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#### Question 1:

Give an example of prokaryotic cell.

Bacterium is a typical prokaryotic organism.

#### Question 2:

What are congenital disease?

Congenital diseases are those diseases which are present from the birth of an individual. An example of congenital disease is albinism.

#### Question 3:

To take the boat away from the bank of a river, the boatman pushes the bank with an oar. Why?

According to Newton's third law of motion, when we apply a force on some object, then in return the object applied an equal and opposite reaction force on us. Following this law, the boatman pushes the bank with an oar. The pushing of the bank with an oar is an action force applied by the boatman in order to get an equal and opposite reaction force from the bank, which helps the boat to move forward.

#### Question 4:

How does the kinetic energy of a moving body depend on its (i) speed, and (ii) mass?

(i) The kinetic energy of a body is directly proportional to the square of its velocity. i.e.

(ii) The kinetic energy of a body is directly proportional to its mass. i.e.

#### Question 5:

Distinguish between symptoms and signs of a disease.

The symptoms and signs of a disease can be distinguished as follows:

 Symptoms of a disease Signs of a disease 1. These provide an indication about the presence of number of diseases. These provide information regarding the presence of a specific disease. 2. Symptoms represent the collective diseases present in a certain organ or part of the body. These are specific for different diseases.

#### Question 6:

Is air a mixture or a compound? Give three reasons for your answer.

Air is a mixture, not a compound because of the following reasons:
1. Air can be separated into its constituents such as oxygen, nitrogen, etc.
2. Air shows the properties of all the gases present in it. For eg. air supports combustion because of oxygen present in the air.
3. Air has a variable composition because at different places a different amount of gases are present in the air.

#### Question 7:

Give one major drawback of Dalton's atomic theory of matter. Dalton's atomic theory says that atoms are indivisible. Is this statement still valid? Give reasons for your answer.

The drawback of Dalton's atomic theory:
According to the theory, an atom is the smallest indivisible particle of an atom. However, it is now known that atoms can further be subdivided into elementary particles like electrons, protons, and neutrons.

"Dalton's atomic theory says that atoms are indivisible" this statement is not valid now as it is found that an atom has a complex structure and is composed of subatomic particles such as electrons, protons, and neutrons.

#### Question 8:

What do you understand by the term "electronic configuration" of an element? Write down the electronic configuration of oxygen (At. No. = 8).
An element has an atomic number 12. How many electrons will be present in the K, L and M energy shells of its atom?

Distribution of electrons around the nucleus in certain shell/orbits is known as the electronic configuration of an element.
Electronic configuration of oxygen = 2, 6

Electronic configuration of the element with atomic number 12 will be 2, 8, 2.
Electrons present is K subshell will be 2.
Electrons present is L subshell will be 8.
Electrons present is M subshell will be 2.

#### Question 9:

The speed-time graph of a car is given alongside. The car weighs 100 kg. (i) What is the distance travelled by car in the first two seconds?
​(ii) What is the braking force applied at the end of 5 seconds to bring the car to a stop within one second?

(i) Area of the speed-time graph gives the total distance travelled.

In the first two seconds, the graph covers an area of a right-angled triangle with a height of 15 units and a base of 2 units.

So, the distance travelled by car in the first two seconds = Area of the triangle formed =

Hence, the car travels 15 m in the first two seconds.

(ii) The slope of the speed-time graph at any moment of time gives the value of the acceleration.

The slope of the graph at the end of the 5 seconds = $\frac{15-0}{6-5}=15$

So, acceleration at the end of the 5 seconds = 15 m/s2

The braking force applied = Mass of the car $×$ Acceleration of the car =

Hence, the braking force applied at the end of the 5 seconds is equal to 1500 N.

#### Question 10:

State two applications of universal law of gravitation. What happens to the gravitational force between two objects when the distance between them is:
(i) Doubled?
​(ii) Halved?

Applications of Universal law of gravitation:

(i) The universal law of gravitation is used to calculate the value of acceleration due to gravity on planets.
(ii) It is also used to study the motion of the satellites around the planets, mainly earth.

The gravitational force of attraction between two bodies of mass 'm1' and 'm2' separated by a distance of 'r'$\mathrm{G}\frac{{m}_{1}{m}_{2}}{{r}^{2}}$

So, the gravitational force $\propto \frac{1}{{r}^{2}}$

(i) As the gravitational force is indirectly proportional to the distance of separation, so if the distance between the objects is doubled, then the force of gravitation becomes one-fourth of the original value.

(ii) If the distance is halved, then the force of gravitation becomes 4-times the original value.

#### Question 11:

Define the term 'work'. Write the formula for the work done on a body when a force acts on the body in the direction of its displacement. Give the meaning of each symbol which occurs in the formula.

In relation to physics, if a body is displaced by a force in the direction of force, the work is said to be done. In simple words, work is something you do when you use a force to displace any object in the direction of the force.

The mathematical formula to calculate the work done is given by the product of displacement produced in a body and the force acting on it in the direction of the displacement. i.e. $W=F×s$

(Where 'W' is the work done, 's' is the displacement and 'F' is the force in the direction of displacement of the body)

#### Question 12:

Name the plant tissue which show the following features?
(i) made up of living cells showing thickening: provide mechanical support to the plant.
(ii) made up of dead cells showing thickening: provide mechanical support to the plant; are made of one type of cells.
​(iii) made up of living cells containing green coloured chloroplasts, possesses intercellular space.

(i) Collenchyma tissues are composed of living cells. These tissues are responsible for providing mechanical strength to the aerial parts of the plants.

(ii) Sclerenchyma tissues are formed by dead cells, which help in secondary growth of the plant. These tissues also provide mechanical strength to the plants and generally, a single type of cell is present in these type of tissues.

(iii) Parenchyma tissues consist of loosely packed living cells with large intercellular spaces. Certain parenchyma tissues called chlorenchyma contain chloroplast, which helps in photosynthesis.

#### Question 13:

Write down differences between bryophytes and pteridophytes.

The differences between bryophytes and pteridophytes have been summarised as follows:

 Bryophytes Pteridophytes 1. Gametophytic plant body is present. 1. Sporophytic plant body is present. 2. True leaves and stems are absent. 2. True stem and leaves are present. 3. Rhizoids help in the fixation of plant body. 3. Roots are used for the fixation of plant body. 4.These are non-vascular. 4. These are vascular.

#### Question 14:

What do you mean by biological weathering? Give two examples.

Biological weathering refers to the breakdown of large rocks into smaller minerals by living organisms. This process is responsible for the formation of soil. Two particular examples of biological weathering includes weathering induced by bryophytes and lichens.

Lichens induce biological weathering by extracting minerals from the rocks, which results in the formation of small cervices that serve as the site for soil formation.

Bryophytes can readily grow in the cervices formed by lichens and further deepen the cervices. These deeper cervices form cracks. In due course of time, because of growth of bigger plants, rocks eventually pulverise and form soil.

#### Question 15:

Define intercropping. How does it differ with mixed cropping?

Intercropping is a practice in which two or more different types of crops are grown in a single field. The crops are grown in defined patterns of rows and it is primarily aimed to increase the productivity per unit area.

Intercropping differs from mixed cropping in the following ways:

 Intercropping Mixed cropping 1. In this process, seeds are not mixed prior to sowing. 1. In this cropping procedure, seeds of different crops are thoroughly mixed before sowing. 2. The crops are organised in fixed rows. 2. There is no fixed pattern for the crops. 3. The primary aim is to increase the per unit area productivity. 3. It is primarily aimed to reduce the chances of crop failure.

#### Question 16:

(i) What is meant by 'diffusion'? Give one example of diffusion in gases.
(ii) Why do gases diffuse very fast?
​(iii) Name two gases of air which dissolve in water by diffusion. What is the importance of this process in nature?

(i) Diffusion: The spreading out and mixing of a substance with another substance due to the motion of its particle is called diffusion.
For eg. When food is cooked some of the substance in food release gases having the smell of food in them. These gases move very quickly and mix up with air by diffusion when these gases reach our nose we get the smell of food cooking in the kitchen.

(ii) Diffusion is the mixing of particles of one substance with others. The molecules in gases are far apart from each other due to negligible forces of attraction between them. Thus, gases have very low densities, therefore, they have a maximum rate of diffusion among solids, liquids, and gases.

(iii) The two gases of air which dissolve in water by diffusion are oxygen and carbon dioxide.
1. Carbon dioxide dissolves in water. Fizzy drinks contain carbon dioxide dissolved in water. In nature, this process helps to regulate the world's carbon dioxide supplies as the gas dissolves in water.
2. Oxygen also dissolves in water. It is this dissolved oxygen that makes the aqueous life to breathe.

#### Question 17:

(i) Describe Thomson's model of the atom. Which subatomic particle was not present in Thomson's model of the atom?
​(ii) The mass number of an element is 18. It contains 7 electrons. What is the number of protons and neutrons in it? What is the atomic number of the element?

(a) Thomson's model of an atom is similar to the model of plum pudding or watermelon. He said that the positive charge in an atom is spread all over like the red edible part of the watermelon. Also, electrons are embedded in the positively charged sphere like the seeds in the watermelon.
Explanation of Thomson' model:
(i) An atom consists of a positively charged sphere with electrons embedded in it.
(ii) The negative and positive charges present inside an atom are equal in magnitude. Therefore, an atom as a whole is electrically neutral.

Thomson only explained about electrons and protons, he did not take neutrons into consideration.

(b)
Mass number of element = 18
Number of electrons = 7
Number of protons = 7
Number of neutrons = Mass number - number of protons
Number of neutrons = 18 - 7
Number of neutrons = 11
Atomic number of this element = 7

#### Question 18:

(i) What do you understand by the kinetic energy of a body?
(ii) A body is thrown vertically upwards. Its velocity goes on decreasing. What happens to its kinetic energy as its velocity becomes zero?
​(iii) A horse and a dog are running with the same speed. If the weight of the horse is ten times that of the dog, what is the ratio of their kinetic energies?

(i) The energy possessed by a body by virtue of its motion is known as the kinetic energy of the body. If the mass of a body is 'm' and it is moving with a speed 'v', then the kinetic energy of the body = $\frac{1}{2}m{v}^{2}$

(ii) When a body is thrown in an upward direction, its velocity decreases continuously but its potential energy goes on increasing. As the body reaches a maximum height, the kinetic energy gradually gets converted into potential energy. At the highest point where the velocity of the body becomes zero, the potential energy of the body becomes maximum. Because all of the kinetic energy gets converted into potential energy.

(iii) Let the speed of the horse and the dog = v

If the mass of the dog = m

Then the mass of the horse = 10m

The ratio of kinetic energies of the horse and the dog = $\frac{\left(K.E.{\right)}_{\mathrm{Horse}}}{\left(K.E.{\right)}_{\mathrm{Dog}}}=\frac{\frac{1}{2}\left(10m\right){v}^{2}}{\frac{1}{2}m{v}^{2}}=\frac{10}{1}$

Hence, the ratio of their kinetic energies is 10:1.

#### Question 19:

(i) An electric bell is suspended by thin wires in a glass vessel and set ringing. Describe and explain what happens if the air is gradually pumped out of the glass vessel.
​(ii) Why cannot a sound be heard on the moon? How do astronauts talk to one another on the surface of moon?

(i) Sound waves are longitudinal waves, which require a medium to travel. If the air inside the glass vessel is pumped out gradually then a vacuum is created around the ringing bell. As the sound waves generating from the ringing bell do not find any medium to travel in their surroundings, therefore the sound of the ringing bell fades and after some time it stops coming.

(ii) Sound waves are longitudinal waves, which require a medium to travel. On the moon, there is no atmosphere, so in the absence of atmosphere, the sound waves find no medium to travel from one place to another. That's why no sound can be heard on the surface of the moon. The Astronauts use the radio waves to communicate on the surface of the moon.

#### Question 20:

Explain various modes of transmission of infectious diseases.

The different modes of transmission of infectious diseases are as follows:

1. Air-borne transmission: In this type of transmission, the pathogenic organisms are transmitted through the air. The transmission basically occurs through droplets released by an infected person. Examples of air-borne transmission include cough, cold and influenza.

2. Water-borne transmission: This type of transmission occurs through contaminated water. Typical examples of diseases that are transmitted through contaminated water are cholera and amoebiasis.

3. Sexual transmission: This type of transmission occurs through sexual contact between an infected and a healthy person. Some of the examples of diseases that are transmitted through sexual transmission  are Acquired Immunodeficiency Syndrome (AIDS) and syphilis.

4. Formite: In some cases, articles that come in contact with an infected person can also act as source of infections. These articles are collectively referred to as formites and can include garments, utensils and door handles.

5. Vectors: Vectors are living organisms that can transmit pathogens from one organism to another. Some of the examples of vectors are house fly, sand fly and Anopheles.

#### Question 21:

Explain ozone layer, its depletion and effects of ozone depletion.

Ozone layer is a protective gaseous covering which protects the Earth surface from harmful ultraviolet radiations. Ozone layer depletion is the process, which leads to gradual reduction in the thickness of ozone layer in the stratosphere. The most prominent reason for the ozone layer depletion is the use of chloro fluoro carbons.

The depletion of ozone layer can have following consequences:
1. It can lead to an increase in the incidents of skin cancer in human population.
2. Most of the phytoplanktons and zooplanktons will not be able to survive.
3. The crop yield will be severely affected.

#### Question 22:

Why is a liquid (the hydraulic fluid) used to operate the brakes in a car?

The liquid is used to operate the brakes in cars, as hydraulic fluids respond faster than air because it is almost incompressible. It provides a better transmission of force or pressure applied to the hydraulic brakes.
Hydraulic fluid can work properly even at high temperature.

#### Question 23:

When 5 g of calcium is burnt in 2 g of oxygen, then 7 g of calcium oxide is produced. What mass of calcium oxide will be produced when 5 g of calcium is burnt in 20 g of oxygen? Which law of chemical combination will govern your answer?

$2\mathrm{Ca}+{\mathrm{O}}_{2}\to 2\mathrm{CaO}$
From the equation, the number of moles of CaO formed is equal to the number of moles of calcium used.
So, from the question,
Mass of calcium = 5 g
Mass of oxygen = 2 g
Mass of calcium oxide = 7 g
Number of moles of oxygen = $\frac{1}{2}$ Number of moles of calcium
Moles of calcium = $\frac{5}{40}$ = 0.125
Moles of oxygen = $\frac{2}{32}$ = 0.0625
So, the number of moles of calcium is greater than the number of moles of oxygen.
Hence, the oxygen atom is the limiting reagent.

Therefore, the number of moles of calcium oxide = number of moles of oxygen.
Now, when 5 g of calcium is used in 20 g of oxygen, then
The number of moles of oxygen = $\frac{20}{32}=0.625$
The number of moles of calcium = $\frac{5}{40}$ = 0.125
Here, the number of moles of calcium is lesser than the number of moles of oxygen.
So, here calcium is the limiting reagent.
Number of moles of calcium oxide = $\frac{7}{56}=0.125$
The number of moles of calcium oxide = Number of moles of calcium.
So, the weight of calcium oxide formed in the second case is 7 g.

Law of conservation of mass is applicable to this equation.

#### Question 24:

A car travels 100 km at a speed of 60 km/h and returns with a speed of 40 km/h. Calculate the average speed for the whole journey.

Time taken by the car to cover the first 100 km of the journey =

Time taken by the car to return the same distance =

Total distance travelled by car in the whole journey = 200 km

Total time taken by the car to complete the whole journey =

The average speed of the whole journey =

Hence, the car covers the whole journey at an average speed of 9.6 km/h.

#### Question 25:

Draw the sketches of two waves A and B such that wave A has twice the wavelength and half the amplitude of wave B. In the above figure, two waves are shown. Wave A has twice the wavelength and half the amplitude of wave B.

#### Question 26:

What will happen if (a) Bone is dipped in hydrochloric acid (HCl); (b) Bone is dried?

(a) Bone dipped in hydrochloric acid (HCl) will become demineralize that is, the calcium phosphate mineral present in the bone will react with HCl to form calcium chloride and calcium phosphate.

(b) A dried bone will become brittle because of the loss of collagen.

#### Question 27:

Conduct a survey in your neighbourhood to find out what the three most common diseases are. Suggest three steps that could be taken by your local authorities to bring down the incidence of these diseases.

The most common diseases that can be found in the neighbourhood are as follows:
1. Tuberculosis
2. Jaundice
3. Typhoid

The local authorities can take following steps to decrease the incidence of these diseases:
1. The sewage should be properly disposed.
2. The supply of safe drinking water should be ensured.
3. The authorities should ensure that the surroundings are kept clean.

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