THE 6 MAIN FACTORS THAT AFFECT SOLUBILITY - CHEMISTRY - 2025

The main factors affecting solubility are polarity, the common ion effect, temperature, pressure, the nature of the solute, and mechanical factors. Solubility is the ability of a solid, liquid, or gaseous chemical (called the solute) to dissolve in a solvent (usually a liquid) and form a solution.

The solubility of a substance depends fundamentally on the solvent used, as well as on temperature and pressure. The solubility of a substance in a particular solvent is measured by the concentration of the saturated solution.

A solution is considered saturated when the addition of additional solute no longer increases the concentration of the solution.

The degree of solubility varies widely depending on the substances, from infinitely soluble (completely miscible), such as ethanol in water, to slightly soluble, such as silver chloride in water. The term "insoluble" is often applied to poorly soluble compounds (Boundless, SF).

Certain substances are soluble in all proportions with a given solvent, such as ethanol in water, this property is known as miscibility.

Under various conditions, equilibrium solubility can be exceeded to give a so-called supersaturated solution (Solubility, SF).

Main factors affecting solubility

1- Polarity

In most cases, solutes dissolve in solvents that have a similar polarity. Chemists use a popular aphorism to describe this characteristic of solutes and solvents: "like dissolves like."

Nonpolar solutes do not dissolve in polar solvents and vice versa (Educating online, SF).

2- Effect of the common ion

The common ion effect is a term that describes the decrease in the solubility of an ionic compound when a salt containing an ion that already exists in chemical equilibrium is added to the mixture.

This effect is best explained by Le Châtelier's principle. Imagine if the slightly soluble ionic compound calcium sulfate, CaSO ₄, was added to the water. The net ionic equation for the resulting chemical equilibrium is as follows:

CaSO4 (s) ⇌Ca2 + (aq) + SO42− (aq)

Calcium sulfate is slightly soluble. At equilibrium, most of the calcium and sulfate exists in the solid form of calcium sulfate.

Suppose the soluble ionic compound copper sulfate (CuSO ₄) was added to the solution. Copper sulfate is soluble; Therefore, its only major effect on the net ionic equation is the addition of more sulfate ions (SO ₄ ^2-).

CuSO4 (s) ⇌Cu2 + (aq) + SO42− (aq)

Sulfate ions dissociated from copper sulfate are already present (common to) in the mixture from the slight dissociation of calcium sulfate.

Therefore, this addition of sulfate ions emphasizes the previously established equilibrium.

Le Chatelier's principle dictates that the additional stress on this side of the equilibrium product results in the equilibrium shift towards the reactants side to relieve this new stress.

Due to the shift towards the reactant side, the solubility of slightly soluble calcium sulfate is further reduced (Erica Tran, 2016).

3- Temperature

Temperature has a direct effect on solubility. For most ionic solids, increasing the temperature increases how quickly solution can be made.

As the temperature increases, the solid particles move faster, which increases the chances that they will interact with more particles of the solvent. This results in an increase in the rate at which a solution is produced.

Temperature can also increase the amount of solute that can be dissolved in a solvent. Generally speaking, as the temperature increases, more solute particles dissolve.

For example, adding table sugar to water is an easy method of making a solution. When that solution is heated and sugar is added, it is found that large amounts of sugar can be added as the temperature continues to increase.

The reason for this is that as the temperature increases, the intermolecular forces can break more easily, allowing more solute particles to be attracted to the solvent particles.

There are other examples, however, where increasing the temperature has very little effect on how much solute can be dissolved.

Table salt is a good example: you can dissolve about the same amount of table salt in ice water as you can in boiling water.

For all gases, as the temperature increases, the solubility decreases. Kinetic molecular theory can be used to explain this phenomenon.

As the temperature increases, the gas molecules move faster and are able to escape from the liquid. The solubility of the gas then decreases.

Figure 1: graph of solubility vs. temperature.

Looking at the graph below, ammonia gas, NH3, shows a strong decrease in solubility as temperature increases, while all ionic solids show an increase in solubility as temperature increases (CK-12 Foundation, SF).

4- Pressure

The second factor, pressure, affects the solubility of a gas in a liquid but never of a solid that dissolves in a liquid.

When pressure is applied to a gas that is above the surface of a solvent, the gas will move into the solvent and occupy some of the spaces between the solvent particles.

A good example is carbonated soda. Pressure is applied to force the CO2 molecules into the soda. The opposite is also true. When the pressure of the gas decreases, the solubility of that gas also decreases.

When you open a soda can, the pressure in the soda drops, so the gas immediately begins to come out of solution.

Carbon dioxide stored in the soda is released, and you can see the fizz on the surface of the liquid. If you leave an open can of soda for a period of time, you may notice that the drink becomes flat due to the loss of carbon dioxide.

This gas pressure factor is expressed in Henry's law. Henry's law states that, at a given temperature, the solubility of a gas in a liquid is proportional to the partial pressure of the gas above the liquid.

An example of Henry's law occurs in diving. When a person dives into deep water, the pressure increases and more gases dissolve in the blood.

While rising from a deep water dive, the diver needs to return to the surface of the water at a very slow speed to allow all the dissolved gases to leave the blood very slowly.

If a person ascends too fast, a medical emergency can occur due to gases leaving the blood too quickly (Papapodcasts, 2010).

5- Nature of the solute

The nature of the solute and solvent and the presence of other chemicals in the solution affect the solubility.

For example, more sugar can be dissolved in water than salt in water. In this case, the sugar is said to be more soluble.

Ethanol in water are completely soluble with each other. In this particular case, the solvent will be the compound that is found in greater quantity.

The size of the solute is also an important factor. The larger the solute molecules, the greater its molecular weight and size. It is more difficult for solvent molecules to surround larger molecules.

If all the above mentioned factors are excluded, a general rule of thumb can be found that larger particles are generally less soluble.

If the pressure and temperature are the same as between two solutes of the same polarity, the one with smaller particles is usually more soluble (Factors Affecting Solubility, SF).

6- mechanical factors

In contrast to the dissolution rate, which depends mainly on temperature, the recrystallization rate depends on the concentration of solute at the surface of the crystal lattice, which is favored when a solution is immobile.

Therefore, agitation of the solution prevents this accumulation, maximizing dissolution. (tipes of saturation, 2014).

References

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2. CK-12 Foundation. (SF). Factors Affecting Solubility. Recovered from ck12.org.
3. Educating online. (SF). Factors affecting solubility. Recovered from solubilityofthings.com.
4. Erica Tran, DL (2016, November 28). Solubility and Factors Affecting Solubility. Recovered from chem.libretexts.org.
5. Factors Affecting Solubility. (SF). Recovered from sciencesource.pearsoncanada.ca.
6. (2010, March 1). Factors Affecting Solubility Part 4. Recovered from youtube.com.
7. Solubility. (SF). Recovered from chemed.chem.purdue.ed.
8. tipes of saturation. (2014, June 26). Recovered from chemistry libretex.org.