Solubility

Solubility is the largest amount of solute (grams of substance) that can be dissolved in 100 gr. of solvent at a fixed temperature, to form a saturated solution in a certain amount of solvent.

Summary

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  • 1 Solubility process
  • 2 Physical properties
  • 3 Types of solutions regarding solubility.
  • 4 Factors that condition or modify the solubility.
    • 1 The Temperature
    • 2 The Pressure
    • 3 Chemical Nature of Solute and Solvent
  • 5 External links
  • 6 Sources

Solubility process

Substances do not dissolve to the same extent in the same solvent. In order to be able to compare the ability of a solvent to dissolve a given product, a quantity that is called solubility is used. The ability of a certain amount of liquid to dissolve a solid substance is not unlimited. By adding solute to a given volume of solvent, a point is reached from which the solution no longer admits solute (an excess of solute would deposit at the bottom of the container). It is said then that it is saturated. Well, the solubility of a substance with respect to a determined solvent is the concentration that corresponds to the saturation state at a given temperature.

Solubility of solids in liquids vary greatly from one system to another. Thus, at 20 ºC the solubility of sodium chloride (NaCl) in water is 6 M and in ethyl alcohol (C 2 H 6 O), at that same temperature, it is 0.009 M. When the solubility is greater than 0.1 M, usually considers the substance as soluble in the solvent considered; below 0.1 M it is considered as poorly soluble or even insoluble if it is far from this reference value.

The solubility depends on the temperature; hence its value is always accompanied by the working temperature. In most cases, solubility increases with increasing temperature. These are processes in which the system absorbs heat to support the solvation phenomenon with an extra amount of energy. In others, however, dissolution is accompanied by a release of heat and solubility decreases with increasing temperature .

 

Physical properties

Another physical property that allows to know the type of bond is the solubility. Ionic bond compounds are soluble in water and those with a covalent bond dissolve in other covalent compounds. This property has several exceptions, the fundamental one is that substances that have molecules with many oxygen atoms and that are not macromolecules are soluble in water because the oxygen atoms unite with the hydrogen atoms of the water. Virtually all compounds formed with elements to the right of the periodic table have covalent bonds. In ordinary life they would be all paints, solvents, fats, hydrocarbons, sugar, alcohol, etc.

Types of solutions regarding solubility.

  • Unsaturated Solution: “It is the one in which the amount of dissolved solute is less than that indicated by its solubility” this solution is recognized experimentally by adding a small amount of solute and it will dissolve.
  • Saturated Solution: “It is one in which the amount of dissolved solute is equal to that indicated by its solubility.” This type of solution is recognized experimentally by adding a small amount of solute and will not dissolve.
  • Oversaturated Solution: “is that in which the amount of dissolved solute is greater than that indicated by its solubility”. This type of solution is experimentally recognized for its great “instability” since when shaking it or adding a small crystal of solute (seed crystal or crystal seed) the crystallization of excess dissolved solute is caused.

Factors that condition or modify the solubility.

As verified through daily experiences, there are substances that are very soluble in water (sugar), others that are very poorly soluble and others that are practically insoluble (oil), therefore, solubility does not always have a fixed or constant value, but depends on certain factors that will of the solubility a value that can be increased or decreased depending on the modifying factor and these are the following:

Temperature

This factor only modifies the solubility of solid and gaseous solutes, the liquids do not undergo any alteration in their solubility, only until they are miscible with each other (mixed). In the case of solids: in general an increase in temperature will cause an increase in solubility although there are cases where the solubility suffers a small variation and even cases where the temperature decreases with increasing solubility.

In the case of gases: an increase in temperature always produces a decrease in solubility and vice versa. If a small quantity of carbonated drink is placed in a container, when heated, an effervescence derived from the escape of gas (carbon dioxide) from the solution is immediately observed. If water is heated, it loses the air dissolved in it.

The pressure

This factor does not produce any alteration in the solubilities of solids and liquids. The pressure considerably modifies the solubility of a gas and acts as follows: “An increase in pressure will always produce an increase in the solubility of the gas and vice versa, provided that the temperature remains constant” (the temperature also modifies the solubility of a gas). This measurement is known in mathematical terms as “Henry’s law” which says: “The solubility of a gas is directly proportional to the pressure of the gas on the surface of the liquid at constant temperature”.

This can be easily verified with the following experience:

The drinks and the champagne contain a dissolved gas (carbon dioxide) at a high pressure, especially the champagne, which is why when they are opened there is a decrease in pressure and the gas violently escapes from the solution. This can be avoided to a certain degree by cooling, since as one can easily see in the case of gases, their solubility varies contrary to pressure and temperature .

Chemical Nature of Solute and Solvent

We can take this factor in simple terms in the following sense:

“A substance could be very soluble in a certain solvent, but this does not guarantee that it is in other solvents”, to exemplify what has been said, one must observe the solubility of sugar and iodine (in g / 100g of solvent at 20ºC) , using water and alcohol as solvents .

It can be clearly seen that sugar is very soluble in water but poorly soluble in alcohol, while iodine is very poorly soluble in water but very soluble in alcohol .

In reality, “Chemical Nature” has to do with the type of “Chemical Union or Link” that the solute and solvent have, this can be summarized in the following sentence:

“The similar dissolves the similar”

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