Question

Give reasons for the following

(i) Carbon acts as an abrasive and also as a lubricant. 

(ii) Sodium carbonate is prepared by Solvay process but the same process is not extended to the manufacture of potassium carbonate. 

(iii) Sulphur melts to a clear mobile liquid at 119°C, but on further heating above 160°C, it becomes viscous.

Answer 1

(i) Carbon exhibits allotropy and exists in various allotropic forms such as diamond, graphite, coke, charcoal etc. 

In diamond crystal, each carbon atom is bonded covalently to each of the four other carbon atoms situated around it tetrahedrally. So, hybridisation of each carbon is sp³ and breakdown of this crystal would involve the rupturing of many extremely stable carbon carbon bonds, due to this diamond is hardest natural substance and used as an abrasive. 

Graphite consists of two dimensional sheet-like polymeric structure in which the carbon atoms are joined together in hexagonal rings. These layers are held together by weak van der Waals' forces and can easily slip over one another, due to this graphite shows lubricating property. 

(ii) Solvey process cannot be used for the manufacture of potassium carbonate as potassium bicarbonate is very soluble and does not crystallise like sodium bicarbonate. 

(iii) The sulphur molecule consists of a ring of eight atoms. On heating the ring tends to break and chains of sulphur atoms are produced. When sulphur melts (on heating at 119°c because it is the m.p. of S), the St rings slip and roll over one another easily. It gives rise to a clear mobile liquid. When liquid sulphur is heated, the rings are broken into very long molecules. These long molecules becomes entangled with one another and make the sulphur very viscous.