Science:2017:CBSE:[All India]: Set - I
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Q1
Write the molecular formula of the 2nd and the 3rd member of the homologous series whose first member is methane.
Marks:1View AnswerAnswer:
Molecular formula of the 2nd member is C2H6 (Ethane)
Molecular formula of the 3rd member is C3H8 (Propane)
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Q2
When a cell reproduces, what happens to its DNA?
Marks:1View AnswerAnswer:
When a cell reproduces, its DNA replicates itself/doubles.
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Q3
In the following food chain, 100 j of energy is available to the lion. How much energy was available to the producer?
Marks:1View AnswerAnswer:
According to 10% law, in a food chain, only 10% of the total energy is transferred to the next topic level.
Let the deer have x Joule of energy. Then:
10% of x=100 J
x=1000 J
Thus, the deer has received 1000 J of energy from the plant.
In the same way,
10% of x=1000 J
x=10000 J
Thus, the energy available to the producer (plants) is 10000 J.
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Q4
An object is placed at a distance of 30 cm from a concave lens of focal length 15 cm. List four characteristics (nature, position, etc.) of the image formed by the lens.
Marks:2View AnswerAnswer:
Given that the object is placed at a distance 30 cm from lens which means that the object is placed between the optical centre and infinity. The ray diagram for the given case is as follows:
Four characteristics of the image formed are,
(i) The image is formed on the same side as that of the object.
(ii) The image formed is virtual.
(iii) The image formed is erect.
(iv) The image formed is diminished.
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Q5
State two advantages of conserving (i) forest, and (ii) wild-life.
Marks:2View AnswerAnswer:
Advantages of conserving forests:
i) Forests add aesthetic beauty to the environment.
ii) They provide us valuable things which are essential for us.
Advantages of conserving wild-life:
i) Wild-life is essential for the maintenance of biodiversity.
ii) Wild-life gives way to ecotourism, which is a potential source of income in many countries.
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Q6
Explain two main advantages associated with water harvesting at the community level.
Marks:2View AnswerAnswer:
Following are the advantages associated with water harvesting at the community level:
i) It helps to recharge the underground water table.
ii) The stored water is used by the plants in and around the area.
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Q7
Write the structural formula of ethanol. What happens when it is heated with excess of conc. H2SO4 at 443K? Write the chemical equation for the reaction stating the role of conc. H2SO4 in this reaction.
Marks:3View AnswerAnswer:
When ethanol is heated at 443K with excess of conc. H2SO4,ethene(C2H4) is obtained.In this reaction, sulphuric acid act as a dehydrating agent.
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Q8
Distinguish between esterification and saponification reactions with the help of chemical equation for each. State one use of each (i) esters, and (ii) saponification process.
Marks:3View AnswerAnswer:
Esterification
Saponification
Carboxylic acids react with alcohol in the presence of an acid catalyst to give an ester. This reaction is called the esterification reaction.
It is reverse of esterification reaction.It is the process of hydrolysis of an ester(fats) in the presence of Conc. NaOH to form soap
(i) Use of ester: Esters are used as flavouring agents.
(ii) Use of saponification process: Saponification processis used in making soaps.
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Q9
Write the number of periods and groups in the Modern Periodic Table. How does the metallic character of elements vary on moving (i) from left to right in a period, and (ii) down a group? Give reason to justify your answer.
Marks:3View AnswerAnswer:
Number of periods in modern periodic table=7
Number of groups in modern periodic table=18
(i) Metallic character of elements decreases from left to right in a period. This is because tendency of an element to lose electrons decreases as we move from left to right.
(ii)Metallic character of elements increases as we move down the group. This is because tendency of an element to lose electrons increases as we move down the group.
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Q10
Na, Mg and Al are the elements of the 3rd period of the modern table having group number 1, 2 and 13 respectively. Which one of these elements has the (a) highest valency, (b) largest atomic radius, and (c) maximum chemical reactivity ? Justify you answer stating the reason for each.
Marks:3View AnswerAnswer:
(a)Valency of Na = 1
Valency of Mg = 2
Valency of Al = 3
Among these, Al has highest valency of 3.
(b) Electronic configuration of Na – 2,8,1
Electronic configuration of Mg – 2,8,2
Electronic configuration of Al – 2,8,3
Elements Na, Mg and Al belong to the 3rd period of the modern periodic table. On moving from left to right in the period (Na to Al), atomic size decreases because outer electrons are added to the same valence shell and effective nuclear charge increases.
Therefore, Na has the largest atomic size.
(c) Na has the maximum reactivity.
This is because Na has to lose only one electron from its valence shell to acquire noble gas configuration whereas Mg and Al have to lose 2 and 3 valence electrons respectively which requires more energy.
