Note on Compounds of Mercury

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Compound of mercury

Calomel or mercurous chloride (Hg2Cl2)

Mercury (I) chloride is the chemical compound with the formula Hg2Cl2. it is also termed as calomel (i.e. a mineral form, rarely found in nature) or mercurous chloride, this is dense white or yellowish-white in appearance and characterized by odorless solid and is the principal example of a mercury (I) compound. It is a component of reference electrodes in electrochemistry.

Chemical formula


Molar mass

472.09 g/mol


White solid


7.150 g/cm3

Melting point

525°C (977°F; 798K) (triple point)

Boiling point

383°C (721°F; 656K) (sublimes)

Solubility in water

0.2 mg/100 mL


insoluble in ethanol, ether

Refractive index


The synonyms of mercury chloride are as follows:

  • dimercury dichloride
  • mercury(I) chloride
  • calomel
  • dimercury dichloride
  • mercurous chloride
  • mercury chloride

The oxidation number of mercury in dimercury dichloride is 1.

  • Formula: Hg2Cl2
  • Hill system formula: Cl2Hg2
  • Formula weight: 472.085
  • Class: chloride
  • Colour: white
  • Appearance: Crystalline solid and an addition to sensitivity of light
  • Melting point: 525°C (turns yellow on heating)
  • Boiling point: 383°C
  • Density: 7160 kg m-3

Element percentages in dimercury dichloride







Calomel contains 84.98% mercury and 15.02% chlorine within it.


  • From mercurous nitrate Hg2(NO3)2: mercurous chloride can be prepared by treating mercurous nitrate with sodium chloride.

Hg2(NO3)2 + 2NaCl → Hg2Cl2 + 2NaNO3

  • From mercuric chloride HgCl2: when mercuric chloride and mercury react in the iron vessel the ratio 4:3, calomel is obtained.

HgCl2 + Hg → Hg2Cl2

  • From mercuric sulphate HgSO4: When mercuric sulphate is treated with sodium chloride in the presence of mercury, calomel is obtained.

HgSO4 + NaCl + Hg → Hg2Cl2 + Na2SO4

Physical properties of mercurous chloride

  • It is amorphous powder.
  • It is insoluble in water, acid but soluble in aquaregia.

Chemical properties

  • Action with heat: when heated calomel gives mercuric chloride and mercury.

Hg2Cl2 → HgCl2 + Hg

  • Action with ammonia: it reacts with ammonia to form black ppt. of amine complex of mercuric chloride.

Hg2Cl2 + 2NH3 → {Hg(NH2)Cl + Hg} + NH4Cl


  • It is used as fungicide.
  • It is used to make electrodes.
  • It is used as the purgative in medicine.
  • It is used for making other compounds of mercury.

Mercuric chloride or corrosive sublimate (HgCl2)

Mercury(II) chloride or mercuric chloride (archaically, corrosive sublimate) is the chemical compound of mercury and chlorine with the chemical formula HgCl2. . Once used as a treatment for an STD disease i.e. syphilis, it is no longer used for medicinal purposes because of toxicity of mercury and the availability of superior treatments.

Chemical formula


Molar mass

271.52 g/mol


colorless or white solid




5.43 g/cm3

Melting point

276°C (529°F; 549K)

Boiling point

304°C (579°F; 577K)

Solubility in water

3.6 g/100 mL (0 °C)
7.4 g/100 mL (20 °C)
48 g/100 mL (100 °C)


4 g/100 mL (ether)
soluble in alcohol, acetone etc.
slightly soluble in benzene, pyrimidine


3.2 (0.2M solution)

Refractive index


The synonyms of mercuric chloride are as below:

  • mercury dichloride
  • mercury(II) chloride
  • corrosive sublimate
  • mercuric chloride
  • mercury chloride

The oxidation number of mercury in mercury dichloride is 2.

  • Formula: HgCl2
  • Hill system formula: Cl2Hg1
  • Formula weight: 271.495
  • Class: chloride
  • Colour: white
  • Appearance: crystalline solid
  • Melting point: 280°C
  • Boiling point: 303°C
  • Density: 5600 kg m-3

Element analysis

Element percentages in mercury dichloride







Mercuric chloride contains 26.12% chlorine and 73.88% mercury within it.


  • From Hg: when excess chlorine is passed over mercury, mercuric chloride is obtained.

Hg + Cl2 → HgCl2

  • From mercuric sulphate: it is prepared by heating mercuric sulphate with sodium chloride in the presence on MnO2.

HgSO4 + 2NaCl → HgCl2 + Na2SO4

  • From aquaregia: aquaregia dissolves mercury and forms mercuric chloride.

HNO3 + 3HCl + Hg → NOCl + HgCl2 + 2H2O

Physical properties of mercuric chloride

  • It is the white crystalline solid.
  • It is soluble in water.
  • It is poisonous in nature.
  • It is corrosive in nature.

Chemical properties

  • Action with SnCl2: at first mercuric chloride gets reduced to mercurous chloride and then to metallic chloride.

2HgCl2 + SnCl2 → Hg2Cl2 + SnCl4

Hg2Cl2 + SnCl2 → 2Hg + SnCl4

  • Action with KI: with HgCl2, it forms mercuric iodide and with excess KI it forms complex potassium mercuric chloride.

HgCl2 + 2KI → HgI2 + 2KCl

HgI2 + 2KI → K2HgI4

Nessler’s reagent: the alkaline solution of potassium mercuric iodide is known as Nessler's reagent. It is used to detect ammonia.

  • Action with ammonia: it forms white ppt. of amine complex of mercuric chloride.

HgCl2 + NH3 → Hg(NH2)Cl + NH4Cl

  • Action of heat; it decompose to Hg & Cl2.

HgCl2 → Hg + Cl2


  • It is used to prepare Kessler's reagent.
  • It is used for the preparation of calomel.
  • It is used as antiseptic for sterilizing surgical instruments.


poulse, tracy. Introduction to chemistry. u.s.a: flexbook, 2010.

Pathak, Sita Karki. The Text Book of Chemistry. kathmandu: Vidhyarthi Pustak Bhandar, 2012.

  • Mercurous chloride is an amorphous powder which is insoluble in water but soluble in aquaregia.
  • Mercurous chloride is used as the fungicide,used as the purgative in medicine,and also used to prepare other compounds of mercury.
  • Mercuric chloride is a white crystalline solid, which is soluble in water and poisonous and corrosive in nature.
  • Mercuric chloride is used to prepare nessler's reagent, used for preparing calomel and also used as antiseptic for sterilizing surgical instruments.

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