The order of strength of Lewis acid character for boron halides is, BF₃ < BCl₃ < BBr₃ < BI₃ (due to back bonding)

The approximate composition of cement is:
Lime (CaO) = 50 – 60%
Silica (SiO₂) = 20 – 25 %
Alumina (Al₂O₂₃) = 5 – 10 %
Magnesia (MgO) = 2 – 3%
Ferric oxide (Fe₂O₃) = 2 – 3%

H₃BO₃ acts as a Lewis acid and accepts OH^- ions to form [B(OH)₄]^-

AlCl₃ exists as a dimer (Al₂Cl₆). It is a strong Lewis acid as it has an incomplete octet and has a tendency togain electrons. AlCl₃ undergoes hydrolysis easily and forms an acidic solution. AlCl₃ + 3H₂O→Al(OH)₃+ 3HCl
Option(c) is true that AlCl₂₃ sublimes at 180ºC.

AlI₃, on reaction with CCl₄, gives the AlCl₃

4AlI₃+3CCl₄ → 4AlCl₃+3CI₄

The stability of dihalides (MX₂) increases down the group. Except C and Si, the other members form dihalides.

The stability of +2 oxidation state increases and +4 oxidation state decreases as we move down in Group-14. So Pb(IV) is strongly oxidising and also I^- is strong reducing agent. Hence PbI₄ is non existing.

Alum furnishes Al³⁺ ions which bring about coagulation of negatively charged clay particles, bacteria etc.

\(\underset{anhydrous}{Na_{2}B_{4}O_{7}}\overset{\Delta }{\rightarrow}\underset{sod.metaborate}{2NaBO_{2}}+\underset{Boric\: anhydride}{B_{2}O_{3}}\)

\(CuO+B_{2}O_{3}\rightarrow \underset{cupric\: meta\: borate(Bluebeed)}{Cu(BO_{2})_{2}}\)

Alums are double salts. They have general formula M₂SO₄M₂(SO₄)₃.H₂O where M is monovalent cation and M'is trivalent cation.
∴Na₂SO₄.Al₂(SO₄)₃.24H₂O is alum.