MgCO 3 (s) + CO 2 (g) + H 2 O(l) →
Mg(HCO 3 ) 2 (aq)On heating, this process is reversed.
Magnesium hydrogencarbonate is
one of the compounds responsible
for temporary *hardness in water.
magnesium hydroxide A white
solid compound, Mg(OH) 2 ; trigonal;
r.d. 2.36; decomposes at 350°C. Mag-
nesium hydroxide occurs naturally as
the mineral bruciteand can be pre-
pared by reacting magnesium sul-
phate or chloride with sodium
hydroxide solution. It is used in the
reÜning of sugar and in the process-
ing of uranium. Medicinally it is
important as an antacid (milk of
magnesia) and as a laxative.
magnesium oxide (magnesia)A
white compound, MgO; cubic; r.d.
3.58; m.p. 2800°C. It occurs naturally
as the mineral periclaseand is pre-
pared commercially by thermally de-
composing the mineral *magnesite:
MgCO 3 (s) →MgO(s) + CO 2 (g)It has a wide range of uses, including
reÛective coatings on optical instru-
ments and aircraft windscreens and
in semiconductors. Its high melting
point makes it useful as a refractory
lining in metal and glass furnaces.
magnesium peroxide A white
solid, MgO 2. It decomposes at 100°C
to release oxygen and also releases
oxygen on reaction with water:
2MgO 2 (s) + 2H 2 O →2Mg(OH) 2 + O 2The compound is prepared by react-
ing sodium peroxide with magne-
sium sulphate solution and is used as
a bleach for cotton and silk.
magnesium sulphate A white sol-
uble compound, MgSO 4 , existing as
the anhydrous compound (rhombic;
r.d. 2.66; decomposes at 1124°C) and
in hydrated crystalline forms. The
monohydrate MgSO 4 .H 2 O (mono-
clinic; r.d. 2.45) occurs naturally as
the mineral kieserite. The common-
est hydrate is the heptahydrate,
MgSO 4 .7H 2 O (rhombic; r.d. 1.68),
which is called Epsom salt(s), and oc-
curs naturally as the mineral ep-
somite. This is a white powder with a
bitter saline taste, which loses 6H 2 O
at 150°C and 7H 2 O at 200°C. It is
used in sizing andÜreprooÜng cotton
and silk, in tanning leather, and in
the manufacture of fertilizers, explo-
sives, and matches. In medicine, it is
used as a laxative. It is also used in
veterinary medicine for treatment of
local inÛammations and infected
wounds.magnetic momentThe ratio be-
tween the maximum torque (Tmax)
exerted on a magnet, current-
carrying coil, or moving charge situ-
ated in a magneticÜeld and the
strength of thatÜeld. It is thus a
measure of the strength of a magnet
or current-carrying coil. In the Som-
merfeld approach this quantity (also
called electromagnetic moment or
magnetic area moment) is the ratio
Tmax/B. In the Kennelly approach the
quantity (also called magnetic dipole
moment) is Tmax/H.
In the case of a magnet placed in a
magneticÜeld ofÜeld strength H, the
maximum torque Tmaxoccurs when
the axis of the magnet is perpendicu-
lar to theÜeld. In the case of a coil of
N turns and area A carrying a current
I, the magnetic moment can be
shown to be m = T/B = NIA or m = T/H
= μNIA. Magnetic moments are meas-
ured in A m^2.
An orbital electron has an orbital
magnetic moment IA, where I is the
equivalent current as the electron
moves round its orbit. It is given by I
= qω/2π, where q is the electronic
charge and ωis its angular velocity.
The orbital magnetic moment is
therefore IA = qωA/2π, where A is the
orbital area. If the electron is spin-337 magnetic moment
m