Salt (chemistry)

2008/9 Schools Wikipedia Selection. Related subjects: Chemical compounds

A magnified crystal of a salt ( halite/sodium chloride)

A salt, in chemistry, is defined as the product formed from the neutralisation reaction of acids and bases. Salts are ionic compounds 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 (CH₃COO⁻) and monoatomic ions such as fluoride (F⁻), as well as polyatomic ions such as sulfate (SO₄²⁻).

There are several varieties of salts. Salts that produce hydroxide ions when dissolved in water are basic salts and salts that 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 centre in the same molecule but are not considered to be salts. Examples include amino acids, many metabolites, peptides and proteins.

When salts are dissolved in water, they are called electrolytes, and are able to conduct electricity, a property that is shared with molten salts. 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.

Properties

Table salt and peppercorns.

Colour

Salts 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. Of course, some salts are inherently opaque.

Salts exist in all different colors, e.g. yellow (sodium chromate), orange ( potassium dichromate), red ( mercury sulfide), mauve ( cobalt chloride hexahydrate), blue (copper sulfate pentahydrate, ferric hexacyanoferrate), green ( nickel oxide), colorless ( magnesium sulfate), white, and black ( manganese dioxide). Most minerals and inorganic pigments as well as many synthetic organic dyes are salts.

Taste

Different salts can elicit all five basic tastes, e.g. salty (sodium chloride), sweet ( lead diacetate; but which will cause lead poisoning if ingested), sour ( potassium bitartrate), bitter ( magnesium sulfate), and umami or savory ( monosodium glutamate).

Odour

Salts of strong acids and strong bases (" strong salts") are non- volatile and odorless, while 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 presence of water, since hydrolysis is the other half of the reversible reaction equation of formation of weak salts.

Nomenclature

Various salt minerals

Chemical composition of sea salt

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 include:

• ammonium NH₄⁺
• calcium Ca²⁺
• iron Fe²⁺ and Fe ³⁺
• magnesium Mg²⁺
• potassium K⁺
• pyridinium C₅H₅NH⁺
• quaternary ammonium NR₄⁺
• sodium Na⁺

Common salt-forming anions (and the name of the parent acids in parentheses) include:

• acetate CH₃COO⁻ (acetic acid)
• carbonate CO₃²⁻ ( carbonic acid)
• chloride Cl⁻ (hydrochloric acid)
• citrate HOC(COO⁻)(CH₂COO⁻)₂ ( citric acid)
• cyanide C≡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)

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 base and an acid anhydride, e.g. 2 NaOH + Cl₂O → 2 NaClO + H₂O

• An acid and an 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) + NaNO₃(aq)