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CBSE Class 10 Science Exam 2018: Important Long Answer Type Questions

Jan 29, 2018 17:27 IST
    CBSE Class 10 Science Important Questions
    CBSE Class 10 Science Important Questions

    Here we are providing a set of important long answer type questions to prepare for CBSE class 10 Science board exam 2018. All these questions have been provided with appropriate answers in a simple language to help you learn easily and effectively. In CBSE class 10 Science exam 2018, question numbers 16 to 21 in Section-A will be of long answer type asked for 5 marks each. Important questions given here will help you familarise the important topics to prepare for the final exam. You may also assess your preparedness by practicing these questions.

    Given below are some very important long answer type questions to prepare for CBSE class 10 Science exam 2018:

    Question 1. (a) Write chemical equations for the reactions involved in obtaining pure alumina  from the mineral bauxite which has impurities of iron oxide and silica.
    (b) Draw a labelled diagram of the electrolytic tank cell used for the extraction of aluminium from alumina.


    The reaction involved in obtaining pure alumina from the mineral bauxite having impurities of iron oxide and silica is:

    The iron oxide and silica (sand) present in the bauxite ore do not dissolve in sodium hydroxide solution. Therefore, they are separated by filtration.

    (b) Given below is the labelled diagram of the electrolytic tank cell used for the extraction of aluminium from alumina:

    Question 2. (a) What are ‘magnetic field lines’? How is the direction of a magnetic field at a point determined ?

    (b) Draw two field lines around a bar magnet along its length on its two sides and mark the field directions on them by arrow marks.


    (a) The magnetic field lines are the pictorial representation of the strength and direction of the magnetic field. 

    The direction of the magnetic field at a point can be found by placing a small magnetic compass at that point. The north end of the needle of  a compass indicates the direction of magnetic field at a point where it is placed.

    (b) Magnetic field lines of a bar magnet emerge from the north pole and terminate at the south pole. Inside the magnet, the field lines emerge from the south pole and terminate at the north pole, as shown in the given figure.

    Question 3. (a) Mention the pH range within which our body works. Explain how antacids
    give relief from acidity. Write the name of one such antacid.
    (b) Fresh milk has a pH of 6. How does the pH will change as it turns to curd? Explain your answer.
    (c) A milkman adds a very small amount of baking soda to fresh milk. Why does this milk take a longer time to set as curd?
    (d) Mention the nature of toothpastes. How do they prevent tooth decay?


    (a) Our stomach has pH equal to 2. Antacids neutralizes excess of acid in our body and gives relief from hyperacidity. Sodium hydrogencarbonate is one of such antacid.
    (b) pH will decrease as it turns to curd because curd is acidic due to the presence of lactic acid.
    (c) It takes longer time to set as curd as bacteria do not work well in presence of sodium hydrogencarbonate, i.e. fermentation will take place slowly.
    (d) Toothpastes are basic in nature. They neutralize the acid formed in mouth which causes tooth decay.

    Question 4. 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 compound is added in a small amount to almost all vegetable dishes during cooking. Oxides of elements A and B are basic in naturewhile those of E and F are acidic. The oxide of D is almost neural. Based on the aboveinformation answer the following questions:

    (i) To which group or period of the periodic table do the listed elements belong?

    (ii) What would be the nature of compound formed by a combination of elements B and F?

    (iii) Which two of these elements could definitely be metals?

    (iv) Which one of the eight elements is most likely to be found in gaseous state at room temperature?

    (v) If the number of electrons in the outermost shell of elements C and G are 3 and 7 respectively, write the formula of the compound formed by the combination of C and G.


    (i) The listed chemicals belong to third period; it includes Na, Mg, Al, Si, P, S, Cl& Ar.

    (ii) Compound formed by the combination of B (Mg) and F (S) will result in formation of salt called MgSO4

    (iii) Compound A (Na) and B (Mg) are definitely metals as their oxides are basic in nature.

    (iv) Element H (Ar), is likely to be found in gaseous state at room temperature as it is the 8th element of the group so it would have 8 electrons in its outermost shell which is the electronic configuration of an Nobel gas.. 

    (v) They will form AlCl3 (Aluminium chloride)

    Question 5. (a) List the three events that occur during the process of photosynthesis. Explain the role of

    stomata in this process.
    (b) Describe an experiment to show that “sunlight is essential for photosynthesis”.


    (a) Three most important steps of photosynthesis are:

    • Absorption of light energy by chlorophyll
    • Conversion of light energy to chemical energy and splitting of water molecules into hydrogen and oxygen
    • Reduction of carbon dioxide to carbohydrates

    Stomata are present in enormous amount over leafs, this tiny pores helps in the exchange of gaseous by photosynthesis. Since large amount of water can also be lost through these stomata, the plant closes these pores when it does not need carbon dioxide for photosynthesis, opening and closing of pours is regulated by guard cells, it  swell up when water flows inside them, causing the stomata pore to open. Similarly the pore closes if the guard cells shrink.

    (b) Experiment to prove that “sunlight is essential for photosynthes” is explained below:

    (i) Select two healthy plants.

    (ii) Keep them in a dark room for 3 days.

    (iii) Now place them in different air tight chambers, one supplied with potassium hydroxide, which is used to absorb carbon dioxide.

    (iv) Now keep the plants in sunlight for 2 hours.

    (v) Pluck leaf from both plants and check the presence of starch.

    (vi) Higher starch content in the jar with potassium hydroxide indicates that in the presence of light the photosynthesis have occurred so higher starch content is found. 

    Question 6. (a) How does Mendel’s experiment show that traits may be dominant or recessive?      

    (b) How traits get expressed from parents to offsprings? Explain with an example.

    (c) Why are traits acquired during the lifetime of an individual not inherited?


    (a) Mendel in his experiment on the inheritance of traits selected and crossed tall pea plant with dwarf pea plant. The F1 progeny had Tt genotype but the plant was tall. This shows that ‘T’ is dominant trait whereas ‘t’ is recessive.

    (b) Traits are passed through DNA. When an egg is fertilized with sperm, the resulting offspring takes 50 percent of its DNA from each parent. This resulting combination of two halves of DNA determines what specific traits the child will have. The expression of traits in an offspring is determined by dominant and recessive genes. If one or both parents pass on a dominant gene to the child, then the dominant gene will be expressed. However, a recessive gene is expressed only in case when both parents pass on the recessive genes to the offspring.

    For example, in case of eye colour, the gene for brown eyes is dominant and that for blue eyes is recessive. So if either parent passes on the brown eye gene, then it will be expressed in the child. On the other hand for the expression of blue eye trait, both the parents should pass blue color traits. Thus people with blue eyes have two recessive genes, but people with brown eyes can have one dominant and one recessive gene or both dominant genes.

    (c) Only the traits which are passed through DNA of germ cells are inherited to the offsprings  during sexual reproduction. On the other hand the traits acquired during lifetime do not pass on to the DNA of germ cells that’s why they cannot be inherited.

    Question 7. (a) Write the chemical equation for the following reactions:

    (i) Methane burns in air

    (ii) Ethanoic acid reacts with sodium carbonate

    (iii) Ethanoic acid reacts with sodium hydroxide

    (b) Why is a mixture of acetylene and oxygen instead of acetylene and air, used for welding purposes.


    (a) Chemical equations for given reactions are:

    (i) Burning of methane in air:

    (b) Acetylene being an unsaturated hydrocarbon, burns incompletely in air due to which a sooty flame is produced with very low temperature. On the other hand when a mixture of acetylene and pure oxygen is burnt, then it gives rise to the complete combustion of acetylene to produce a blue flame with a very high temperature which can be used to weld the metals.

    Question 8. A person uses concave lens spectacles. What vision defect does he have? What could be the possible reasons causing this defect?

    Draw a diagram
    (i) To show the image formed by an eye suffering from vision defect.
    (ii) To show the correction of this eye defect using the lens.

    A person using concave lens spectacles is suffering from myopia (short-sightedness).

    The possible reasons causing myopia are:

    • Convering power of eye lens has been increased due to its short focal length.
    • Eye ball has become longer.

    (i) Diagram showing the myopic eye:

    (ii) Diagram to show the correction of myopia:

    Question 9. (a) What is ovulation?

    (b) How is it beneficial for the foetus to have a circulatory system that is not directly attached to the circulatory system of mother?

    (c) What changes occur at the time of birth?


    (a) When a girl child is born, her ovaries already contain thousands of immature ova (or eggs) which are contained in immature follicles. At the age of puberty, one follicle develops into a mature ovum (or egg). It pinches off from the surface of the ovary and enters the fallopian tube. This process is termed as ovulation.

    (b) The circulatory system of the foetus and that of the mother are connected by an umbilical cord that attaches the placenta to the fetus. The separate circulatory system facilitates quick diffusion of nutrients, metabolic waste and respiratory gases between the foetus and the mother. Also placenta acts as a shield for the baby against bacteria and other pathogens.

    (b) At the time of birth the strong muscles in the walls of the uterus undergo the rhythmic contractions  that are initiated by decreased progesterone secretion and the secretion of oxytocin hormone by the pituitary gland. These rhythmic contractions followed by labour pains result in the dilation of cervix. When the cervix is fully dilated, contractions help the baby to move from the uterus into the vagina through wbich the baby finally steps into the world. After the child delivery, the placenta is expelled out.

    Question 10. (a) What is astigmatism? How is it caused? How can it be corrected?

    (b) A person is unable to see both far as well as near objects. What is the defect he is suffering from? What could be the reason for this defect? What type of spectacles should be worn by him to correct his vision.


    (a) Astigmatism: The defect by which the person is not able to differentiate horizontal and vertical position, is called astigmatism.

    Causes of Astigmatism:
    Astigmatism is caused by an unevenly shaped cornea. The uneven shape causes the rays of light to focus on two points on the retina, causing distortion and blurring of images.

    Correction of astigmatism:

    This defect of vision can be rectified by using cylindrical lenses. 

    (b) The defect of eye in which a person is unable to see both far as well as near objects clearly is called presbyopia.

    Causes of presbyopia:

    This defect of vision commonly occurs to old people because of the following reasons:

    • Weakening of the ciliary muscles
    • Reduction in the flexibility of the eye lens

    Correction of presbyopia:

    This type of defect can be corrected by using spectacles having bi-focal lenses in which upper part consists of a concave lens to correct myopia (shortsightedness) used for distant visions and the lower part consists of a convex lens to correct hypermetropia (longsightedness) sused for reading purposes.

    Question 11. (a) Why does carbon generally forms covalent compounds.

    (b) Also state reasons to explain why covalent compounds:
    (i) are bad conductors of electricity?
    (ii) have low melting and boiling points?

    (c) Name an allotrope of carbon that is a good conductor of electricity.


    (a) Carbon being tetravalent, is neither capable of losing all of its four valance electrons nor it can easily accept four electrons to complete its octet because both of these conditions are energetically less favourable. Therefore carbon atom prefers to complete its octet by sharing of electrons and hence forms covalently bonded compounds.

    (b) (i) Covalent compounds are bad conductors of electricity because they are formed by sharing of electrons. So they don't have any free electron that is required for carrying electricity. Thus they are bad conductors of electricity. 

    (ii) CovaLent compounds are formed by the sharing of electrons, hence the intermolecular forces are relatively weaker than the electrostatic forces existing between the ionic compounds that are formed by the transferring of electrons. Due to the weak intermolecular forces, less energy is needed to break the covalent bonds hence they have low melting and boiling points

    (c) Structure of graphite accounts for it being a good conductor of electricity. In its structure each carbon atom is directly attached to three carbon atoms through covalent bonds. Therefore, out of the four valence electrons in a carbon atom, only three are used for bonding and the fourth electron remains free and can move from one carbon atom to the other. These free electrons make graphite a good conductor of electricity.

    Question 12. (a) Explain the term “rancidity”. Name and explain the type of chemical reactions responsible for causing rancidity.

    (b)Write three methods for preventing rancidity of food.


    (a) Rancidity: Rancidity can be defined as the chemical decomposition of fats and oils due to hydrolysis or auto-oxidation or microbial activity, producing aldehydes, hydroxyl acids, keta acids, and other compounds. This causes a considerable damage to the natural structure of fats and oils, producing undesirable odors, flavors and discoloration  in the food containing them; making them unfit for consumption.

    For Example: When butter is kept open for a long time, then its smell and taste gets changed (due to the presence of butyric acid (a four-carbon acid)).

    Factors responsible for causing rancidity are light, oxygen, trace elements such as iron and zinc, salt, water, bacteria, and molds etc.

    Types of chemical reactions responsible for causing rancidity:

    (i) Oxidation Reaction:  Oxidation is said to have occurred when a substance reacts with oxygen in the air forming oxides. It causes the formation of peroxide at the double bonds (C=C) of fat molecules with subsequent breakdown of these peroxides to form aldehydes, ketones and acids of lower molecular weight. This results in off-flavor, off-odour and sometimes change in colour.

    (ii) Hydrolysis: Hydrolysis can be defined as the breaking down of a chemical compound into two or more simpler compounds by reacting with water. It decomposes a lipid into its component fatty acids and glycerol.

                    C-O-CO-R + H2O →  C-O-H + HO-CO-R

    (b) Following are the methods for preventing rancidity of food: (write any three)

    • In order to prevent rancidity due to oxygen or water vapours present in a container, vacuum packing should be used or the packing should be filled with inert gases. For Example: Potato Chips are usually packed in thick foil packets filled with Nitrogen.
    • Refrigeration of foods also reduces the rate of most of the reactions that cause rancidity.
    • Reducing the water content in foods by drying or smoking and then storing them in a moisture free environment helps to reduce the chances of rancidity.
    • By adding anti-oxidants (butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT) to the food containing fats and oils.
    • Keeping food in air tight containers also helps to reduce oxidation.

    Question 13. (a) Define 1 watt.

    (b) State the commercial unit of electric energy. Express it in terms of SI unit of energy.

    (c) An electric refrigerator rated as 750 W operates 8 hours per day. What is the cost of the energy to operate it for a month of June at Rs. 2.50 per kWh?


    (a) Watt: Watt is the SI unit of electric power. It is the power consumed by a device that carries 1 A of electric current when operated at a potential difference of 1 V.

    Thus, 1 W = 1 volt × 1 ampere = 1 V A

    (b) Commercial Unit of Electric Energy:

    The unit ‘watt’ is very small. So, practically a much larger unit called ‘kilowatt’ is used. It is equal to 1000 watts. Since electrical energy is the product of power and time, the unit of electric energy is, therefore, watt hour (Wh).

    One watt hour is the energy consumed when 1 watt of power is used for 1 hour.

    The commercial unit of electric energy is kilowatt hour (kWh), commonly known as ‘unit’. One kilowatt hour is the energy consumed when 1000 watt of power is used for 1 hour.

    1 kW h = 1000 watt × 3600 second

                 = 3.6 × 106 watt second

                 = 3.6 × 106 joule (J) (where joule ‘J’ is the S.I unit of energy.)

    (c)  Given, Power, P = 750 W

            Time, T = 8 hrs per day

    We know that, electrical energy is the product of power and time.

    Therefore, the energy consumed by the refrigerator in one day

                                         = 750 W × 8.0 hour/day

                                        = 6000 W h = 6 kW h

    Now, the total energy consumed by the refrigerator in the month of June (30 Days)

                                        = 30 × Energy/day

                                        = 30 × 6kWh

                                        =180 kWh

    Thus, the cost of energy required to operate the refrigerator for the month of June at Rs. 2.50 per kWh = 180 kW h × Rs 2.50 = Rs 450.00

    Students must practice CBSE Class 10 Science, Important Long Answer Type Questions given here as practice is the key to success.

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