(i) Diamond is very hard.
(ii) The carbon atoms in diamond are sp³ hybridised and bonded to four neighbouring carbon atoms by a bonds with a C-C bond length of 1.54 Å.
(iii) This results in a tetrahedral arrangement around each carbon atom that extends to the entire lattice.
(iv) Since all four valance electrons of carbon are involved in bonding there is no free electrons for conductivity.
(v) Being the hardest element, it used for sharpening hard tools, cutting glasses, making bores and rock drilling.

(i) Anhydrous aluminium chloride is used as a catalyst in Friedel Crafts reactions.
(ii) It is used for the manufacture of petrol by cracking the mineral oils.
(iii) It is used as a catalyst in the manufacture on dyes, drugs and perfumes.

(i) When aluminium metal or aluminium hydroxide is treated with hydrochloric acid, aluminium trichloride is formed.
(ii) The reaction mixture is evaporated to obtain hydrated aluminium chloride.
2Al + 6HCl ⟶ 2AICl₃ + 3H₂
Al(OH)₃ + 3HCl ⟶ AlCl₃ + 3H₂O

(i) Boron trifluoride is obtained by the treatment of calcium fluoride with boron trioxide in presence of conc. sulphuric acid.
B₂O₃ + 3CaF₂ + 3H₂SO₄ \(\overset { \triangle }{ \longrightarrow } \) 2BH₃ + 3CaSO₄ + 3H₂O
(ii) It can also be obtained by treating boron trioxide with carbon and fluorine.
B₂O₃ + 3C + 3F₂ ⟶ 2BF₃ + 3CO
(iii) In the laboratory pure BF₃ is prepared by the thermal decomposition of benzene diazonium tetrafluoro borate.
PhN₂BF₄ \(\overset { \triangle }{ \longrightarrow } \) BF₃ + PhF + N₂

(i) Boric acid is used in the manufacture of pottery glasses, enamels and pigments.
(ii) It is used as an antiseptic and as an eye lotion.
(iii) It is also used as a food preservative.