Compounds of Mercury :

(1) Mercuric oxide, HgO : It is obtained as a red solid by heating mercury in air or oxygen

for a long time at 673 K

or by heating mercuric nitrate alone or in the presence of Hg

When NaOH is added to a solution of HgCI₂ , yellow precipitate of HgO are obtained.

Red and yellow forms of HgO differ only in their particle size. On heating to 673 K, yellow form changes to red form.

(2) Mercuric chloride, HgCl₂ : It is obtained by treating Hg with CI₂ or by heating a mixture of NaCl and HgSO₄ in presence of small amount of MnO₂ (which oxidizes any Hg(I) salts formed during the reaction).

It is a white crystalline solid and is commonly known as corrosive sublimate. It is a covalent compound since it dissolves in organic solvents like ethanol and ether.

It is extremely poisonous and causes death. Its best antidote is white of an egg.

When treated with stannous chloride, it is first reduced to white ppt. of mercurous chloride and then to mercury (black).

With ammonia it gives a white ppt. known as infusible white ppt.

HgCI₂ + 2NH₃ → Hg ( NH₂ ) CI + NH₄ CI

A dilute solution of HgCI₂ is used as an antiseptic.
(3) Mercuric iodide, HgI_2­ : It is obtained when a required amount of KI solution is added to a solution of . HgCI₂

Below 400 K, Hgl₂ is red but above 400 K, it turns yellow

Hgl₂ readily dissolves in excess of KI solution to form the ( Hgl₄ )^2- complex ion.

An alkaline solution of K₂ [ Hgl₄] is called Nessler’s reagent and is used to test NH⁺₄ ions.

It gives a brown ppt. of NH₂ - Hg - O Hg - I (Iodide of Millon’s base) with NH⁺₄ ions.

2K₂ [ HgI₄ ] + NH₃ + 3KOH → NH₂. HgO. Hgl + 7KI + 2H₂ O

It is used in ointments for treating skin infections.

(4) Mercurous chloride, Hg₂Cl₂ : It is obtained as under :

It dissolves in chlorine water forming mercuric chloride.

Hg₂Cl₂ + Cl₂ → 2HgCl₂

With ammonia, it turns black due to the formation of a mixture of finely divided black Hg and mercuric amino chloride.

                                 

It is used to prepare standard calomel electrode and as a purgative in medicine.

(5) Mercuric sulphide, HgS : The solubility product of HgS is lower than that of ZnS and hence it gets precipitated as black solid when H₂S is passed through an acidic solution of any mercury (II) salt.

HgCl₂ + H₂S → HgS + 2HCl

It is insoluble in water and HCl but dissolves in aqua regia (1 part conc.HNO₃ + 3 parts conc. HCl)

HgS+ 2| Cl | → HgCl₂ + S ↓

On sublimation, its colour changes to red and hence it is used as a red pigment.

(6) Mercuric sulphate, HgSO₄ : It is obtained when HgS is treated with conc.H₂SO₄ .

Hg + 2H₂ SO₄ → HgSo₄ + SO₂ + 2H₂O

It is a white solid which decomposes on heating to give mercurous sulphate.

                 

3HgSo₄ → Hg₂ SO₄ + Hg + 2SO₂ + 2O₂

It is used as a catalyst in the hydration of alkynes to give aldehydes or ketones. It is also used as a cosmetic under the name Vermillon and in ayurvedic medicine as makardhwaj.

(7) Amalgams : Mercury forms alloys commonly known as amalgams, with all metals except iron and platinum. Hence it is transported in iron containers.

(8) Alloy of transition metal : See in table discuss earlier in metallurgy.