Q25 of 59 Page 1

A student puts a drop of reaction mixture of a saponification reaction first on a blue litmus paper and then on a red litmus paper. He may observe that:

As a base turns red litmus blue and has no effect on blue litmus. Saponification mixture is also a base and therefore on pouring it on blue litmus, it will remain same and when it is poured on red litmus, it will turn blue.

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23

(a) Define optical center of a spherical lens.

(b) A divergent lens has a focal length of20 cm. At what distance should an. object of height 4 cm from the optical centre of the lens be placed so that its image is formed 10 cm away from the lens. Find the size of the image also.


(c) Draw a ray diagram to show the formation of image in above situation.

 24

What is atmospheric refraction? Use this phenomenon to explain the following natural events.

(a) Twinkling of stars (b) Advanced sun-rise and delayed sun-set


Draw diagrams to illustrate your answers.

 26

For preparing soap in the laboratory we require an oil and a base. Which of the following combination of an oil and a base would be best suited for the preparation of soap?

 27

In the neighborhood of your school, hard water required for an experiment is not available. Select from the following groups of salts available in your school, a group each member of which, if dissolved in distilled water, will make it hard: