In chemistry, salt is a general term used for ionic compounds composed of positively charged cations and negatively charged anions, so that the product is neutral and without a net charge. These ions can be inorganic (Cl-) as well as organic (CH3-COO-) and monoatomic (F-) as well as polyatomic ions (SO42-).
Solutions of salts in water are called electrolytes. Electrolytes as well as molten salts conduct electricity.
Zwitterions are salts that contain an anionic center and a cationic center in the same molecule, examples are the amino acids, many metabolites, peptides, and proteins.
Mixtures of many different ions in solution like in the cytoplasm of cells, in blood, urine, plant saps, and mineral waters usually do not form defined salts after evaporation of the water. Therefore their salt content is given for the respective ions.
Salts are usually solid crystals with a relatively high melting point. However, there exist salts that are liquid at room temperature, so-called ionic liquids. Inorganic salts usually have a low hardness and a low compressibility, similar to table salt.
Salts often have a good solubility in water. During the solvation or hydration process the packed ions are separated by water molecules. However, many salts are almost insoluble in water, e.g. silver chloride, calcium sulfate, and many metal sulfides.
Salts can be clear and transparent (sodium chloride), opaque (titanium dioxide), and even metallic and lustrous (iron disulfide).
Salts exist in all different colors, e.g. yellow (sodium chromate), orange (sodium dichromate), red (mercury sulfide), mauve (cobalt dichloride hexahydrate), blue (copper sulfate pentahydrate, ferric hexacyanoferrate), green (nickel oxide), colorless (magnesium sulfate), white (titanium dioxide), and black (manganese dioxide). Most minerals and inorganic pigments as well as many synthetic organic dyes are salts.
Different salts can elicit all five basic tastes, i.e. salty (sodium chloride), sweet (lead diacetate), sour (potassium bitartrate), bitter (magnesium sulfate), and umami or savory (monosodium glutamate).
Pure salts are usually odorless, while impure salts may smell after the acid (e.g. acetates like acetic acid (vinegar), cyanides like hydrogen cyanide (almonds)) or the base (e.g. ammonium salts like ammonia).
The 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 are:
- ammonium NH4+
- calcium Ca2+
- iron Fe2+ and Fe3+
- magnesium Mg2+
- potassium K+
- pyridinium C5H5NH+
- quaternary ammonium NR4+
- sodium Na+
Common salt-forming anions (and the name of the parent acids in parenthese) are:
- acetate CH3-COO- (acetic acid)
- carbonate CO32- (carbonic acid)
- chloride Cl- (hydrochloric acid)
- citrate HO-C(COO-)(CH2-COO-)2 (citric acid)
- cyanide Câ‰¡N- (hydrogen cyanide)
- hydroxide OH- (water)
- nitrate NO3- (nitric acid)
- nitrite NO2- (nitrous acid)
- oxide O2- (water)
- phosphate PO43- (phosphoric acid)
- sulfate SO42- (sulfuric acid)
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
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).