Solution

As a solution a homogeneous mixture of at least two chemical substances is known in chemistry. However, the process of dissolving is a physical process. Such a solution consists in part from one or more dissolved solid, liquid or gaseous substances ( solvates ) and partly from the usually liquid and the largest part of the solution constituting liquid or solid solvent ( solvent), which in turn a solution can be.

Solutions are not externally visible, as such, because they only form a homogeneous phase: The solutes are dispersed as molecules, atoms or ions in the solvent and statistically homogeneous. For example, they are not separated by filtration.

Properties

The properties are dependent on both a solution of the solute and the solvent. Most properties change in a clear dependence on the concentration of the dissolved substance. This applies, for example, the color intensity of solutions of colored substances ( Lambert- Beer law ), for the viscosity, the electrical conductivity of ionic liquids, or for the refractive index. Therefore, such properties can be analytically use to measure the concentration and for the recovery of salary data. Colligative properties of solutions are the properties whose changes depend only on the number of dissolved particles, not on their chemical properties. These include the lowering of vapor pressure and freezing temperature and the boiling temperature elevation ( the higher the number of dissolved particles ). A distinction between ideal and real solutions. Ideal solutions obey eg the Raoult's laws on the molar freezing point depression or boiling point elevation molar. Real solutions follow these linear laws completely accurate only at " infinite dilution " and, at high concentrations, a saturation effect. It is similar with the other above- mentioned properties of solutions.

The properties of solutions can be classified under physical point of view as follows:

• Colligatively: lowering of vapor pressure, elevation of boiling point, melting point depression, osmotic pressure
• Additive: mass, volume, energy, enthalpy
• Constitutive: viscosity, refractive index
• Combined electrical properties, interfacial properties, solubility

Components

The solvent of a solution ( solvents ) are usually liquids. Solutions with fixed solvents are usually referred to as mixed crystals or, if they have metallic properties than alloys.

The solutes can be:

• Gas (eg air gases such as oxygen, nitrogen, carbon dioxide in water, natural gas in crude oil, hydrogen chloride or ammonia in water)
• Liquid (such as ethanol in water, octane in gasoline)
• Fixed (eg sodium chloride or calcium bicarbonate in water, naphthalene in hexane)

Solubility

Whether and in what amount a substance is soluble in a solvent depends on the solubility of the substance. Is in a solution as much as possible of the substance dissolved, the solution is saturated; Another fabric will now be added to the solution, this leads to the formation of a sediment.

Not all solutions have a limited solubility so can be ethanol and water in any ratio dissolve in each other.

For solutions of gases in liquids, a solution is considered saturated when a diffusion equilibrium between going into solution and the solution leaving the gas molecules is established. From supersaturated gas solutions but only occur from gas bubbles (as in mineral water or sparkling wine), if the sum of all dissolved gases Lösungspartialdrücke is greater than the mechanical pressure at the site of blister formation. A definite limit of the recording of a liquid, a gas, there is not. The " solubility " herein is rather the coefficient relating the amount dissolved using the spent gas pressure in relation.

Separation

In a solution of substances of solute is usually readily extractable again because there seems to be superficially no chemical reaction at a solution.

In fact, the ionic bonds of the crystal are in solving salts, however, very well dissolved, shall have hydration shells of water molecules around the ions are formed (hydration ). Many metal ions form with the water molecules actually quite stable complex cations, eg Hexaaquaeisen (III). The bond formation mentioned must be completely reversible when a mixture of substances is to apply as a solution.

The releasing of the gaseous acid or Baseanhydriden leads to a reaction. Hydrogen chloride dissolves and immediately dissociated almost completely into chloride ions and hydrogen ions, which in turn combine with water immediately to oxonium. Carbon dioxide remains in solution for the most part as a gas on the other hand. However, a small part forms with the water carbonic acid, which in turn dissociates to bicarbonate, carbonate and hydronium. Also these reactions are fully reversible, i.e., the solutions are no additional reagents again separated.

Separating solids from liquids

Evaporation of the liquid solvent causes the solution is gradually saturated and the solid crystallized out as far as it concerns the solution of a sparingly soluble substance. Upon complete evaporation of the solid at the end remains as sediment.

There are solutions of " solids " such as calcium bicarbonate, which decompose during thickening of the solution, and therefore does not exist as solids. In this example, a residue of calcium carbonate, carbon dioxide evaporates during together with the water formed.

A technically increasingly used possibility is to reverse osmosis. Here, the solution is pressed through a semipermeable membrane, which can not be pass ions and larger molecules. This technique is mainly used for water treatment and in particular used for seawater desalination.

Separating liquid mixtures

Liquids can be separated by fractional distillation (largely). This case, use the different boiling points of the substances involved. However, since the boiling of the volatile substance already there is a lower vapor pressure the higher the boiling liquid, always goes a small portion of it with over. Thus can be explained by alcohol distillation purity gain only up to about 96%. We speak with such a mixture of one azeotrope.

Separation of gas and liquid

Heating the solution leads to the escape of the gas, since its solubility decreases with increasing temperature. Distribute Completely out of the solution can be a dissolved gas, but only by the boiling of the liquid, because the vapor pressure then reaches the mechanical pressure and forms bubbles, with which the gas is completely expelled. The partial pressure of the solvent in these bubbles is then 100 % of the pressure in the bubbles.

Gases may each other "crowd out" also from the solution. To do this, the solution of any gas A into contact with any one gas B to bring, for example by bubbling. It then comes to a diffusion process between the bubbles of gas B and the dissolution of the gas A, in the necessarily more and more B goes into solution and more and more A leaves the solution. The " displacement " has nothing to do with varying solubility. One speaks of a true " stripping ". The Aussieden a gas from its solution is also such a Strippungsvorgang in principle.

Alloys

Also, metal melting mostly represent solutions and are called alloys. A plurality of metals or non- metals are dissolved in a main component; For example, there are some molten steel from a solution of chromium, vanadium and carbon in iron. Alloys are also counted among the " homogeneous mixtures ".

Glass

Glasses, since it is supercooled liquid mixtures at them, also be interpreted as solutions.

Limiting cases

The dissolution of a metal in an acid solution process is not in the proper sense, since in this case a chemical reaction occurs.

There are also boundary cases in which a reversible chemical reaction, and simultaneously a solution process takes place. Examples are:

• The dissolution of sodium in liquid ammonia
• The solution of carbon dioxide in water, an equilibrium with the formation of carbonic acid and its dissociation products ( bicarbonate and carbonate ions) is formed, which disappears when the carbon dioxide leaves the solution ( eg by blowing with another gas )

In geology

In geology is further differentiated in the weathering processes of congruent and incongruent solution. From a congruent solution is called at a uniform and thus complete dissolution of the rock, for example when the solution weathering of halite or limestone, the latter accompanied by the setting of a reversible Dissoziationssystems of carbonic acid ( see above). From an incongruous solution one speaks at a selective dissolution of individual minerals or ions from the rock mass, such as in the wake of silicate weathering.