason for the following
Chemical reactions occur.
Chemical reactions occur due to the redistribution of electrons.
Ionic compounds have high melting point and boiling point.
Ans.
Ionic compounds are crystalline, hard and brittle obviously large amount of energy is
required to separate the ions. So ionic compounds have high melting point and boiling point.

Covalent compounds are soft solids , liquids or gases at room temperature.
Ans.
Covalent compounds formed by sharing of electron between two atoms and they
are very unstable. So Covalent compounds are soft solids , liquids or gases at room temperature.
Q VI

Ans.
Answer the following
How do elements that are not stable try to attain stability ?
Elements try to attain stability either by

Transfer of electrons( by donating or gaining) from other atoms .

Sharing of electrons with other atoms.

What is one of the most important factor for the formation of molecules of compounds and
molecules
Ans. The most important factor for the formation of molecules of compounds and molecules is its
electronic configuration.

Ans.

Draw a diagram representing the ionic bondformation in a sodium chloride molecule.
Refer page no 114
figure 7.2 Ionic bond formation in NaCl
Define the following

Chemical bond-----A chemical bond is defined as a mutual attraction between two atoms
resulting from a redistribution of the electrons in their outermost shell.

Ionic bond------The chemical bond formed as a result of the transfer of electrons from one
atom of an element to another is called an electrovalent bond or ionic bond.

Covalent bond ----The chemical bond form between two or more combining atoms by
mutual sharing of one or more electrons of non metallic atoms is called covalent bond or
molecular bond.

Draw diagrams to show the formation of
Ans.
a)
A single covalent bond-__________
Refer page no 117
b) a double covalent bond__________Refer page no 118
c)
fig 7.7
fig 7.10
a triple covalent bond --in suitable molecule____Refer page no 119
fig 7.11
6.
Ans .
What are ionic compounds ? Give two examples.
Compounds formed by an ionic or electrovalent bond are called ionic compounds.
Ex
7
Ans.
Potassium chloride ( KCl)
What are covalent compounds ? Give two examples.
Compound formed due to covalent bonds is called covalent compound.
Ex
8.
Magnesium oxide ( MgO),
Hydrogen , Chlorine
Make a list of four differences between ionic compounds and covalent compounds.
Ionic compounds
Covalent compounds

Crystalline ,hard and brittle at
or gases at room
Soft solids or liquids
room temperature
temperature

Good conductors ofelectricity
electricity
Bad conductor of
in solution or molten state

High melting point and boiling
and boiling point
point

Very stable
Very unstable
Low melting point
Chemical Reaction
Q
I Choose the correct answer
 Two spoon of potassium chlorate is taken in a test tube and heated.
Potassium chloride is obtained in the test tube, oxygen is evolved. This is
an example for chemical decomposition .
 The gas used to extinguish fire is carbon dioxide.
 The substance used as catalyst in the preparation of oxygen is
manganese dioxide.
 Hydrochloric acid is treated with a base , sodium hydroxide. It gives a
salt .sodium chloride and water is an example for neutralization.
Q II
Match the following
A
B
 Dipping an iron nail in copper
displacement
chemical
sulphate solution
 Burning magnesium in air
chemical combination
 Mixing sodium sulphate solution
Solution and barium chloride
Solution
double displacement
 Heating potassium chlorate
chemical decomposition
Q
IV
Answer the following
 Write the difference between physical change and chemical change
Ans.
Physical change
Chemical change
 No new substance is formed
formed
New substance is
 The original substance can be
cannot be
The original substance
obtained easily by physical
physical
obtained easily by
methods
methods
Q
2. Define chemical reaction. Name the types of chemical reactions.
Ans. The process which brings about a chemical change is called chemical
reaction
Typesof chemical reactions are 1) Chemical combination
2) Chemical decomposition 3) Chemical displacement 4) Chemical double
displacement
3.
What is chemical combination? Give example.
Ans. When two or more substance combine to form a single product, the
reaction is called chemical combination.
Ex
Carbon + Oxygen
→Carbon dioxide
Hydrogen + Oxygen
→Water
4
What is chemical displacement? Give example.
Ans. A chemical change in which a more active element displaces the less active
element from a compound is called chemical displacement.
Ex
Copper sulphate + Iron → Copper + Iron sulphate
 What is neutralization? Give an example
Ans. The process of reaction between acid and base to form salt and water is
called neutralization reaction.
ExSodium hydroxide + hydrochloric acid →sodium chloride + Water
(NaOH)
NaCl
+
(HCl )
→
+ H2 O
 What is double displacement reaction? Give example.
Ans. A chemical change in which two compounds in solution react to form two
new compounds by mutual exchange of radicals is known as double displacement
reaction.
Ex
Sodium sulphate + Barium chloride
chloride
→
Barium sulphate + Sodium
 Describe the method of preparation of oxygen in the laboratory from
potassium chlorate, with the help of neat labelled diagram.
Ans. For figure refer page no 136
fig 8.3
Aim---Preparation of oxygen
Chemicals----Potassium chlorate and manganese dioxide
Apparatus-----Test tube,jar, rubber cork, delivery tube beaker
Procedure-----Take the mixture Potassium chlorate and manganese dioxide
In a test tube in the ratio 4: 1 Close the mouth of test tube with rubber cork.
Arrange the apparatus as shown in the figure. Heat the mixture of Potassium
chlorate and manganese dioxide in the test tube. After sometime a gas is
liberated. It can be identified by the formation of bubbles in the glass jar.The
collected gas in the jar is Oxygen.
 How is oxygen useful to us?
Ans.
Uses of oxygen.
1)
Oxygen is essential for breathing and burning.
2)Liquid oxygen is used in rockets as oxidizer 3) It is carried in cylinder by
mountaineers , sea divers and astronauts for respiration.
9.
Describe the preparation of carbon dioxide in the laboratory, with
the help of a neat labelled diagram.
Ans. For figure refer page no 139
fig 8.4
Aim---preparation carbon dioxide
Apparatus--- Flask, delivery tube , glass jar and thistle funnel
Procedure—Take some marble chips in a flask. Close its mouth with two
holed rubber cork. Introduce a thistle funnel through one hole and a
delivery tube through another hole. Connect the delivery tube to a glass jar
as shown in the figure. Pour dilute sulphuric acid on the marble chips
through thistle funnel. The reaction takes place. A gas is liberated, it is
collected in the jar by upward displacement of air. The collected gas is
carbon dioxide.
10
Write any two uses of carbon dioxide ?
A.
a) The green leaves of plants absorb carbon dioxide from the atmosphere
and produce starch. This process is called photosynthesis.
b) It is used in the manufacture of aerated drinks ( soft drinks ) such as soda
water.
c) It is used in fire extinguishers.
d) solid carbon dioxide is used as a refrigerant.
11
What happens when a burning splinter is introduced into the jar containing
carbon dioxide ? why ?
A.
Carbondioxide is neither combustible gas nor a supporter of combustion.So
when a burning splinter is introduced into the jar containing carbon dioxide
It puts off fire.
Give reason
 There is fire accident. People put sand on it.
Ans . There is fire accident. People put sand on it. Because sand is
neither combustible gas nor a supporter of combustion.
 When carbondioxide is passed through lime water ,it turns milky.
Ans.
When carbondioxide is passed through lime water ,it turns
milky.Beacuse white insoluble calcium carbonate will form.
 A house should be ventilated properly.
Ans.
Because to get fresh air a house should be ventilated properly.
 While diluting an acid, acid should be poured to water slowly and stirred
well.
 During the preparation of oxygen in the laboratory, manganese dioxide is
added to potassium chlorate.
A Because it acts as catalyst.
Electrical Appliances in Every -day Life
I choose the correct answer

An electrical appliance which works on heating effect of electrical current is…. Electric iron

Magnetic effect of electrical current increases with increases in ……….. turns in the wire coil

The process of depositing ordinary metal wire with the help of electric current is called
…..electro plating

A basic semiconductor device is ….. diode
II fill in the blanks

An electric bulb converts electrical energy into heat and light energy.

A fuse wire is generally, made by an alloy of tin and lead.

The flow of electric current through a solution causes chemical change.

The electricity carrying conductor creates magnetic field.

Commonly used semiconductors are silicon and germanium.
III match the following
A
B
1.
electroplating
prevents rusting
2.
electromagnet
extracts iron ore
3.
electric fuse
protects electric devices
4.
electric resistance
filament of bulb
5.
Q IV

electric motor
glows the
runs most of the machine
Answer the following
What is meant by heating effect of electric current ? Name few appliances which make use this
effect .
Ans. A current carrying having resistance get heated up. This phenomenon is called heating effect
of electric current. Appliances which work on heating effect of electric current are Iron box ,electric
heater, Water heater, Microwave oven etc

How does the filament of an electric bulb glow ? Write the disadvantage of an incandescent
bulb.
Ans. When electric current flow through the filament , the filament quickly heats up due to its high
resistance and bulb begins to glow. Incandescent bulb wastes most of useful electrical energy in the
form of unwanted heat.

What is a fuse? Why it is essential ?
Ans.
Fuse is a safety device. It is used to protect electric current components against excess
flow of electric current. It is essential because in the event of excess current, the fuse element melts
and open the circuit there by protecting it from damage.

How can you say that C.F.L and L.E.D light sources are economical ?
Ans.
Compact Fluorescent Lamp(C.F.L) and Light Emitting Diodes(L.E.D) are more economical
sources of light and last longer. These convert most of the electrical energy into required light energy
with negligible waste of electric current.

Ans.
Explain with a diagram how a piece of iron can be converted into a temporary magnet ?
Aim – To convert piece of iron to temporary magnet
Requirements – nail, copper wire, cell
Procedure – Now connect the free ends of the wire to the terminals of a dry cell through a
switch. Bring some iron pins near the tip of the nail. Switch on the circuit. Nail act as a
magnet and attract the pins.
For diagram refer page no 62
fig 4.18

What are the uses of electromagnets ?
Ans.
Electromagnets are used in magnetic separation of iron ore and also in the separation of
magnetic materials from the junk. Strong electromagnets are used in cranes to lift very heavy loads
in factories and industries.

What is electroplating? Why are articles electroplated?
Ans. The process of depositing a layer of any desired metal on another material by means of
electricity is called electroplating. Articles are electroplated to prevent them from rusting or to
make them look attractive.

What are the advantages of electronic devices over electrical appliances?
Ans. Electronics does jobs that electricity alone cannot do. Today electronics deals with
design, development and application in commucation, entertainment, medical science, industry
and defense etc. Electronic devices are tools that manipulate electric current in sophisticated
ways.

What are semiconductors? How they are differ from conductors and insulators.

A semiconductor is a substance whose conductivity lies between that of a good conductor and
an insulator. Semiconductors are the basic building blocks of electronic components which
constitutes electronic devices and appliances.

Make a list of semiconductor devices which are commonly used in modern electronic gadgets.
Ans. Semiconductor devices commonly used in modern electronic gadgets are transistor,
integrated circuits, microprocessors, microchips.
Q V

Give reason
A magnetic needle of a compass placed close to an electric circuit deflects.
Ans. A magnetic needle of a compass placed close to an electric circuit deflects. Because the
current carrying conductor produces magnetic field around it.

A computer that was connected temporarily to an electric circuit without a fuse got burnt
Ans . Because if the electric current exceeds the prescribed limit , the wire get over heated due to
heating effect of electric current. The developed heat will melt the conducting wires of circuit and cause
the fire accidents.

An ornaments loses its glitterness quickly.
Ans. An ornaments loses its glitterness quickly, because a thin layer of noble metal is
coated on the surface of an ordinary base metal.