The word ‘salt’ is our daily companion. Table salt is the seasoning we add to meals in order to preserve them or improve their taste. Table salt (NaCl-, sodium chloride) is also a salt in chemical terms. In chemistry, when we say ‘salt’, we aren’t referring to a single substance but to the whole range of compounds with a characteristic structure and properties.
The structure and classification of salts
Salts are chemical compounds with a crystalline structure. They consist of metal cations (or the cation of ammonium NH4+) and acid radical anions. To give an example, in the well-known table salt, also known as sodium chloride (NaCl), the cation is the sodium ion Na+, and the anion is the chloride ion Cl–. The vast majority of salts features a crystalline structure. However, nature knows certain molecules of salt that are not crystalline despite being solids. An example of such a substance is tin(II) diphosphate(V): Sn2P2O7. There are also salts that are in liquid form at room temperature, such as antimony(V) fluoride: SbF5.
Based on the structure of a particular salt, we differentiate:
These are derivatives of hydracids (aqueous solutions of the relevant hydrides of elements from groups 16 and 17 in the periodic table).
Double and triple salts
They are distinguished by the fact that their structure contains, respectively, two or three different cations linked with the acid radical.
Also referred to as acidic salts. They are produced of such acids in which not all hydrogen atoms have been substituted by metal cations.
Basic salts are such salts in which not all hydroxide atoms have been substituted by acid radical anions.
Hydrates are hydrated salts, i.e., salts that are additionally bound with one or more water molecules in their crystal latices.
Naming of salts
The name of a salt is formed based on the name of the acid (acid radical donor), by adding the name of the metal and including its valence.
The names of salts differ depending on whether the salt derives from an oxyacid or a hydracid:
- the names of hydracid salts end with –ide (e.g., sulphide, iodide, chloride, etc.);
- the names of oxyacids end with –ate (e.g., sulphate(VI), sulphate(IV), nitrate(V), etc.).
When we form the names of salts, we should always include the valence of the acid radical and of the metal contained in the compound.
For double and triple salts, when creating their chemical names, we should list the cations in alphabetical order, linked with the conjunction ‘and’. In addition, the names are preceded with a prefix that defines the number of metal atoms in the molecule, such as di-, tri-, etc.
For the names of acid salts, we mustn’t forget to attach the prefix ‘hydrogen-‘, which indicates that the salt molecule contains atom(s) of hydrogen.
When determining the names of basic salts, we should first state the name of the acid radical anion, then the number of hydroxide ions, and finally the name of the metal.
The names of hydrated salts (hydrates) must include the full name of the salt and then the number of linked water molecules.
In chemistry, it is very popular to use customary names of various salts. The best example can be sodium acid carbonate, commonly known as baking soda used for baking cakes and other confections, as an additive in carbonated drinks or in drugs the combat hyperacidity. Calcium sulphate(VI) –water (1/2) is the systematic name for crystal gypsum, while sodium nitrate(V) is the popular ‘Chile saltpetre’. Customary names have become so popular that they are familiar household names.
The properties of salts
Salts form crystals, which have an ionic structure (they are built of ions). Most of them do not have any colour (very often the white colour of the salt crystals is caused by the degree of fragmentation, while in fact the crystals are colourless). However, there is a large group of these compounds that have a particular colour, for example:
- the salts containing copper cations are normally green or blue,
- nickel salts are characterised by their green colour,
- iron causes its compounds to turn brown or green.
The colour depends on the valence which the chemical element has in a particular compound. The colours of hydrated salts are often different from those of anhydrous salts. To give an example, cobalt(II) chloride (CoCl2) is blue, but as its molecules bind with water (i.e., turn into the hydrated form), it becomes pink.
The solubility of salts in water varies. Most of them, such as nitrates(V) or sodium, potassium, or ammonium salts, are very soluble in water, where they dissociate. There is also quite a large group that forms insoluble deposits. If we are unsure which compounds are freely soluble and which are not, we should use the solubility tables.
Ionic dissociation of salts relates to their electrical conductivity and consists in the decomposition of molecules into cations and anions in water solutions. The salts that dissociate have the ability to transfer electric charges, in other words, to conduct current. However, we should note that current is also conducted by molten salts.
The application of selected salts
The chemical compounds called salts are undoubtedly present in each area of our lives, no matter their diversity and properties. Below we present selected applications of three sample salts.
Without a doubt, the most familiar salt is the sodium chloride (NaCl) that we all know as table salt. In day-to-day life, it is used as a seasoning for foods. Table salt also has preserving properties. In addition, it is used in the tanning and glass-making industries. The aqueous solution of sodium chloride is the so-called physiological solution, utilised in pharmacy and medicine.
Calcium carbonate (CaCO3) is a popular ingredient used in the manufacturing of toothpastes, construction mortars and paints. At schools, calcium carbonate is commonly applied as chalk. This particular salt is also an important ingredient of fertilisers due to its ability to increase the pH of soil (the neutralisation of the acids it contains).
Given its oxidation properties, potassium tetraoxomanganate(VII) (KMnO4) is an important component of substances used for disinfecting or washing wounds. It can also be used to produce oxygen (on a laboratory scale).