Properties of mercury oxide and the reaction of its breakdown

Obtaining mercury oxide illustrated by an experiment


Mer­cury ox­ide is a bi­na­ry com­pound of oxy­gen and mer­cury, with the for­mu­la HgO. In nor­mal con­di­tions it is a sol­id, loose sub­stance, and de­pend­ing on the de­gree of dis­per­sion it is red or yel­low – the main and most im­por­tant mer­cury ox­ide. It is hard­ly ever found in na­ture, one ex­cep­tion be­ing the rare min­er­al of mon­troy­dite. Us­ing mer­cury ox­ide, in 1774 the sci­en­tist Joseph Priest­ley dis­cov­ered oxy­gen (the re­ac­tion of the break­down of mer­cury ox­ide).

Prop­er­ties of mer­cury ox­ide

Yel­low HgO is a more chem­i­cal­ly ac­tive sub­stance, which breaks down at a tem­per­a­ture of 332 °С, and turns red when heat­ed. Red mer­cury ox­ide breaks down at a tem­per­a­ture of 500 °С, and when heat­ed it changes its col­or to black (re­versible re­ac­tion).

Mer­cury ox­ide (II) is poor­ly sol­u­ble in wa­ter, and the sub­stance shows weak base prop­er­ties. It dis­solves in con­cen­trat­ed so­lu­tions of al­ka­lis, form­ing hy­drox­o­com­plex­es. Yel­low HgO in­ter­acts with NH₃ with the for­ma­tion of Mil­lon’s base, and the re­ac­tion of the equa­tion is

2HgO + NH₃ → [Hg₂N]OH · H₂O + Q

This sub­stance en­ters into a re­ac­tion with acids, form­ing cor­re­spond­ing salts. It is used to ob­tain mer­cury, and also in some types of elec­tric cells. Mer­cury ox­ide has a strong tox­ic ef­fect.

Ob­tain­ing mer­cury ox­ide il­lus­trat­ed by an ex­per­i­ment

Mer­cury ox­ide (II) is a use­ful reagent, which can be used in the lab­o­ra­to­ry to ob­tain var­i­ous mer­cury salts – for ex­am­ple mer­cury chlo­ride or ac­etate (II). Mer­cury ac­etate (II) is used in or­gan­ic syn­the­sis, for ex­am­ple for ob­tain­ing alu­minum iso­propy­late, and with Hg­Cl₂ ac­tive mag­ne­sium amal­gam can be ob­tained.

To con­duct the ex­per­i­ment the fol­low­ing equip­ment is re­quired:

  • a flask with a ground glass joint;
  • test tube;
  • re­flux con­denser;
  • a frit­ted glass fil­ter;
  • a con­ic flask.

Reagents used:

  • ni­tric acid 65%;
  • mer­cury;
  • caus­tic soda;
  • sodi­um chlo­ride or hy­drochlo­ric acid

Safe­ty rules for the ex­per­i­ment

As ni­tric ox­ides (II) and (IV) are poi­sonous and have a car­cino­genic ef­fect, cau­tion must be ob­served. Mer­cury salts are tox­ic for hu­man be­ings, and also dan­ger­ous for the en­vi­ron­ment. Poi­sonous mer­cury ni­trate can eas­i­ly be ab­sorbed through the skin. You must work with a fume hood and a re­flux con­denser, as the re­leased gas­es of­ten con­tain mer­cury fumes, which are dan­ger­ous in them­selves. The syn­the­sis should be car­ried out with ex­treme cau­tion. A fa­tal dose of mer­cury ni­trate is from 0.2 to 0.4 gr. Here you’ll find safe chem­istry ex­per­i­ments to do at home.

[Deposit Photos]

Process of syn­the­sis of mer­cury ox­ide

Sus­pend 30 g (0.15 of a mole) of mer­cury in a test tube. Into a flask of 250 ml with a re­flux con­denser, pour 60 ml (0.9 of a mole) of HNO₃. Add mer­cury to the acid in small por­tions with a pipette, and the re­ac­tion will take place. Af­ter all the mer­cury is added, the re­flux con­denser is put on again. The so­lu­tion heats and “boils” from the vig­or­ous re­lease of ni­tro­gen diox­ide. As the re­ac­tion ends, the re­lease of brown gas stops, and the so­lu­tion in the flask be­comes col­or­less. Equa­tion of the re­ac­tion:

Hg + 4H­NO₃ => Hg(NO₃)₂ + 2NO₂ + 2H₂O

[Deposit Photos]

Ni­tric acid is used in abun­dance to avoid the for­ma­tion of mer­cury ni­trate (I). The liq­uid cools, and HCl or NaCl is added to it – this is a test for the pres­ence of mer­cury (I) – Hg₂²⁺. When a sed­i­ment of Hg₂­Cl₂ set­tles in the so­lu­tion, mer­cury (I) is present. A lit­tle ni­tric acid must be added to the so­lu­tion, then it must be heat­ed. If the test for the pres­ence of mer­cury (I) is neg­a­tive, then 250 ml of 4 M sodi­um hy­drox­ide is slow­ly added to the so­lu­tion. An or­ange sed­i­ment of mer­cury ox­ide (II) HgO forms, which is fil­tered. The equa­tion of the re­ac­tion:

Hg(NO₃)₂ + 2NaOH => HgO + 2NaNO₃ + H₂O

The prod­uct is rinsed with wa­ter in the fil­ter and dried to a con­stant mass in a dessi­ca­tor over sil­i­ca gel. The re­lease of mer­cury ox­ide (II) is 32.467 g.

Safe­ty rules for con­duct­ing ex­per­i­ments with mer­cury ox­ide must be strict­ly ob­served.

De­con­tam­i­na­tion of mer­cury waste

The en­tire fil­trate and rinsed wa­ter is col­lect­ed in a large cup, if nec­es­sary the so­lu­tion is brought to an al­ka­li and sodi­um sul­fide is added to it. Black mer­cury sul­fide HgS forms, which can be poured down the drain.


Sol­u­ble mer­cury salts must not be poured down the sink. The ob­tained mer­cury ox­ide is stored in tight­ly sealed jars.

Re­ac­tion of the break­down of mer­cury ox­ide

Ob­tain­ing oxy­gen in the lab­o­ra­to­ry is based on the break­down of loose com­pounds which con­tain oxy­gen in their com­po­si­tion. These sub­stances in­clude Berthol­let’s salt, potas­si­um per­man­ganate, sodi­um hy­drox­ide and mer­cury ox­ide. When these sub­stances are heat­ed, they break down with the re­lease of oxy­gen. The re­ac­tion of the break­down of mer­cury ox­ide can be demon­strat­ed in an ex­per­i­ment.

For the ex­per­i­ment, we use a test tube of high-melt­ing glass with a length of 17 cm and di­am­e­ter of 1.5 cm, with a bent low­er end of a length of 3 cm. In the low­er end, place 3-5 g of red mer­cury ox­ide. In the tilt­ed test tube fas­tened to a stand, place a rub­ber stop­per with a gas tube, through which the oxy­gen re­leased in heat­ing goes to the crys­tal­liz­er with wa­ter.

When mer­cury ox­ide is heat­ed to 500 °С, we ob­serve the re­lease of oxy­gen from the gas tube, and drops of metal­lic mer­cury form on the walls of the test tube. Oxy­gen dis­solves poor­ly in wa­ter, so it is col­lect­ed us­ing the method of wa­ter dis­place­ment af­ter the com­plete re­moval of oxy­gen from the test tube.

Af­ter the ex­per­i­ment is com­plet­ed, we must first take the tube out of the crys­tal­liz­er with wa­ter, then turn off the burn­er, and only open the test tube af­ter it has cooled down com­plete­ly (mer­cury fumes are very poi­sonous). In­stead of a test tube, you can use a re­tort with a re­cep­ta­cle for mer­cury. From 10 g of red mer­cury ox­ide, 500 ml of oxy­gen is ob­tained. The equa­tion of the re­ac­tion of the break­down of mer­cury ox­ide:

2HgO = 2Hg + O₂ - 2x25 kcal

Warn­ing! Sub­stances of this ex­per­i­ment are tox­ic and high­ly dan­ger­ous for your health. Do not try this at home. Only un­der pro­fes­sion­al su­per­vi­sion.