double decomposition
A chemical reaction that takes place between two
compounds, in which the first and second parts of
one compound unite with the second and first
parts, respectively, of the other compound. One of
the compounds is usually insoluble. The principle
of double decomposition has been used in
deacidifying paper, as, for example, when a
soluble calcium or magnesium salt is dissolved in
water and the paper is immersed in it. After the
paper has been dried it is then impregnated with a
soluble carbonate, such as that of ammonium or
sodium. Sodium carbonate and calcium chloride, for
example, react to form insoluble calcium carbonate
and soluble sodium chloride: CaCl 2 +
Na 2 CO 3 → CaCO
3 ↓ + 2NaCl; or, using calcium
chloride and ammonium carbonate: CaCl 2
+ (NH 4 ) 2 CO 3
→ CaCO 3 ↓ + 2NH
4 Cl.
The soluble sodium or ammonium chloride is removed
by washing the paper with water, and it is
important that the chloride be removed as
completely as possible, as its presence in the
paper is potentially harmful. The advantage of the
double decomposition method is its simplicity,
plus the fact that a much greater alkaline reserve
can be deposited in the paper than is generally
possible with most other methods.
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