Which allotrope of carbon is the hardest natural substance?
A. Diamond
B. Coal
C. Graphite
D. Lamp black
Answer: Option A
Solution (By JKSSB Mock Tests)
Diamond is the hardest known natural material due to its rigid three-dimensional network of covalent bonds with sp³ hybridization, forming a tetrahedral structure. Graphite is soft and slippery because of its layered structure with weak van der Waals forces between layers. Coal is an amorphous form of carbon. Diamond is used in cutting, drilling, and jewelry. Mohs hardness of diamond is 10.
Explanation:
Liquids have a definite volume but take the shape of their container. Solids have the least intermolecular space and highest density. Gases have maximum intermolecular space and are highly compressible. This relates directly to the kinetic theory of matter, which describes particle arrangement and movement in different states.
Explanation:
Calorific value is the amount of heat energy produced by complete combustion of unit mass of a fuel. SI unit is J/kg, often expressed as kJ/kg or MJ/kg. Previously, kcal/kg or cal/g was used. 1 cal = 4.184 J. Higher calorific value indicates better fuel. Example: hydrogen has very high calorific value ~150 MJ/kg. Exam note: Calorific value is measured by bomb calorimeter.
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