CBSE Science Practice Paper : Set XII
16. Explain the working of a dry cell with a neat diagram. 3
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17. At 298 K, the equilibrium concentration of the reactants and products for the reaction :
H2(g) +I2 (g)= 2HI (g) are as follows :
[HI] = 0.49 M, [H2] = 0.080 M, [I2] = 0.060 M.
Calculate the equilibrium constant at this temperature. 3
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18. What are enzymes ? Name the enzymes required for fermentation of sugarcane to ethanol. Which gas is evolved during the process of fermentation ? How is ethanol prepared from ethene ? Give two uses of ethanol. 3
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19. Derive Len’s formula. Define magnification. Relate HI, v,/for a lens. Use it to find the position of an Image formed by a convex lent of focal length 20 cm and magnified 2 times.
Derive mirror formula. Define magnification. Relate m , v and/for a concave mirror. Use it to find the
position of an image formed by a concave mirror of focal length 20 cm and magnified 2 times. 3+2
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20. Write three physical and three chemical properties of methanoi. Write chemical reactions involved. 5
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21. What is the source of free oxygen given out by the plants ? 1
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22. Name a phytohormone which is a growth inhibitor. 1
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23. What is self-pollination and cross-pollination ? 1
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24. Write short notes on : (a) tubectomy and (b) vasectomy. 2
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25. Explain why there are changes of accumulation of harmful chemicals in the body of human beings. 2
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26. Give the difference between point sources and non-point sources. 3
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27. Write the functions of (a) Testis, {b) Ovaries, © Epididymis. 3
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28. What are the various methods of vegetative propagation ? Discuss any one method. 3
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29. Why is the DNA molecule compared to a spiralling staircase ? Which three components make up the nucleotide ? 3
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30. Draw a well-labelled diagram to explain the excretory system of man.
Draw a well-labelled diagram to explain the respiratory system of man. 5
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