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Q. 12. Soap does not work well with hard water. Name the class of compounds which can be used as cleansing agents in place of soap. Write the name of one such compound. Explain in brief the echanism of its cleansing action when used in hard water. 3

Ans. Soap does not work well with hard water. This difficulty is overcome by using another kind of cleansing agent called detergents.

A detergent is the sodium salt of a long chain benzene sulphonic acid.

A common detergent is sodium n-dodecyl benzene sulphonate. Detergents can lather well even in hard water because they do not form insoluble calcium or magnesium salts on reacting with the calcium and magnesium ions present in hard water. A detergent works by making the oil and grease particles of dirty clothes dissolve in water through the formation of miscelles.

Q. 13. A student focussed the image of a candle flame on a white screen by placing the flame at various distances from a convex lens. He noted his observations in the following table:

Analyse the above table and answer the following questions:
(a) What is the focal length of the convex lens?
(b) Which set of observations is incorrect and why?
(c) Draw a ray diagram to show the image formation for any correct set of observations. 3

Science 2009 Question Papers Class X
Delhi		Outside Delhi		Compartment Delhi		Outside Delhi		Foreign
	Set 1 (PDF)		Set 1 (PDF)		Set 1		Set 1		Set 1 (PDF)
					Set 2		Set 2
					Set 3		Set 3