Here, we are going to share Class 10th Board exam Paper with solutions of Science (Code 086) Set 2 conducted on 3rd March 2015.

Section A

1. Write the number of covalent bonds in the molecule of propane, C3H8.
► 10 (eight C–H bonds and two C–C bonds)

2. Where is DNA found in a cell?
► Nucleus

3. The first trophic level in a food chain is always a green plant. Why?


Green plants are always in first trophic level because they are producers. They convert the energy of the Sun into a usable form of energy for the next trophic level.

4. The absolute refractive indices of glass and water are 4/3 and 3/2, respectively. If the speed of light in glass is 2 × 108 m/s, calculate the speed of light in
(a) vacuum
(b) water


Note: In the given question, the refractive indices of glass and water are wrong. The question is solved accroding to correct values.

5. We often observe domestic waste decomposing in the bylanes of our homes. List four ways to make the residents aware that improper disposal of wastes is harmful to the environment and also for their own health.


Improper disposal of wastes is harmful to the environment and for our own health in the following ways:
(i) Accumulated wastes serves as a breeding ground for mosquitoes and thereby, creates favourable conditions for the spread of various diseases.
(ii) It releases harmful gases in the environment and makes the environment unclean and unhygienic for all living organisms.
(iii) It flows to water bodies along with the rain water and becomes a threat to aquatic organisms.
(iv) It reduces the fertility of the soil and results in soil pollution.

6. List any two advantages associated with water stored in the ground.


Advantages associated with water store in the ground:
(i) Water does not evaporate and spread out to recharge wells.
(ii) It does not get contaminated by pollutants.

7. What is meant by homologous series of carbon compounds? Classify the following carbon compounds into two homologous series and name them.
C3H4, C3H6, C4H6, C4H8, C5H8, C5H10


Homologous series is a group of organic compounds having a similar structure and similar chemical properties in which the successive compounds differ by a CH2 group.
a) Alkenes (CnH2n): Propene (C3H6), Butene (C4H8), Pentene (C5H10)
b) Alkynes (CnHn-2) : Propyne (C3H4), Butyne (C4H6), Pentyne (C5H8)

8. List two tests for experimentally distinguishing between an alcohol and a carboxylic acid and describe how these tests are performed.


Carboxylic acid can be distinguished from an alcohol by performing the following tests:
(i) Litmus Test: Take 2 ml of alcohol and a carboxylic acid in test tubes. Put 1 or 2 drop of blue litmus solution into each test tube. The liquid which show no effect is alcohol and liquid which turns red is carboxylic acid.
(ii) Sodium hydrogen carbonate test: When a solution of baking soda is treated with a few drops of a carboxylic acid, brisk effervescence is observed due to the evolution of carbon dioxide gas. When this gas is passed in lime-water, the later turns milky. Whereas no effervescence occurs upon treating an alcohol with baking soda solution.

9. The elements 4Be, 12Mg and 20Ca, each having two valence electrons in their valencshells, are in periods 2, 3 and 4, respectivy, of the modern periodic table. Answer the following questions associated with these elements, giving reason in each case:
(a) In which group should they be? 
(b) Which one of them is least reactive?
(c) Which one of them has the largest atomic size?


(a) Group II
Because Be, Mg and Ca each have two valence electrons and a group number is determined by the number of electrons in the outermost shell.

(b) Be is the least reactive because the tendency to lose electrons increases down the group. Thus, reactivity increases down the group.

(c) Ca has the larges atomic size because as we move down the group the size of an atom increases.
10. Taking the example of an element of atomic number 16, explain how the electron configuration of the atom of an element relates to its position in the modern periodic table and how valency of an element is calculated on the basis of its atomic number.

Atomic number of the element = 16
Electronic configuration = 2, 8, 6
The period number is equal to the number of shells which starts filling up in it.
The atom of an element has three shells. So, the period numbis 3.
The atom of an element has six valence electrons in the outermost shell. So, the group number of the element will be 16 (6 + 10).
The valency of an element is determined by the number of valence electrons present in the outermost shell. The atom of an element has six valence electrons in the outermost shell, so the valency of the element is 2.
You can download full solutions in PDF via link given below:

Download Science (Set 2) board paper with solutions 2015
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