What are salts?

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Autoren: Hans-Jürgen Schwarz, NN
back to Fundamentals

Abstract

Salts are built up of metal kations (e.g. K⁺, Ca²⁺) or an alkaline group like NH₄⁺) and an acid radikal like nitrate, that are held together by ionic bonds. They are crystalline.

The setting of salts

Salts normally consist of positive charged ions, cations, and negative charged ions, anions, that build a crystal lattice. In addition, there often in water (H₂O) in the lattice.

The crystal lattice of NaCl

Salts arise e.g. from a neutralisation reaction, when an acid meets a base.

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle Na^+ + OH^- + H_3O^+ + Cl^- \rightarrow NaCl + 2H_2O}

    base    +        acid          →          salt   +   water

The result in not always a neutral salt. There are also alkaline or acidic salt, depending on the strength of the participating acids and bases:

1. strong acid + strong base Base → neutral salt + H₂O

2. strong acid + weak base → acidic salt + H₂O

3. weak acid + strong base → alkaline salt + H₂O

e.g.

1. Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle H_3O^+ + Cl^- + Na^+ + OH^- \rightarrow NaCl + H_2O }

2. Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle H_3O^+ + Cl^- + NH_3 \rightarrow NH_4Cl + H_2O }

3. Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle H_3O^ + HCO_3^- + 2Na^+ + 2OH^- \rightarrow Na_2CO_3 + 3H_2O }

The chemical notation of salts is written like this:

(positive ion (kation))_x (negative ion (anion))_y • nH₂O; e.g. CaCl₂ • 6 H₂O ,

x,y,n being the number of the ions or water molecules

Salts consist of metal cations or an alkaline group and an acid radical, that are bound together with ionic bonds. They are crystalline.

Which ions build up salts?

Structure damaging salts

Over 90% of the structure damaging salts consist of these
anions

• Sulphate SO₄^2-
• Nitrate NO₃-
• Chloride Cl^-
• Carbonate CO₃^2-

And the kations

• Sodium Na⁺
• Potassium K⁺
• Calcium Ca²⁺
• Magnesium Mg²⁺

further anions are:

• Acetate CH₃COO^-
  
• Formiate HCOO^-
• Oxalate C₂O₄^2-
• Phosphate PO₄^3-

further kations are:

• Ammonium NH₄⁺
  

Salt properties

Salts have certain properties that help to explain their behaviour in solution and as structure damaging salts on objects. These are:

• solubility
• deliquescence humidity
• electrical conductivity
• thermal conductivity
• solidity
• density
• hardness
• melting point
• thermal extension
• colour
• brightness
• luminescence

Density

The density is a location-independant size of every mineral. It is measured in [g/cm³]. The density is only valid for a certain phase; after conversion to another modification, the density changes, e.g.:
Calcite (trigonal): 2.71 g/cm³
Aragonite (rhombic): 2.04 g/cm³

Solidity

Beside density, solidity and hardness determine the physical properties of a salt/mineral. Both refer to the cohesion properties that depend on the sort and strength of bounds in the crystal lattice and are thus subject to anisotropism. Also, cohesion properties depend on errors in the crystal structures, i.e. measures of solitidity and hardness differ from theoretical value. Like most minerals, salts are quite brittle. When a crystal breaks through, the fraction can be unconformable, conchoidal or splintered, or it can have an even cleavage face. The reason for cleavability is found in the crystal structure; it’s simple longitudinal to the crystal lattice, when the allocation is especially dense and there are only few bounds per unit area. Cleavability along planes with hydrogen bonds or Van der Waals-bonds is almost always perfect, especially when equally loaded parts meet by dislocation along the lattice plane by mechanical stress, with the effect of repulsion.

Such lattice planes are also important crystal planes, so that cleavability indicates the symmetry and microstructure of a mineral. It’s especially nice to watch the cleavage cracks when studing minerals with a microscope.

The hardness of minerals:

Intrusion on the surface of a mineral can occur by scratching, impressing a punch, grinding or planing. Subsequently different defined hardness can be detected and measured with varying methods. The indicated values can be compared with each other. In mineralogy, the standard method is the scratching method, the so-called scratch hardness according to Mohs. He defined a hardness scale from 1 to 10 on the basis of ten minerals:

hardness	mineral	hardness	mineral
1	talc	6	potassic felspar
2	gypsum	7	quartz
3	calcite	8	topaz
4	fluorite	9	corundum
5	apatite	10	diamond

The anisotropy that has to be considered when defining the scratching hardness is mostly irrelevant when using the quite rough graduation according to MOHS (e.g. Al₂SiO₅ - Disten).

Melting point

The most structure damaging salts don’t melt. They decompose before reaching the melting point, or a transmutation changes the salt into another mineral, so that the melted mass has a different composition than the salt had.

Thermal conductivity

In a crystal the thermal conductivity is directional and behaves like the optical orientation of light. Directions with dense packed atoms have a greater conductivity than those with less dense packed atoms, so that different circumstances exist within one crystal system. Compared with crystals, the thermal conductivity in amorph solid bodies is isotropic (conduction in all directions of space with the same speed), having only a short-range order of the components and a missing long-range order.

Thermal extension

Like most materials, minerals extend while warming. In crystals this thermal dilatation is much lower than in liquids. The thermal extension also depends on the anisotropy, i.e crystals extend differently in the diverse directions. Some minerals even have a negative extension in one direction, i.e they contract. The irregular deformation of crystals by changes of temperature is the main reason for so-called temperature weathering. In technics, a ceramic glass without thermal extension can be produced by a specific controlling of the cooling down process and crystal growth within the glass bulk.

Electrical conductivity

The conductivity of ion crystalls should be understood as a ion conduction. Crystal errors slip through the lattice. In other words, ideal crystals don’t conduct electricity. As crystall errors are a thermodynamic size (concentration equilibrum) regulated by pressure, temperature and crystal composition, hardly any perfect crystals exist. A thermal induced oscillation of the crystal components around the rest position of an ideal lattice site induce some of the components to leave their site and leave a vacancy. They move to an intermediate lattice site or to the crystal surface. With rising temperature the number of these defects increases. Ion conduction only takes place at high temperatures, because only then the concentration of crystal errors and the thermic energy are large enough to induce an electral conductivity (semiconductor property).
Only in a solved condition then hydrated ions are good electric conductors, thus ion conductors.

Colour

The colour of a mineral is quite unimportant for the salts dealt with here. They are all white. Plaese notice that some coloured (idiochromatic) minerals exist, whose colouring elements (chromophores) like Cr, Mn, Fe, Cu etc.) are an important feature. Allochromatic minerals gain their colour by integrating idiochromatic particles or certain micro elements in their lattice. These other-directed colours an untypic for the mineral and can save varying appearences. The colour of anisotropic minerals changes with the line of sight. This effect called pleochroism is quite distinct in some minerals.

Brightness

The brightness of a mineral is an optical effect induced by the reflection of light. High reflections cause a high brightness that can be described as a metal, diamond or glass brightness. The specific surface form also has an influence on the brightness (greasy, silky, pearly brightness).

Luminescence

Luminescence (lat. lumen = cold light) is the property of minerals to converse applied energy into visible light. Radio luminescence is the stimulation by radioactive radiation, Thermo luminescence is the stimulation by the supply of heat and UV-fluorescence is the stimulation by ultra-violet rays. Phosphorescence is a light phenomenon in minerals that continues after the time of energy input. It is a temperature-dependent process, where energy is stored on an intermediate level and emitted by increasing the temperature. Some electrons stay longer on the intermediate level, so that the glow continues for some time after switching off the stimulating rays and fades away slowly.
Consequences from birefringence, different forms of refraction and other optical properties are used to identify minerals with a polarising microscope. You can find a detailed description in that chapter.

an atom or molecule with a negative electrical charge.

the property of a material being directionally dependent, as opposed to isotropy, which implies identical properties in all directions

The relationship between a mineral's crystallographic axes and optical indicatrix.

the property of exhibiting different colors as a function of the vibration direction

the mathematical difference between the largest and smallest refractive index for an anisotropic mineral