# CBSE Sample Papers for Class 10 Science Paper 8

## CBSE Sample Papers for Class 10 Science Paper 8

These Sample papers are part of CBSE Sample Papers for Class 10 Science. Here we have given CBSE Sample Papers for Class 10 Science Paper 8.

Time allowed : 3 hours
Maximum marks : 80

General Instructions

• The question paper comprises two sections, A and B. You are to attempt both the sections.
• All questions are compulsory.
• All questions of Section-A and B are to be attempted separately.
• There is an internal choice in two questions of three marks each and one question of five marks.
• Question numbers 1 and 2 in Section-A are one mark question. They are to be answered in one word or in one sentence.
• Question numbers 3 to 5 in Section-A are two marks questions. These are to be answered in 30 words each.
• Question numbers 6 to 15 in Section-A are three marks questions. These are to be answered in about 50 words each.
• Question numbers 16 to 21 in Section-A are 5 mark questions. These are to be answered in 70 words each.
• Question numbers 22 to 27 in Section-B are based on practical skills. Each question is a two marks question. These are to be answered in brief.

SECTION – A

Question. 1.
Where are guard cells found and what is their function ?

Question. 2.
What is the difference between gustatory receptor and olfactory receptor ?

Question. 3.
What is meant by strong acids and weak acids ? Classify the following into strong acids and weak acids:
HCl,CH3COOH, H2SO4, HNO3, H2CO3,H2SO3.

Question. 4.
What is colour-blindness ? What kind of retinal cells are lacking in person suffering from this defect ?

Question. 5.
Why are antacids used when people suffer from indigestion ?

Question. 6.
How would the strength of magnetic field due to a current-carrying loop be affected if :
(a) the radius of the loop is reduced to half of its original value, and
(b) the strength of the current through the loop is doubled ?
OR
What is a solenoid ? Draw magnetic field lines due to a current-carrying solenoid. Write three important features of the magnetic field obtained.

Question. 7.
Find the equivalent resistance across the two ends A and B of the circuit.

Question. 8.
Properties of the elements are given below. Where would you locate the following elements in the periodic table
(a) A soft metal stored under kerosene
(b) An element with variable (more than one) valency stored under water.
(c) An element which is tetravalent and forms the basis of organic chemistry.
(d) An element which is an inert gas with atomic number 2.
(e) An element whose thin oxide layer is used to make other elements corrosion resistant by the process of ‘anodising’.

Question. 9.
What is periodicity in properties of elements with reference to the Modem periodic table ? Why do all the elements in a particular group have similar properties ? How does the tendency of elements to gain electrons change as we move from left to right along a period ? What are the reasons for the change ?

Question. 10.
Write one feature which is common each of the following pairs of terms or organs ?
(a) Gills and lungs
(b) Chlorophyll and haemoglobin
(c) Arteries and veins
OR
Bring out the differences between arteries and veins.

Question. 11.
How was Mendel able to show that the traits inherited in his experiments were either dominant or recessive ?

Question. 12.
An object of 5.0 cm in length is placed at a distance of 20 cm in front of a convex mirror of radius of curvature 30 cm. Find the position of the image, its nature and size.

Question. 13.
(a) Name the following compounds :

(b) What will be the formula and electron dot structure of cyclopentane ?
OR
What are hydrocarbons ? Distinguish between saturated and unsaturated hydrocarbons. How are alkenes converted into alkanes ?

Question. 14.
Why plants appear to bend towards the light ?

Question. 15.
Write a note on fossil fuels and state the consequences of their use on the environment ?

Question. 16.
Draw a labelled diagram of an electric motor. Explain its principle and working. What is the function of a split ring in an electric motor ?

Question. 17.
List the merits and drawbacks of Mendeleev periodic table.
OR
Atoms of eight elements A, B, C, D, E, F, G and H have the same number of electronic shells but different number of electrons in their outermost shell. It was found that elements A and G combine to form an ionic compound. This compoundis added in a small amount to almost all vegetable dishes during cooking. Oxides of elements A and B are basic in nature while those of E and F are acidic. The oxide of D is almost neutral. Based on the above information answer the following questions :

(a) To which group or period of the Periodic Table do the listed elements belong ?
(b) What would be the nature of compound formed by a combination of elements B and F ?
(c) Which two of these elements could definitely be metals ?
(d) Which one of the eight elements is most likely to be found in gaseous state at room temperature ?
(e) If the number of electrons in the outermost shell of elements C and G be 3 and 7 respectively, write the formula of the compound formed by he combination of C and G.

Question. 18.
What are the different methods of contraception ?

Question. 19.
(a) List three common refractive defects of vision. Suggest the way of correcting these defects.
(b) About 45 lac people in the developing countries are suffering from corneal blindness. About 30 lakh children below the age of 12 years suffering from this defect can be cured by replacing the defective cornea with the cornea of a donated eye. How and why can students of your age involve themselves to create awareness about this fact among people ?

Question. 20.
What is thermal decomposition ? Are decomposition reactions opposite of combination reactions ? Show with examples.

Question. 21.
What are the conventional sources of energy ? What precautions should be taken while constructing a dam to generate hydroelectricity ?
OR
Human beings are on top of any food chain. Is this harmful for humans in the long run ? Does the term biological magnification apply here ?

SECTION – B

Question. 22.
10 ml of a solution of NaOH is completely neutralised by 8 ml of a given solution of HCl. If we take 20 ml of the same solution of NaOH find the amount of HCl solution required to neutralise it.

Question. 23.
Is respiration an exothermic reaction ? Why ?

Question. 24.
A student observed permanent slide of yeast reproduction.
(a) What method would he have seen ?
(b) Draw a labelled diagram of his observation.

Question. 25.
When we chew bread it tastes sweet. Why ? What is the role of saliva in digestion of food ?

Question. 26.
Find the focal length of the concave mirror in the experimental set-up shown in figure.

OR
In an experiment to trace the path of a ray of light passing through a rectangular glass slab, angle of incidence in air was given as 60°. Two students P and Q found angle of refraction as 42° and 65° respectively. Who is right and why ?

Question. 27.
An ammeter has 20 divisions between mark 0 and mark 2 on its scale. Find the least count of the ammeter.

SECTION – A

Guard cells are kidney shaped cells found on either side of the stomata in the epidermis of the leaves. They help in opening and closing the stomatal pore by shrinking and enlarging.

Gustatory and olfactory receptors are special nerve cells found in the sense organs like the tongue and nose respectively that help to detect information from the environment and make the body respond to it suitably. The information from the environment is detected by specialized tips of the nerve cells in these parts.

A strong acid is one that completely ionises in water to form a large amount of hydrogen ions whereas a weak acid only partially ionises in water and thus produces a small amount of hydrogen ions.
Strong acids : HCl, H2SO4, HNO3
Weak acids : CH3COOH, H2CO3.

The defect of the eye due to which a person is unable to distinguish between certain colours, is known as colour blindness. Cone shaped retinal cells are responsible for making a person differentiate between colours. The colour blind persons do not possess cone cells that respond to certain colours.

When people suffer from indigestion antacids like milk of Magnesia are used because they have a pH which makes them alkaline. This basic nature of the antacids helps to neutralize the acidity in the stomach thereby facilitating digestion.

(a) If the radius of current loop is reduced to half of its original value, the strength of magnetic field is doubled as magnetic field (B) is inversely proportional to the radius (r) of loop.
(b) As strength of magnetic field is proportional to current (I) flowing through the loop (B α I), the magnetic field is doubled if current is doubled.

OR
A solenoid is a coil of large number of circular turns of wire wrapped in the shape of a cylinder. On passing electric current, a magnetic field is developed. Magnetic field lines are drawn below. The field is along the axis of solenoid such that one end of solenoid behaves as north pole and the other south pole. Thus, field of a solenoid is similar to that of a bar magnet. Important features of magnetic field due to a current carrying solenoid are:

1. Magnetic field lines inside the solenoid are nearly straight and parallel to its axis. It shows that magnetic field inside a solenoid is uniform.
2. Magnetic field of solenoid is identical to that due to a bar magnet with one end of solenoid behaving as a north pole and other end as a south pole.
3. A current-carrying solenoid exhibits the directive and attractive properties of a bar magnet.

Equivalent resistance of R1 and R2 joined in parallel is R12, where

Similarly R56 and R78 are in series and make a resistance R10 = 2 Ω
Finally R9 and R10 are connected in parallel between the points A and B are hence equivalent resistance R is given as

(a) Sodium (Na) Group 1 and Period 3 or Potassium (K) Group 1 and Period 4
(b) Phosphorus (P) Group 15 and Period 3
(c) Carbon (C) Group 14 and Period 2
(d) Helium (He) Group 18 and Period 1
(e) Aluminium (Al) Group 13 and Period 3.

The occurrence of elements with similar properties after certain regular intervals when they are arranged in increasing order of their atomic number is called periodicity. The periodic repetition of the properties is due to the recurrence of similar valence shell configuration at regular intrervals. The elements in a group have same valence electrons and so they have similar properties. In a period, tendency to gain electrons increases from left to right. This happens because the hold of the nucleus on the outermost electrons becomes weak and so the electrons are lost easily.

(a) Both gills and lungs are respiratory organs which helps in gaseous exchange. Gills are found in fishes whereas lungs in human beings.
(b) Both chlorophyll and haemoglobin have porphyrin structure that binds to a central metal atom to form a complex. In chlorophyll central metal atom is Mg and haemoglobin is iron.
(c) Both arteries and veins carry blood. They are blood vessels.
OR
There is a well-developed circulatory system in human beings. It consists of a large number of blood vessels and a powerful pumping organ called the heart. The heart lies slightly to the left of the thoracic region and it is the size of a person’s closed fist. The different blood vessels are the arteries, veins and the capillaries. The capillaries are extremely thin-walled blood vessels that form a link between the arteries and the veins and function as the main path of exchange between them.

Arteries

1. Arteries have thick, elastic walls.
2. Arteries carry blood from the heart to different parts of the body
3. Arteries carry pure or oxygenated blood.
4. Blood flows at high pressure in the arteries.
5. Arteries are placed deep inside muscles.

Veins

1. Veins have thin walls.
2. Veins collect blood from different parts of the body and bring it to the heart.
3. Veins carry impure or deoxygenated blood.
4. Blood flows at lower pressure in the veins.
5. Veins are placed superficially.

While doing his experiments, Mendel found that there are some traits that are inherited but are not expressed. He called them recessive characters. Mendel clearly explained this with the help of the monohybrid cross.

In a monohybrid cross performed by Mendel, a tall plant (homozygous) was crossed with a dwarf plant (homozygous). In the F1 generation all plants produced were tall in appearance. Next these tall plants were self- pollinated. In the F2 generation it was found that tall and dwarf plants appeared in the ratio of 3 :1. This clearly showed that the dwarf character was not lost but suppressed in the Fa generation and expressed itself in the F2 generation.

(a) (i) Bromoethane, (ii) Formaldehyde, (iii) Cyclohexene.
(b) The molecular formula is C5H10. Its structural formula is:
Cyclopentane (C5H10)

OR
Hydrocarbons are organic compounds made up of carbon and hydrogen. There are two kinds of hydrocarbons, saturated hydrocarbons and unsaturated hydrocarbons.

(i) Saturated hydrocarbons: These include the alkanes in which we find only single bond between the carbon atoms. Their general formula is CnH2n+2. The simplest alkane is methane – CH4. Its structural formula is

(ii) Unsaturated hydrocarbons : These are the compounds of carbon having double and triple bonds between the carbon atoms. They include the alkenes and alkynes. The alkenes (CnH2n) have double bonds in them. The alkynes (CnH2n-2) have triple bonds in them.

The reaction which converts unsaturated hydrocarbons (alkenes) to saturated hydrocarbons (alkanes) is called hydrogenation. It helps to obtain ghee from oil.

Phototropism means the response of plant parts towards sunlight. Here sunlight is the stimulus, while the shoot system responds positively to sunlight the root system responds negatively to sunlight. Different plant hormones help to coordinate growth, development and responses to the environment. They are synthesised at places away from where they act and simply diffuse to the area of action.

When growing plants detect light, a hormone called auxin which is synthesised at root tips helps the cells grow longer. When light is coming from one side of the plant, auxin diffuses towards the shady side of the shoot. This concentration of the auxin stimulates the cells to grow longer on the side of the shoot which is away from light. Thus, the plant appears to bend towards light.

In ancient times, wood was the most common source of energy with forests being in plenty around us. With industrial revolution, coal became a new source of energy. With increasing industrialisation life became comfortable and coal and petroleum became the new energy sources. There was an increased usage of these fossil fuels which unfortunately were non-renewable. Once people realised they are soon coming to the end of our coal and petrol resources they started looking for alternate sources of energy.

Fossil fuels have several disadvantages too. They lead to air pollution due to their fumes. The oxides of carbon, nitrogen and sulphur that are released on burning fossil fuels are acidic oxides. They mix with water in the atmosphere and form acids like carbonic acid, nitric acid and sulphuric acid which come down as acid rain. Acid rain affects the soil by affecting the lives of many organisms in the soil and making the soil unfit for growth of plants. In addition release of carbon dioxide and carbon monoxide from vehicle exhausts leads to the greenhouse effect on the earth’s surface. It raises the global temperature and causes global warming.

Electric motor : A labelled diagram of an electric motor has been shown in Fig.

Principle : A current – carrying conductor, when placed in a magnetic field, experiences a force. If the direction of the field and that of the current are mutually perpendicular then force acting on the conductor will be perpendicular to both and will be given by Fleming’s left-hand rule. Due to this force the conductor begins to move, if it is free to do so.

Working : Let current in the coil ABCD of motor enters from the source battery through the conducting brush X, flows along ABCD and finally flows back to the battery through brush Y. On applying Fleming’s left-hand rule we find that force acting on arm AB due to magnetic field pushes it downwards. But the force acting on arm CD pushes it upwards. Thus, the coil and the axle rotate anticlockwise. Due to action of split ring commutator at half rotation split rings P and Q change their contacts with brushes. Now, P makes contact with Y and Q with X. As a result, current begins to flow in coil along DCBA. As a result, now AB arm is being pushed upward and arm CD downward by the magnetic force. Consequently the coil rotates half a turn more in the same direction. This reversing of current direction is repeated at each half rotate and so the coil continues to rotation in the same direction.

Merits of Mendeleev Periodic Table :

1. At some places the order of atomic weight was changed in order to justify the chemical and physical nature.
2. Mendeleev left some gap for new elements which were not discovered at that time.
3. One of the strengths of Mendeleev’s periodic table was that, when inert gases were discovered they could be placed in a new group without disturbing the existing order.

Drawbacks of Mendeleev Periodic Table :

1. Position of hydrogen: Hydrogen resembles alkali metals (forms H+ ion just like Na+ ions) as well as halogens (forms H ion similar to Cl ion). Therefore, it could neither be placed with alkali metals (group I) nor with halogens (group VII).
2. Position of isotopes: Different isotopes of same elements have different atomic masses, therefore, each one of them should be given a different position in the periodic table. On the other hand, because they are chemically similar, they had to be given same position.
3. Anomalous pairs of elements : At certain places, an element of higher atomic mass has been placed before an element of lower mass. For example, Argon (39.91) is placed before potassium (39.1).

OR

1. A and B belong to group 1 and 2 because they form basic oxides. C belongs to group 13 as it has 3 valence electrons. D belongs to group 14 as it forms almost neutral oxide. E and F belong to group 15 and 15 as they form acidic oxides, G belongs to group 17 as it has 7 valence electrons and H belongs to group 18. They belong to 3rd period of the Period Table because AG is NaCl, added in a small amount to almost all vegetable dishes during cooking and Na and Cl belong to 3rd period.
2. Ionic compounds will be formed because ‘B’ is metal and ‘F’ is non-metal. ‘B’ can lose two ‘ electrons and ‘F’ can gain two electrons. .
3. A and B are definitely metals as they form basic oxides.
4. G and H are gaseous at room temperature.
5. CG3 is the formula of the compound formula by combination of C and G.

The different methods of contraception are :

1. Those that create a mechanical barrier so that the sperm does not reach the egg. They include condoms on the penis or similar coverings on the vagina.
2. A second category acts by changing the hormonal balance of the body so that eggs are not released and fertilisation does not occur. These are drugs taken as oral pills. However as they change hormonal balance they cause side effects too.
3. There are contraceptive devices placed in the uterus such as the loop or the copper T to prevent pregnancy. This could cause side effects like irritation in the uterus.
4. Surgery is used to create blocks in the vas deferens of the male so that sperm transfer is prevented. Surgery is also used to block the fallopian tube so that the egg does not reach the uterus. While surgical methods are safe in the long run surgery itself could cause infections and other problems if not done properly.

(a) The three common refractive defects of vision are :

1. Myopia (or near sightedness): It is corrected by using concave lenses of appropriate power.
2. Hypermetropia (or far sightedness): It is corrected by using convex lenses of appropriate power.
3. Presbyopia: It is corrected by using bifocal lenses of appropriate power in which the upper part consists of a concave lens (to correct myopia) and lower part consists of convex lens (to correct hypermetropia).

(b) Eyes of a dead person can be donated to the person having corneal blindness, it will help him/ her to see the world. We can also register ourselves and encourage others also to register to eye donation camps who can preserve our eyes after our death and donate them to the needy.

Thermal decomposition is a decomposition reaction carried out by heating. The decomposition of calcium carbonate into calcium oxide and carbon dioxide on heating is an example of thermal decomposition. The calcium oxide formed is also called lime or quicklime.
CaCO3(s) $$\underrightarrow { on\quad heating }$$ CaO(s) + CO2(g)

Yes decomposition reactions can be regarded as opposite of combination reactions. Decomposition reactions require energy in the form of heat, light or electricity to break down the reactants. Such reactions which absorb energy are called endothermic reactions. On the other hand, in combination reactions heat is given out. Such reactions are called exothermic reactions. Here are some examples.

1. Glucose combines with oxygen to form carbon dioxide and water with the liberation of energy.
C6H12O6(aq) + 6O2(g) → 6CO2 (aq) + 6H2O(1) + Energy
2. Methane or natural gas burning in air forms carbon dioxide and water.
CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)

In ancient times, wood was the most common source of heat energy. Soon the energy of flowing water and wind was also used to generate power. Coal and petroleum, well known as fossil fuels started serving mankind right after the industrial revolution. However they are non-renewable resources, Moreover, burning fossil fuels leads to certain problems too like air pollution, global warming and acid rain. Hence, man is making use of water and wind to fulfil his energy requirement.

Hydroelectricity or hydel electricity is the generatipn of electricity using the power of water. It is a means to generate electricity either from the kinetic energy of flowing water or the potential energy of falling water. In our country we primarily focus on storing huge quantities of water by building dams across rivers. The water from the high levels in the dam is carried through pipes to a turbine at the bottom of the dam. As rains regularly fill up the dams one need not worry about the generation of electricity. However, while constructing dams one has to keep in mind a few things.

1. Not to affect large areas of agricultural land
2. To safeguard the human habitation around the dam
3. To ensure that vegetation is not submerged under water in the dams giving rise to air pollution on rotting.

OR
Human beings are on top of any food chain. This means the maximum concentration of several harmful chemicals get accumulated in the human bodies. This is also called biological magnification. We have seen how water is polluted by different substances. Pesticides and some chemicals are used to protect crops from pests. These enter the soil and the water that flows on this soil. When this water is absorbed by the roots of plants it enters the plants. Similarly when aquatic animals drink water some chemicals enter their bodies too. These chemicals then enter the food chain. As they are not degraded they are gradually accumulated in each trophic level. Finally all of them reach the human bodies. Food grains like rice, wheat, vegetables, fruits, and meat contain varying amounts of pesticide residues. It is difficult to completely remove them by washing. Hence any organism at the top of the food chain is likely to get a fairly large amount of the harmful substances that enter the food chain at every level. So the effect of the various pollutants are maximised in human beings.

SECTION – B

10 ml of NaOH solution is completely neutralized by 8 ml of HCl.
We now have 20 ml in NaOH solution. How much of HC1 will be required.
10 ml of NaOH = 8 ml of
HCl 20 ml of NaOH = x ml of HCl
$$\frac { 10 }{ 20 }$$ = 8/x or x = $$\frac { 8 x 20 }{ 10 }$$
=16 ml

Yes, respiration is an exothermic reaction because during the breakdown of glucose – C6H12O6 in the presence of O2 heat is generated. The reaction results in the formation of CO2 and H2O. The energy released during respiration helps the body to perform all its activities.

(a) The student observed the process of budding in yeast.

When we chew bread it is mixed with saliva in our mouth. Saliva contains the enzyme ptyalin which converts starch to sugar. Ptyalin converts bread particles into sugar and this is responsible for the sweet taste we get in our mouth. Saliva helps to bring together all the chewn food particles and makes it into a ball like structure called bolus which is then swallowed and pushed into the oesophagus.

Least count = $$\frac { Range }{ Total\quad number\quad of\quad divisions }$$ = $$\frac { 2-0 }{ 20 }$$ = 0.1 A.