Characteristics of magnesium, qualitative reactions

General properties of magnesium and its compounds


Mag­ne­sium is an el­e­ment of the sec­ond group of the third pe­ri­od of the Pe­ri­od­ic Ta­ble (atom­ic num­ber: 12). It is an al­ka­line earth met­al (oth­er mem­bers of this group in­clude beryl­li­um Be, cal­ci­um Ca, stron­tium Sr, bar­i­um Ba and ra­di­um Ra).

In na­ture, mag­ne­sium is en­coun­tered in the form of a min­er­al – for ex­am­ple, the abun­dant min­er­als dolomite Ca­CO₃·Mg­CO₃ and mag­ne­site Mg­CO₃.

Phys­i­cal prop­er­ties

Mag­ne­sium is a sil­very white met­al with high elec­tri­cal con­duc­tiv­i­ty. In or­di­nary con­di­tions, its sur­face is cov­ered with an ox­ide film, MgO, which can be re­moved by heat­ing in air to 600 ᵒC (1112 ᵒF). Com­bus­tion of mag­ne­sium af­ter the dis­in­te­gra­tion of the ox­ide film starts im­me­di­ate­ly with a blind­ing white flash.


3 sta­ble iso­topes of mag­ne­sium may be en­coun­tered in na­ture (iso­topes have an iden­ti­cal atom­ic num­ber, but dif­fer­ent mass num­bers) – with mass num­bers 24, 25 and 26 (the most com­mon iso­tope is the first – its con­tent in a mix­ture of iso­topes reach­es 78-79%). The long­est-lived ra­dioac­tive iso­tope of mag­ne­sium is Mg with an atom­ic num­ber of 28 (half-life – 20.9 hours).

Ob­tain­ing mag­ne­sium

There are 2 meth­ods for ob­tain­ing mag­ne­sium – by elec­trol­y­sis of a flux of its salt and ther­mic re­duc­tion by coal. Elec­trol­y­sis of mag­ne­sium bro­mide takes place as fol­lows:

К(-): Mg²⁺ + 2е = Mg⁰;

A(+): 2Br⁻ - 2е = Br₂;

Mg­Br₂ = Mg + Br₂.

Mag­ne­sium is ob­tained ther­mal­ly from the ox­ide by the re­duc­tion by coal or sil­i­con with heat­ing:

MgO + C = Mg + CO ((so that the mag­ne­sium is not con­tam­i­nat­ed with im­pu­ri­ties, only its ox­ide must be present in the re­ac­tion mix­ture – if a mix­ture of ox­ides is re­duced, all met­als present will be re­duced by coal).

Mag­ne­sium can be re­duced from dolomite with­out mix­tures of cal­ci­um by sil­i­con:

  1. Ca­CO₃·Mg­CO₃ = CaO + MgO + 2CO₂ (when dolomite is baked, 2 ox­ides form – mag­ne­sium and cal­ci­um);

  2. CaO + 2MgO + Si = 2Mg + Ca­SiO₃ (in the re­duc­tion of a mix­ture of cal­ci­um and mag­ne­sium ox­ides, mag­ne­sium can be ob­tained with­out a mix­ture of cal­ci­um, as cal­ci­um ox­ide turns to sil­i­cate).

Magnesium oxide [Wikimedia]

Chem­i­cal prop­er­ties of mag­ne­sium

In the com­bus­tion of mag­ne­sium in air, 2 com­pounds can form – mag­ne­sium ox­ide and mag­ne­sium ni­tride (re­ac­tions take place with the re­lease of a great deal of heat and light):

  • 2Mg + O₂ = 2MgO;

  • 3Mg + N₂ = Mg₃N₂.

Mag­ne­sium can also burn in car­bon diox­ide:

2Mg + CO₂ = 2MgO + C.

Mag­ne­sium does not self-com­bust, so heat­ing is re­quired to car­ry out these re­ac­tions. With heat­ing, mag­ne­sium re­acts with many non-met­als:

  • Mg + S = MgS (mag­ne­sium sul­fide forms);

  • Mg + Cl₂ = Mg­Cl₂ (mag­ne­sium chlo­ride forms);

  • Mg + H₂ = MgH₂ (mag­ne­sium hy­dride forms).

Magnesium hydride 3D structure [Wikimedia]

With heat­ing, mag­ne­sium re­acts with wa­ter, with the for­ma­tion of mag­ne­sium ox­ide or hy­drox­ide and hy­dro­gen:

  • Mg + 2H₂O = Mg(OH)₂ + H₂.

Mag­ne­sium also dis­solves in di­lut­ed acids, form­ing salt and hy­dro­gen gas:

  • Mg + H₂­SO₄ = Mg­SO₄ + H₂;

Click here for in­ter­est­ing ex­per­i­ments with metal­lic mag­ne­sium.

Con­cen­trat­ed sul­fu­ric acid re­acts with mag­ne­sium as fol­lows:

  • 4Mg + 5H₂­SO₄ = 4Mg­SO₄ + H₂S + 4H₂O.

De­pend­ing on the con­cen­tra­tion of ni­tric acid, the fol­low­ing re­ac­tions with mag­ne­sium are pos­si­ble:

  • 4Mg + 10H­NO₃ = 4Mg(NO₃)₂ + N₂O + 5H₂O (if the acid is di­lut­ed);

  • 2Mg + 6H­NO₃ = 2Mg(NO₃)₂ + N₂O + NO + 3H₂O (if the acid is of 30% con­cen­tra­tion).


Qual­i­ta­tive re­ac­tions to mag­ne­sium

There are sev­er­al meth­ods for de­ter­min­ing the pres­ence of mag­ne­sium ions in so­lu­tions of mag­ne­sium salts:

  1. When car­bon­ate ions re­act with mag­ne­sium ions, mag­ne­sium hy­drox­y­car­bon­ate forms in a sol­u­ble sed­i­ment:

2Mg²⁺ + 2CO₃²⁻ + H₂O = (MgOH)₂CO₃ + CO₂;

The sed­i­ment dis­solves in acids:

(MgOH)₂CO₃ + 4Н⁺ = 2Mg²⁺ + 3H₂O + CO₂.

  1. When hy­drophos­phate ions re­act with mag­ne­sium ions in the pres­ence of am­mo­ni­um chlo­ride and am­mo­nia, a crys­talline sed­i­ment of dual mag­ne­sium-am­mo­ni­um phos­phate forms:

Mg²⁺ + HPO₄²⁻ + NH₃·H₂O = MgN­H₄PO₄ + H₂O.

The re­ac­tion can be ex­am­ined by crys­talline mi­cro­chem­i­cal anal­y­sis – it can be car­ried out on a glass slide, and the crys­tals that form can be ob­served un­der a mi­cro­scope.

When car­bon­ate ions re­act with a so­lu­tion of mag­ne­sium salt, the white sed­i­ment Mg­CO₃ forms:

Mg²⁺ + CO3²⁻ = Mg­CO₃ (but many oth­er al­ka­line earth met­als give the same re­ac­tion).

MgCO₃ [Wikimedia]

Or­gan­ic com­pounds can also give a qual­i­ta­tive re­ac­tion to mag­ne­sium – when mag­ne­son-I or II re­acts with mag­ne­sium ions in the ab­sence of am­mo­ni­um ions, the so­lu­tion turns blue.

Oxyquino­line in an al­ka­line medi­um gives a green­ish-yel­low crys­talline sed­i­ment when it re­acts with mag­ne­sium ions.

Wa­ter hard­ness

Along with cal­ci­um ions, the pres­ence of mag­ne­sium ions in wa­ter de­ter­mine its hard­ness. Tem­po­rary wa­ter hard­ness is caused by the pres­ence of mag­ne­sium and cal­ci­um hy­dro­car­bon­ates (Mg(HCO₃)₂ и Ca(HCO₃)₂). This type of hard­ness can be elim­i­nat­ed by boil­ing, as these hy­dro­car­bon­ates de­com­pose when heat­ed:

  • Mg(HCO₃)₂ = Mg(OH)₂ + CO₂;

  • or 2Mg(HCO₃)₂ = (MgOH)₂CO₃ + 3CO₂ + Н₂О;

  • Ca(HCO₃)₂ = Ca­CO₃ + CO₂ + H₂O.

Con­stant hard­ness is caused by the pres­ence of mag­ne­sium and cal­ci­um salts – for ex­am­ple sul­fates and chlo­rides Mg­Cl₂ (Ca­Cl₂), Mg­SO₄ (Ca­SO₄).

Mag­ne­sium salts have found ap­pli­ca­tion in medicine – for ex­am­ple mag­ne­sium ox­ide MgO neu­tral­izes ex­cess acid of the stom­ach juice. Mag­ne­sium ox­ide is also used in the man­u­fac­ture of re­frac­to­ry cru­cibles, in pho­tog­ra­phy, and in bat­ter­ies.