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CBSE 10th Standard Science Subject Chemical Reactions and Equations HOT Questions 3 Mark Questions With Solution 2021

Published on: 29/05/2021

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Std: 10th Standard CBSE - Science

chapter: - HOT Questions

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Questions:

Generic 3

5 x 3 = 15

FeSO₄.7H₂O, green colour crystals on heating, changes colour. Why?

Blue crystals of copper sulphate on heating in a dry test tube become colourless. Give reasons.

A teacher took few crystals of sugar in a dry test tube and heated the test tube over flame. The colour of sugar turned black. Explain why?

Mohan took pure water for the electrolytic decomposition of water but did not see any bubble near the electrodes. Explain why?

Give six uses of decomposition reaction.

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Answers Key:

Generic 3

5 x 3 = 15

The green colour of ferrous sulphate crystals is due to the presence of 7 water molecules (water of crystallization). :It loses the water of crystallization on heating, thus leading to change in colour.

The blue colour of copper sulphate is due to its crystalline nature which holds 5 water molecules (water of crystallization). On heating, the water molecules disappear and anhydrous copper sulphate (white in colour) is left back.

Sugar is a complex compound which on heating undergoes decomposition. Water from sugar gets evaporated thereby leaving behind only black carbon in the test tube.

Pure water has covalent bond and therefore, does not allow the electricity to flow through it and does not get decomposed. On adding a few drops of dil. acid in it, the free ions are obtained and electricity flows through water to dissociate hydrogen and oxygen.

(i) It helps all the living matter to return back to nature, after death.
(ii) Management of garbage
(iii) Photosynthesis \(\rightarrow\) decomposition of water
(iv) Photography \(\rightarrow\) decomposition of silver bromide
(v) Used in chemical industry to obtain elements from complex compounds.
(vi) Decomposition of agricultural waste leads to formation of compost.