What is a liquid solution 1



A solution in chemistry is a homogeneous mixture consisting of two or more chemically pure substances. It contains one or more dissolved substances (the solutes) and a solvent (which can itself be a solution), usually the substance that is present in larger quantities. The solution can be solid, liquid or gaseous.

Solutions cannot be recognized as such from the outside, since by definition they only have one phase and the dissolved substances are therefore evenly distributed in the solvent.

Properties of solutions

The properties of a solution are equally dependent on the dissolved substances and the solvent. Most of the properties change depending on the concentration of the dissolved substance. This applies e.g. B. for the color intensity of solutions of colored substances (Lambert-Beer law), for the electrical conductivity of ion solutions or for the refractive index. Therefore, such properties can be used analytically to measure the concentration.

A distinction is made between ideal solutions and real solutions. Ideal solutions obey z. B. Raoult's laws on the lowering of the molar melting point or the increase in the molar boiling point. Real solutions only follow these linear laws exactly with "infinite dilution" and show a saturation effect at higher concentrations. It is similar with the other properties of solutions mentioned above.

Components of solutions

The solvents are usually liquids. The dissolved substances can be:

solubility

Whether and to what extent a substance is soluble in a certain solvent depends on its solubility (see there for details). If as much of the substance as possible is dissolved in a solution, this solution is called saturated. If it contains too much of a substance, a sediment is formed or the excess remains as saturation. Not all solutions have a limit of the solubility.

In the case of solutions of gases in liquids, a solution is considered to be saturated if there is a diffusion equilibrium between gas molecules going into solution and gas molecules leaving the solution. However, gas bubbles only emerge from oversaturated gas solutions (as in mineral water or sparkling wine) if the sum of the solution partial pressures of all dissolved gases is greater than the mechanical pressure at the location of the bubble formation. There is no definite limit to the absorption capacity of a liquid for a gas. The “solubility” here is rather the coefficient that relates the amount dissolved to the gas pressure used.

Separate solutions

When dissolving substances, the dissolved substance is usually easy to extract again, since in the case of a solution no chemical reaction appears to take place.

In fact, when salts are dissolved, the ionic bonds of the crystal are indeed broken and hydration shells of water molecules are formed around the ions (hydration). Many metal ions even form very stable complex cations with the water molecules (e.g. hexaqua iron (III)). The linkages mentioned must be completely reversible if a substance mixture is to be regarded as a solution.

A reaction also occurs when gaseous acid or base anhydrides are dissolved. Hydrogen chloride dissolves and immediately dissociates almost completely into chloride ions and hydrogen ions, which in turn combine immediately with water to form oxonium. In contrast, the majority of carbon dioxide remains dissolved as a gas. However, a small part forms carbonic acid with the water, which in turn dissociates into hydrogen carbonate, carbonate and oxonium. These reactions are also completely reversible, i. H. the solutions can be separated again without additional reagents.

Separation of solids in liquids

Evaporation of the liquid pure substance causes the solution to gradually become oversaturated and the solid to crystallize out, provided that it is a solution of a substance with limited solubility. When the evaporation is complete, the solid remains as a sediment at the end.

There are solutions of "solids" such as B. Calcium hydrogen carbonate, which disintegrate when the solution is thickened and therefore do not even exist as a dry substance. In this example, a residue of calcium carbonate is created while carbon dioxide evaporates with the water.

Reverse osmosis is a technology that is increasingly being used. The solution is pressed through a semipermeable membrane that prevents ions and larger molecules from passing through. This technology is mainly used for water treatment and especially for seawater desalination.

Separation of liquid mixtures

Liquids can (never completely) be separated by fractional distillation. The different boiling points of the substances involved are used. But since there is a lower vapor pressure of the higher-boiling liquid even when the more volatile substance is boiling, a small proportion of it is always transferred with it. Alcohol can only be obtained up to approx. 96% purity by distillation. Such a mixture is called an azeotrope.

Separation of gas and liquids

Heating the solution leads to the escape of the gas, since its solubility decreases with increasing temperature. A dissolved gas can only be completely expelled from the solution by boiling the liquid, because the vapor pressure then reaches the mechanical pressure and forms bubbles with which the gas is completely expelled, since the partial pressure of the solvent in these bubbles is ultimately 100% of the Pressure in the bladders.

Gases can also "displace" one another from the solution. To do this, the solution of any gas A must be brought into contact with any gas B, e.g. B. by bubbling. A diffusion process then occurs between the bubbles of gas B and the solution of gas A, in which necessarily more and more B goes into solution and more and more A leaves the solution. The “displacement” has nothing to do with different solubility. One speaks more appropriately of a "stripping". Boiling a gas out of its solution is, in principle, also such a stripping process.

Alloys

Metal melts are also mostly solutions and are referred to as alloys. Several metals or non-metals are dissolved in one main component; For example, some steel melts consist of a solution of chromium, vanadium and carbon in iron.

Glass

Because glasses are supercooled mixtures of liquids, glasses can also be viewed as solutions.

Borderline cases

The dissolution of a metal in an acid is not a dissolution process in the strict sense, as it is a chemical reaction.

But there are also borderline cases in which a reversible chemical reaction and a dissolution process take place at the same time. examples are

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

Chemical solution in geology

In geology one also differentiates between the weathering processes of congruent and incongruent solution. One speaks of a congruent solution when the rock is uniformly and thus completely dissolved, for example when halite or limestone is weathered in solution, the latter being accompanied by the setting of a reversible dissociation system of the carbonic acid (see above). In contrast, one speaks of an incongruent solution when there is a selective solution of individual minerals or ions from the rock, for example in the course of silicate weathering.

See also

Category: Chemical Solution