What are salts?: Difference between revisions
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==Salt properties == | ==Salt properties == | ||
Salts have certain properties that help to explain their behaviour in solution and as | Salts have certain properties that help to explain their behaviour in solution and as "damaging" salts on objects. These are: | ||
*[[solubility]] | *[[solubility]] | ||
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*thermal extension | *thermal extension | ||
*colour | *colour | ||
* | *luster | ||
*luminescence | *luminescence | ||
Revision as of 14:47, 14 June 2011
<bibimport />
Autoren: Hans-Jürgen Schwarz, NN
back to Fundamentals
Abstract[edit]
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[edit]
Salts[1] 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.
Salts are often the result of 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 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. 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 (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?[edit]
More than 90% of the "damaging" salts consist of these
anions:
- Sulphate SO42-
- Nitrate NO3-
- Chloride Cl-
- Carbonate CO32-
And the cations
- Sodium Na+
- Potassium K+
- Calcium Ca2+
- Magnesium Mg2+
other anions are:
- Acetate CH3COO-
- Formiate HCOO-
- Oxalate C2O42-
- Phosphate PO43-
further cations are:
- Ammonium NH4+
Salt properties[edit]
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[edit]
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[edit]
Beside density, strength 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 anisotropy. Also, cohesion properties depend on defects in the crystal structures, i.e. measures of strength and hardness differ from 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’s simple longitudinal to the crystal lattice, when the allocation is especially dense and there are only few bounds per unit area. Cleavage 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 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:[edit]
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[edit]
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[edit]
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[edit]
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[edit]
The conductivity of ion crystals should be understood as a ion conduction. Crystal errors 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 then hydrated ions are good electric conductors, so called ion conductors.
Colour[edit]
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[edit]
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[edit]
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.