What are solubility and concentration

Solutions are homogeneous mixtures. This page deals with solutions with water as a solvent. However, there are also solutions with other liquid, gaseous or even solid substances.
The maximum amount of a substance that can be dissolved in a certain amount of the solvent under certain circumstances, forming a stable system, is called solubility. In the given circumstances, it is always a question of the prevailing temperature and possibly the pressure present. Any other amount of the substance in relation to the amount of a solvent is called a concentration.

The solubility of a salt can be calculated if none of its ions are also present from other substances. It is A in the case of saltmB.n defined as follows:

ceqstands for the concentration of the cation or the anion of the salt in the equilibrium state (see below).
m and n are for the salt AmB.n known. For NaCl, both parameters would be 1, for Al2(SO4)3 would be m = 2 and n = 3.
KL. is the constant of solubility and is defined as follows: KL. = ceq(A.+)m ceq(B.-)n
If the solubility constant is known, the solubility of the salt can be calculated quickly and easily.

Chemical equilibrium

In the case of reversible reactions, which consist not only of a forward but also of a reverse reaction, there is a characteristic ratio of starting materials and products at a constant temperature. An equilibrium is established in which the speed of the back and forth reaction is the same (vdown = vback).

For the reaction aA + bB <-> cC + dD
with the speed of the forward reaction vdown = kdown. ca(A) . cb(B)
and the speed of the reverse reaction vback = kback. cc(C) . cd(D)

applies to vdown = vback:

c stands for the respective concentration of the substance in the solution.
kdown and kback are the respective rate constants of the reactions.
K is the so-called equilibrium constant or mass action constant.

The equilibrium constant is only valid for a certain temperature and a certain pressure. If those parameters are changed, the value of the constant also changes. If the value of the equilibrium constant is very high, almost only products of the reaction can be found. If the value is very small, the equilibrium is mainly on the side of the educts.

Dissolving a substance in water is possible until the concentration of the substance reaches its solubility. At this point, a phase of undissolved material will form. In the case of solids, this is the soil.
The solution can be oversaturated for a short time, but the excess dissolved substance will quickly precipitate again. A dynamic equilibrium is established in which substance is dissolved at the same rate as it comes out of solution. The solution is saturated.

If temperature and pressure are not specified for a solubility, standard conditions usually apply. The default temperature is 25OC or 20OC usual, while the standard pressure is 1 bar.

The following parameters can influence the solubility of a substance in water:

Substances that consist of ions or molecules that contain charges are called polar. Molecules without these charges are called non-polar. The water molecule is polar because its two hydrogen molecules have a positive partial charge and its oxygen molecule has a negative partial charge. Since like dissolves in like, polar substances such as salts are more soluble in water than non-polar substances such as oil.

The temperature has a clear influence on the solubility of substances. When a substance is dissolved in water, either energy is released (in the form of heat) or energy is required (in the form of heat). The former solution processes are called exothermic, whereas the latter is called endothermic. If such a process is endothermic, the solubility of the substance increases with increasing temperature. This is true for most solids. In exothermic processes, the solubility decreases as the temperature rises, which is normally the case with gases.

Pressure has almost no influence on the solubility of solids in water. This is due to the small change in volume. The volume of gases, on the other hand, is strongly influenced by pressure. Their solubility in water is therefore also very pressure-dependent. The following applies: the higher the pressure, the more gas goes into solution.
Gases with very high solubilities react with the solvent.

PH value
When some substances are dissolved, ions are formed from water molecules. An example of this is chlorine, which causes the formation of H in water+Ions (Cl2 + H2O <-> OCl- + 2H+ + Cl-). Depending on the pH of the solvent, it either contains more H+-Ions or more OH--Ions. This ratio is balanced with neutral solvents.
Pure water is a neutral solvent. If a substance is dissolved in it, the pH value has no influence on its solubility. For example, soils, on the other hand, can be more acidic or basic. In this case, the pH value of the soil influences the solubility of a substance in the soil solution.

Salinity and the presence of other substances
The solubility of substances is also influenced by whether other substances are contained in the water. However, not only can the concentration of one salt influence the solubility of another salt in water, but the salt content can also change the solubility of gases, for example. Oxygen has a lower solubility in salt water than in fresh water.

L.solubility of salts


Ch. E. Mortimer, U. Müller, 2003. Chemistry - The basic knowledge of chemistry.
8th edition, completely revised and expanded. Stuttgart, Georg Thieme Verlag.

Dr. G. den Boef, 1977. Theoretical fundamentals van de analyze in waterige oplossingen.
4e print. Amsterdam / Brussel, Elsevier.

www.wikipedia.org, solubility, chemical equilibrium, even weight reactions. January 2007.

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