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11.(a) What is meant by periodicity in properties of elements with reference to the periodic table? (b) Why do all the elements of the same group have similar properties? (c) How will the tendency to gain electrons change as we go from left to right across a period? Why?

Solution:(a)The trend in which a particular property decreases or increases in a period or a group is called periodicity in properties.(b)All the elements of a group have the same number of valence electrons or the same valency. Hence, all the elements of a

group have similar properties.(c)The tendency to gain electrons will increase as we go from left to right in the periodic table. This is because the electronegativity increases12 (a) Distinguish between esterification and saponification reactions of organic compounds. (b) With a labeled diagram describe an activity to show the formation of an ester.

Solution: Esterification: Formation of an ester from an organic acid and an alcohol in the presence of an acid is esterification.

Saponification: Reaction of esters in the presence of an acid or a base to form alcohol and carboxylic acid is saponification.

(b)Take 1 ml of ethanol and 1 ml of glacial acetic acid in a test tube along with a few drops of concentrated sulphuric acid. Place the test tube in a water bath for about 5 minutes. The formation of ester can be identified by a sweet smell, when this mixture is poured in 20 ml water.13. For which position of the object does a convex lens form a virtual and erect image? Explain with the help of a raydiagram.

Solution:When an object is placed between the optic centre and principal focus of a convex lens, the image of the object formed isvirtual, erect and magnified.15. (a) Distinguish between ionic and covalent compounds under the following properties: (i)Strength of forces between constituent elements (ii) Solubility of compounds in water (iii) Electrical conduction in substances (b) Explain how the following metals are obtained from their compounds by the reduction process: (i) Metal M which is in the middle of the reactivity series. (ii) Metal N which is high up in the reactivity series. Give one example of each.

Solution:(a)(i)Ionic compounds have greater strength of forces of attraction between the constituent elements, while covalentcompounds have lower strength of forces of attraction between the constituent elements.

(ii)Ionic compounds are soluble in water, while covalent compounds are insoluble in water.(iii)Ionic compounds conduct electricity in fused state or in aqueous solution, while covalent compounds do not conductelectricity.

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(b)(i)Metal M, which is in the middle of the reactivity series, are present as carbonates and sulphides. It is easier to obtain ametal from its oxide as compared to its carbonate and sulphide. Therefore, prior to reduction, the metal sulphides andcarbonates must be converted to metal oxides. These oxides are reduced using suitable reducing agents.(ii)Metal N, which is high up in the reactivity series is obtained by electrolytic reduction, that is, by subjecting molten metalsalt to electrolytic refining.

15. (a) Distinguish between ionic and covalent compounds under the following properties: (i)Strength of forces between constituent elements

(ii) Solubility of compounds in water (iii) Electrical conduction in substances (b) Explain how the following metals are obtained from their compounds by the reduction process: (i) Metal M which is in the middle of the reactivity series. (ii) Metal N which is high up in the reactivity series. Give one example of each.

Solution:(a)(i)Ionic compounds have greater strength of forces of attraction between the constituent elements, while covalentcompounds have lower strength of forces of attraction between the constituent elements.(ii)Ionic compounds are soluble in water, while covalent compounds are insoluble in water.(iii)Ionic compounds conduct electricity in fused state or in aqueous solution, while covalent compounds do not conductelectricity.(b)(i)Metal M, which is in the middle of the reactivity series, are present as carbonates and sulphides. It is easier to obtain ametal from its oxide as compared to its carbonate and sulphide. Therefore, prior to reduction, the metal sulphides and

carbonates must be converted to metal oxides. These oxides are reduced using suitable reducing agents.(ii)Metal N, which is high up in the reactivity series is obtained by electrolytic reduction, that is, by subjecting molten metalsalt to electrolytic refining. Question 16

16. (a) What is myopia? State the two causes of myopia. With the help of labelled ray diagrams show(i) the eye defect myopia.(ii) correction of myopia using a lens.(b) Why is the normal eye unable to focus on an object placed within 10 cm from the eye?

Solution:(a) The defect of the eye due to which it cannot focus on the far of f objects but can focus the nearby objects clearly is calledmyopia. When the curvature of the eye ball is more than the normal or if the elongation of the eye ball is also more than thatof the normal, it gives rise to myopia.

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(b) When an object is placed within 10 cm from the eye, the rays from the object are more divergent and hence do not focuson the retina.OR(a)What is dispersion of white light? What is the cause of such dispersion? Draw a diagram to show the dispersion of whitelight by a glass prism.(b)A glass prism is able to produce a spectrum when white light passes through it but a glass slab does not produce anyspectrum. Explain why it is so.

Solution:(a) The phenomenon in which a white light ray splits into its constituent colours while passing through a prism is calleddispersion of light. As the refractive index of each colour is different for a medium, they deviate to different extent whilepassing through the medium and result in dispersion.

(b) In a glass slab, the surfaces where the light ray is incident and emergent are parallel to each other. Hence, even if there isdispersion of light at one surface, while passing through the other surface, a recombination of light of different colours takesplace to nullify the dispersive effect. Hence, the phenomenon of dispersion of light cannot be observed in a glass slab and isobserved only in a prism.

17. Which compounds are responsible for the depletion of ozone layer?Solution:Chlorofluorocarbons (CFCs) are responsible for the depletion of ozone layer.19. Where does digestion of fat take place in our body?

Solution:Complex fats are emulsified by bile salts in duodenum and completely digested in small intestine by the action of lipase

enzyme into fatty acids and glycerol.Question 20

20. How is charcoal obtained from wood? Why charcoal is considered a better fuel than wood?Solution:When wood is burnt in a limited supply of oxygen, water and volatile materials present in it get removed and charcoal is leftbehind as the residue.Charcoal is considered a better fuel wood because:i) Charcoal has higher calorific value than wood.ii) It burns smoothly and produces less smoke.iii) It leaves fewer residues.iv) It is a more compact fuel than wood and has higher percentage of combustible matter in it.v) It has lower ignition temperature than wood.22. What is Chipko Movement? Why should we conserve forests?

Solution:Chipko Andolan is Hug the Trees Movement. It was the result of a grass root level effort to end the alienation of peoplefrom their forests. It originated from an incident in a remote village called Reni in Garhwal, in the early 1970s. We should conserve forests and wildlife for preserving our environment. They together maintain an ecological balance ofnature. We should conserve them for our economic and social growth and to meet our material aspirations.23. Suggest any four changes that you would like to be incorporated in the lifestyle of students of your age to movetowards a sustainable use of available resources?

Solution:i) Rain water harvesting should be done to recharge ground water.ii) Trees should be planted along the school boundary.iii) The fallen leaves of trees should be collected and made into compost, which can be used in the school garden.iv) Fluorescent tubes must be used instead of bulbs as they consume less electricity.24. Describe the role of the following in human beings:(i) Seminal vesicles(ii) Prostate gland

Solution:Seminal vesicle is a sac like structure and secretes seminal fluid. It opens into the urethra.

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Prostate gland is present around the base of seminal vesicle and secretes seminal fluid. It opens into urethra through manyducts.Seminal fluid makes the transport of sperms easier and also provides nutrition.26. Explain how the sex of the child is determined at the time of conception in human beings.

Solution:Conception is a method which involves the entry of male sperm into the vagina of a female. Male gametes contain Xchromosome or Y chromosome. Female gametes contain only X chromosomes. When X gamete of male fuses with (X)

gamete of female, it results into a female child with XX chromosomes. If Y gamete of male fuses with (X) gamete offemale, it results into a male child with XY chromosomes.1. What happens when a small piece of sodium is dropped into ethanol?

Solution:Sodium ethoxide is formed with the liberation of hydrogen gas when a small piece of sodium is dropped intoethanol.

Question 2

2. Which one of these has a higher concentration of H+ ions?

1M HCl or 1MSolution:1M HCl has a higher concentration of H+ ions. As HCl is a stronger acid than CH3COOH, it dissociates in water to a largerextent giving a higher concentration of H+ ions.11. (a) What is a homologous series of substances?(b)In an organic compound, which parts largely determine its physical and chemical properties?(c)Write a chemical equation to represent the reaction of ethanol with acidified solution of potassium dichromate?

Solution:(a)A series of substances in which the same functional group substitutes for hydrogen in carbon chain is called a homologousseries.(b) In an organic compound, the length of carbon chain determines the physical properties, while the chemical properties ofthe compound are determined by the functional groups.

(c)Acidified solution of potassium dichromate reacts with ethanol and oxidizes it to acetic acid.2. What happens when a small piece of sodium is dropped into ethanol?

Solution:When a small piece of sodium is dropped into ethanol, sodium ethoxide is formed along with the liberation of hydrogen gas.