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Q. 12. A convex lens has a focal length of 12 cm. At what distance from the lens should an object of height 6 cm be placed, so that on the other side of the lens its real and inverted image is formed 24 cm away from the lens? What would be the size of the image formed? Draw a ray diagram to show the image formed in this case.

Sol. Focal length of the convex lens, f = + 12 cm
Size of the object, h₁ = 6 cm
Image distance, v = + 24 cm (+ ve sign for real image)
Object distance, u = ? Size of the image, h₂ = ?
According to lens formula

Q. 13. (a) Give a chemical test to distinguish between saturated and unsaturated hydrocarbons.
(b)
(i) Name the products formed when ethanol burns in air.
(ii) What two forms of energy are liberated on burning alcohol?
(c) Why is the reaction between methane and chlorine considered a substitution reaction?

Ans. (a)
(i) The saturated hydrocarbons generally burn in air with a blue, non-sooty flame because the percentage of carbon in the saturated hydrocarbons is comparatively low which gets oxidised completely by the oxygen present in air. The unsaturated hydrocarbons burn in air with a yellow, sooty flame because the percentage of carbon in unsaturated hydrocarbons is comparatively higher which does not get oxidised completely in the oxygen of air.

(b)
(i) When ethanol burns in air it produces carbon dioxide and water along with the liberation of heat and light

C₂H₅
.

(ii) Heat and light are the two forms of energy which are liberated on burning alcohol.

(c) Methane reacts with chlorine in the presence of sunlight to form chloromethane and hydrogen chloride.

Science 2008 Question Papers Class X
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