What are salts?

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Autoren: Hans-Jürgen Schwarz, NN
back to Fundamentals

Abstract

Salts are made of metal cations (e.g. K⁺, Ca²⁺) or an alkaline group (like NH₄⁺) and an anion 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 is often the water molecule (H₂O) in the lattice.

The crystal lattice of NaCl

Salts are often the result of a neutralisation reaction, when an acid meets a base.

$Na^{+}+OH^{-}+H_{3}O^{+}+Cl^{-}\rightarrow NaCl+2H_{2}O$

base    +     acid       →       salt   +   water

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

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

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

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

e.g.

1. $H_{3}O^{+}+Cl^{-}+Na^{+}+OH^{-}\rightarrow NaCl+H_{2}O$

2. $H_{3}O^{+}+Cl^{-}+NH_{3}\rightarrow NH_{4}Cl+H_{2}O$

3. $H_{3}O^{+}HCO_{3}^{-}+2Na^{+}+2OH^{-}\rightarrow Na_{2}CO_{3}+3H_{2}O$

The chemical notation of salts is written like this:

(positive ion (cation))_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.

Composition of salts?

More than 90% of the "damaging" salts consist of these
anions:

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

And the cations

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

other anions are:

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

further cations are:

• Ammonium NH₄⁺
  

Salt properties

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

• solubility
• deliquescence humidity
• electrical conductivity
• thermal conductivity
• strength
• density
• hardness
• melting point
• thermal extension
• colour
• luster
• luminescence

Density

The density is a location-independent property 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

Strength

Beside density, strength and hardness determine the physical properties of a salt/mineral. Both refer to the cohesion properties that depend on the type and strength of the bounds in the crystal lattice and are thus subject to anisotropy. Also cohesion properties depend on defects in the crystal structures, i.e. measures of strength and hardness differ from the theoretical value. Like most minerals salts are quite brittle. When a crystal breaks, the fracture can be irregular, conchoidal or splintery, or it can have an even cleavage face. The reason for cleavage is found in the crystal structure. It is very easy along that kind of crystal lattice planes, where the occupation in the lattice is especially dense and there are only a few bounds per unit area. Cleavage along planes with hydrogen bonds or Van der Waals-bonds is almost always perfect. Particularly if the mechanical stress leads to a shift of the crystal lattice in such a way that blocks are facing the same charge, so that a repulsion is achieved.

Such lattice planes are also important crystal planes, so that cleavage indicates the symmetry and microstructure of a mineral. It’s especially nice to watch the cleavage cracks when studying 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 scales of hardness can be detected and measured with varying methods. The 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:

Mohs hardness	Mineral	Chemical formula	Absolute hardness	Image
1	Talc	Mg3Si4O10(OH)2	1
2	Gypsum	CaSO4·2H2O	3
3	Calcite	CaCO3	9
4	Fluorite	CaF2	21
5	Apatite	Ca5(PO4)3(OH–,Cl–,F–)	48
6	Orthoclase Feldspar	KAlSi3O8	72
7	Quartz	SiO2	100
8	Topaz	Al2SiO4(OH–,F–)2	200
9	Corundum	Al2O3	400
10	Diamond	C	1600

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 "damaging" salts don’t melt. They decompose before reaching the melting point, or a phase change of the salt into another mineral takes place, so that the resulting melt of a salt has a different composition than the original salt.

Thermal conductivity

In a crystal the thermal conductivity is dependant on the direction (anisotropic) 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 conditions exist within one crystal system. Compared with crystals there is in amorphous solids an isotropic thermal conductivity (heat conduction in all directions of space at 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 crystals can be understood as a ion conduction. Crystal defects slip through the lattice. In other words, ideal crystals don’t conduct electricity. As crystal defects are a thermodynamic property (concentration equilibrium) influenced by pressure, temperature and 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. Ion conduction only takes place at high temperatures, because only then the concentration of crystal defects and the thermal energy are large enough to induce an electric conductivity (semiconductor property).
Only in the solved condition hydrated ions are good electric conductors, so called ion conductors.

Colour

The colour of a mineral is quite unimportant for the salts we dealt with They are all white. Please 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 trace elements in their lattice. These colours are not typical of the mineral and can be very different for one and the same mineral. In anisotropic minerals, of course, the colour of the mineral depends on the direction. This effect the pleochroism is in some minerals pronounced.

Luster

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

Luminescence

Luminescence (lat. lumen = cold light) is the property of minerals to convert input energy into visible light. Radio-luminescence: Stimulation by radioactive radiation. Thermoluminescence: stimulation by application of heat. UV fluorescence: excitation by UV light. Phosphorescence is a light phenomenon in minerals that continues after the time of energy input. It is a temperature-dependent process, where the light energy is stored on an intermediate level and the emission takes place by rising 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.
The consequences from birefringence, different forms of refraction and other optical properties are used to identify the salt minerals with a polarising microscope. You will find a more detailed description in the corresponding chapters.

an atom or molecule with a negative electrical charge.

A molecule is an electrically neutral group of at least two atoms.

an atom or molecule with a positive 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