[3 Mark Question Answer]

Question 13 Marks

State in brief the transformation of vegetable matter to different types of coal varying in carbon content. State two uses each of $(a)$ coal $(b)$ coke $(c)$ lampblack or soot.

Answer

Transformation of vegetable matter to coal is due to slow bacterial decomposition of vegetable matter under the influence of heat from within the earth, high pressure of earth $($soil$)$ above it and in absence of oxygen over millions of year. The different stages of transformation of vegetable matter results in different types of coal

$60 \%$C is peat
$65 \%$C is lignite
$85 \%$C is bituminous
$90 \%$C is anthracite

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Question 23 Marks

State the reasons for $(a)$ Use of diamond $–$ as an item of jewellery, $(b)$ Use of graphite $–$
$(i)$ as a lubricant for heated machine parts
$(ii)$ as a lining for crucibles used in manufacture of high grade steel
$(iii)$ as an electrode in electroplating.

Answer

$(a)$ Use of diamond -as an item of jewellery as -diamond has sparkling brilliance due to high refractive index is cut and polished.
$(b) (i)$ Heavy machines which run very fast, when other lubricants are used catch fire, but graphite as lubricant when used parallel layers slide over one and other is non-volatile and non$-$sticky and reduces friction between rotating parts.
$(ii)$ Graphite is used in refractory crucibles as graphite can withstand high temperature is a good conductor of heat.
$(iii)$ Graphite as electrode in electroplating as graphite is good conductor of electricity relatively inert and almost does not react with acids.

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Question 33 Marks

Compare the structure of the crystal of diamond $\&$ graphite with special reference to the reason for diamond being the hardest natural substance while graphite one of the softest. Compare the electrical & thermal conductivity of the two crystalline allotropes of carbon.

Answer

Structure of crystal of diamond : Hardest natural substance is due to $–$ strength and uniformity of the carbon$-$carbon covalent bond $[$tetrahadron structure$]$. It is bad conductor of electricity and heat due to abscence of free electrons in the crystal, i.e. no electron is free to move.
Structure of graphite : “Is soft and greasy due to parallel layers of $C-$atoms held together by weak $\ce{VENDER WAAL}$ forces which slide over one another, i.e. hexagonal structure.
It is good conductor of heat and electricity due to $–$ presence of mobile electrons in the crystal.

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CHEMISTRY [3 Mark Question Answer]

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