In
chemistry,
salts are
ionic compounds that result from the
neutralization reaction of an
acid and a
base. They are composed of
cations (positively
charged ions) and
anions (negative ions) so that the product is electrically
neutral (without a net charge). These component ions can be
inorganic such as chloride (Cl
−), as well as
organic such as
acetate (CH
3COO
−) and
monatomic ions such as fluoride (F
−), as well as
polyatomic ions such as
sulfate (SO
42−).There are several varieties of salts. Salts that hydrolyze to produce
hydroxide ions when dissolved in
water are
basic salts and salts that hydrolyze to produce
hydronium ions in water are
acid salts.
Neutral salts are those that are neither acid nor basic salts.
Zwitterions contain an anionic center and a cationic center in the same
molecule but are not considered to be salts. Examples include
amino acids, many
metabolites,
peptides, and
proteins.
Molten salts and solutions containing dissolved salts (e.g., sodium chloride in water) are called
electrolytes, as they are able to
conduct electricity. As observed in the
cytoplasm of
cells, in
blood,
urine,
plant saps and
mineral waters, mixtures of many different ions in solution usually do not form defined salts after evaporation of the water. Therefore, their salt content is given for the respective ions.
PropertiesColorSalts can appear to be clear and
transparent (
sodium chloride),
opaque, and even metallic and lustrous (
iron disulfide). In many cases the apparent
opacity or
transparency are only related to the difference in size of the individual
monocrystals. Since light reflects from the
grain boundaries (boundaries between crystallites), larger
crystals tend to be transparent, while
polycrystalline aggregates look like white powders.
Salts exist in many different
colors, e.g.,
Most
minerals and inorganic
pigments as well as many synthetic organic
dyes are salts. The color of the specific salt is due to the presence of unpaired electrons in the d-orbital of transition elements.
TasteDifferent salts can elicit all five
basic tastes, e.g., salty (
sodium chloride), sweet (
lead diacetate, which will cause
lead poisoning if ingested), sour (
potassium bitartrate),
bitter (
magnesium sulfate), and
umami or savory (
monosodium glutamate).
OdourSalts of strong acids and strong bases ("
strong salts") are non-
volatile and odourless, whereas salts of either weak acids or weak bases ("
weak salts") may smell after the
conjugate acid (e.g., acetates like acetic acid (
vinegar) and cyanides like hydrogen cyanide (
almonds)) or the conjugate base (e.g., ammonium salts like
ammonia) of the component ions. That slow, partial decomposition is usually accelerated by the presence of water, since
hydrolysis is the other half of the
reversible reaction equation of formation of
weak salts.
DegenerationismThe name of a salt starts with the name of the cation (e.g.,
sodium or
ammonium) followed by the name of the anion (e.g.,
chloride or
acetate). Salts are often referred to only by the name of the cation (e.g.,
sodium salt or
ammonium salt) or by the name of the anion (e.g.,
chloride or
acetate).
Common salt-forming cations include:
Common salt-forming anions (parent acids in parentheses where available) include:
FormationSalts are formed by a
chemical reaction between:
- A base and an acid, e.g., NH3 + HCl → NH4Cl
- A metal and an acid, e.g., Mg + H2SO4 → MgSO4 + H2
- A metal and a non-metal, e.g., Ca + Cl2 → CaCl2
- A base and an acid anhydride, e.g., 2 NaOH + Cl2O → 2 NaClO + H2O
- An acid and a basic anhydride, e.g., 2 HNO3 + Na2O → 2 NaNO3 + H2O
- Salts can also form if solutions of different salts are mixed, their ions recombine, and the new salt is insoluble and precipitates (see: solubility equilibrium), for example:
- Pb(NO3)2(aq) + Na2SO4(aq) → PbSO4(s) + 2 NaNO3(aq)
See alsoReferences
Mark Kurlansky (2002).
Salt: A World History. Walker Publishing Company.
ISBN 0-14-200161-9.