Solubility is the ability of a solute to form a solution in a solvent. The extent of the solute’s solubility in a specific solvent can be determined by its concentration in a saturated solution. There is no more solute dissolving in the solvent and both the substances (solute and solvent) are said to be in solubility equilibrium whereas some solutes and solvents just dissolved in each other and are not saturated called miscible substances.
The solubility of the solutes in a solvent depends on their nature. Mainly solubility depends on the composition of solutes and solvents. In addition, there are factors that affect the rate of solubilities, such as temperature, pressure, common ion, Surface area, and agitation (stirring).
These are the following solubility rules:
Solubility rule 1
- Nitrates, chlorates, and acetates are soluble in water whereas silver acetate is partially soluble in water.
Solubility rule 2
- All the salts of potassium (K), sodium (Na), and aluminum (Al) is soluble in water.
Solubility rule 3
- All metals chlorides, bromides, and iodides are soluble in water except silver, lead, and mercury (l). They are also insoluble in dilute acids. For example, silver iodide (HgI2) is not soluble in water. However, some salts of lead such as lead chloride (PbCl2), (PbBr2), and (PbI2) are soluble in hot water.
Solubility rule 4
- Sulfates of all metals can be dissolved in water except lead, mercury (I), barium, and calcium. Note that, silver sulfate is sparingly soluble in water.
Solubility rule 5
- Phosphates, borates, carbonates, sulfites, arsenates, and chromates of every metal are not soluble in water. Yet, these salts of some metals such as ammonium, sodium, and potassium are soluble in dilute acids.
Solubility rule 6
- Sulfides of all metals except sodium, potassium, and ammonium are insoluble in water. Additionally, salts of barium, calcium, and magnesium sulfides are sparingly soluble in water.
Solubility rule 7
- Sodium, potassium, and ammonium hydroxides are soluble in water whereas calcium and barium hydroxides are moderately soluble. However, oxides and hydroxides of all other metals are insoluble.
Solubility rule 8
- Antimony and bismuth salts hydrolyze in water to give basic salts, which are insoluble in water but soluble in dilute acids.
Solubility rule 9
- When solutions of lead, iron (III), copper, mercury (II), and aluminum salts are boiled in water, they hydrolyze to give precipitations of hydroxides or basic salts.
Solubility of ionic compounds
As mentioned in the standard solubility rules, some of the ionic compounds are soluble in water due to attraction between the positive and negative charges.
For example, in water, oxygen is partially negative while hydrogen is partially positive. The negative part of salt or ionic compound attracts the positive atom (hydrogen atom) in the water while the positive part of the salt attracts the negative atom (oxygen atom) in the water. This phenomenon of encapsulation of ions by water molecules is known as solvation. This results in the dissolution of the salts in the water.
However, there is a limit to the dissolution of salt in water and that concentration is the solubility which is related to the solubility product (Ksp) that shows the equilibrium constant of sparingly or insoluble salts.
Solubility of organic compounds
According to the solubility rules, most organic compounds are non-polar and they dissolve in non-polar solvents. For example, petroleum gel can dissolve in gasoline because both are non-polar.
However, they are not dissolved in water or alcohol, this is because water and alcohol are polar substances. Similarly, polar solutes dissolve in polar solvents. Like water dissolved in alcohol.
According to the solubility rules, the solubility of ions is mentioned below in the solubility chart:
- Blue boxes represent the soluble ions.
- Green boxes represent the sparingly soluble ions.
- Red boxes are the insoluble ions.
- White boxes are unstable ions or unknown.
These are the following applications of the solubility chart which is designed on the solubility rules:
- It has great importance in a large number of scientific disciplines and practical applications.
- Solubility rules give a theoretical explanation of the solubility of chemical substances.
- Solubility is also used in ore processing and nuclear reprocessing.
- It is used to indicate the solute’s polarity. For instance, indigo is expressed as insoluble in water but soluble in chloroform.
- Solubility is useful in the method of mixture separation. For example, sodium chloride and silica can be separated by dissolving in water.
- Solution vs. Suspension: Differences and Examples
- Solubility Product (Ksp): Definition, Applications
- Physical and Chemical Properties of Salts
What are the 3 types of solubility?
Solubility is the mixing or dissolution of one substance (solute) with the other substance (solvent). On the basis of how much solute’s concentration dissolves in the solvent, solubility has three types of solution:
- Saturated solution
- Unsaturated solution
- Supersaturated solution
What is the golden rule of solubility?
The golden rule of solubility is, like dissolves like. For example, polar solutes dissolve in the polar solvent whereas non-polar solutes dissolve in the non-polar solvent.
How do you use the solubility rule table?
The solubility rule table is a chart in which different ions and their solubility is mentioned. These are four boxes in the solubility chart:
- Green boxes represent the soluble ions
- Red boxes represent the insoluble ions
- Yellow boxes are the sparingly soluble ions
- Black boxes are unstable ions
Why are solubility rules so hard to memorize in chemistry?
Solubility rules are according to their experimentation. They are not theoretical but experimental. So if they are memorized with experiments, it will be easy to memorize them.
What is the purpose of solubility tests?
The purpose of the solubility test is the identification and determination of the amount of solute that is dissolved in the solvent.
14.4 Hydrolysis of Salts (openstax.org)