In chemistry, salts are ionic compounds that result from the neutralization reaction of
an acid and a base.
They are composed of related numbers of cations and anions so that the product is electrically
neutral.
These component ions can be inorganic, such as chloride, or organic, such as acetate;
and can be monatomic, such as fluoride, or polyatomic, such as sulfate.
There are several varieties of salts.
Salts that hydrolyze to produce hydroxide ions when dissolved in water are basic salts,
whilst those that hydrolyze to produce hydronium ions in water are acidic salts.
Neutral salts are those that are neither acid nor basic salts.
Zwitterions contain an anionic centre and a cationic centre in the same molecule, but
are not considered to be salts.
Examples include amino acids, many metabolites, peptides, and proteins.
Usually, non-dissolved salts at standard temperature and pressure are solid, but there are exceptions.
Molten salts and solutions containing dissolved salts 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.
Properties Color
Salts can appear to be clear and transparent, opaque, and even metallic and lustrous.
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, larger crystals tend to be transparent, while
the polycrystalline aggregates look like white powders.
Salts exist in many different colors, for example:
yellow orange
red mauve
blue purple
green colorless—may appear white when powdered
or in small pieces 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.
Taste Different salts can elicit all five basic
tastes, e.g., salty, sweet, sour, bitter, and umami or savory.
Odor Salts of strong acids and strong bases are
non-volatile and odorless, whereas salts of either weak acids or weak bases may smell
after the conjugate acid and cyanides like hydrogen cyanide) or the conjugate base 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.
Solubility
Many ionic compounds can be dissolved in water or other similar solvents.
The exact combination of ions involved makes each compound have a unique solubility in
any solvent.
The solubility is dependent on how well each ion interacts with the solvent, so there are
certain patterns.
For example, all salts of sodium, potassium and ammonium are soluble in water, as are
all nitrates and many sulfates - barium sulfate, calcium sulfate and lead(2) sulfate are exceptions.
However, ions that bind tightly to each other and form highly stable lattices are less soluble,
because it is harder for these structures to break apart for the compounds to dissolve.
For example, most carbonate salts are not soluble in water, such as lead carbonate and
barium carbonate.
Soluble carbonate salts are: sodium carbonate, potassium carbonate and ammonium carbonate.
Conductivity Solid salts do not conduct electricity.
However, liquid salts do.
Moreover, solutions of salts also conduct electricity.
Nomenclature The name of a salt starts with the name of
the cation followed by the name of the anion.
Salts are often referred to only by the name of the cation or by the name of the anion.
Common salt-forming cations include: Ammonium NH4+
Calcium Ca2+ Iron Fe2+ and Fe3+
Magnesium Mg2+ Potassium K+
Pyridinium C5H5NH+ Quaternary ammonium NR4+
Sodium Na+ Common salt-forming anions include:
Acetate CH3COO− Carbonate CO32−
Chloride Cl− Citrate HOC(COO−)(CH2COO−)2
Cyanide C≡N− Fluoride F−
Nitrate NO3− Nitrite NO2−
Phosphate PO43− Sulfate SO42−
Formation
Salts 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,
for example: Pb(NO3)2(aq) + Na2SO4(aq) → PbSO4(s) + 2
NaNO3(aq)
See also
References
Mark Kurlansky.
Salt: A World History.
Walker Publishing Company.
ISBN 0-14-200161-9.