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The Concept of Solubility

Solubility of a substance is defined as the amount of the substance that will dissolve in a particular solvent. Let's consider an example in which sucrose is added to water. When we add sucrose and stir, it dissolves in the water. Let's say we keep on adding more and more sucrose to the solution. Since more and more sucrose is dissolving, the solution is not saturated or it is called an unsaturated solution.
It reaches a point where additional amount of sucrose will not dissolve in that solution. At that point the solution is said to be saturated. We have yet another category in this tradition of categorizing solutions. In some cases, certain solutes become more soluble if the solution is heated. Such solutions are called supersaturated solutions. If we carefully and slowly cool down a supersaturated solution without disturbing its contents, normally we will still have a supersaturated solution. At this stage, even the slightest addition of the solute will result in crystallization.

The Reasons for Solubility

One basic rule that you have to keep in mind is "like dissolves like." This means that substances which have similar polarity dissolve one another. We know that water is a polar substance. Since oil is nonpolar, water and oil cannot mix. Solubility helps in maintaining the lowest energy possible when the solute and the solvent are mixed together.

Ionic Solutions

Ionic compounds have very peculiar solubility trends. Some are highly soluble, whereas some others have very little solubility. The solubility of ionic compounds can be explained in terms of the interactions between the ions and the water molecules. Let's take sodium chloride as an example. Sodium chloride has a solubility of 360 g per liter or 36 g per 100 ml at room temperature.
Since water is a polar molecule, when we add sodium chloride to water, the water molecules will orient themselves according to the surrounding ions. Here we have sodium (Na+) and chloride (Cl) ions. So the slightly negative pole (the negative pole is due to the electronegativity of the oxygen atom) of the water molecules will align toward the sodium ions, and the other pole will orient toward the chloride ions. This attraction is otherwise called hydration. Besides hydration, there is another force called lattice energy of the crystal lattice. The greater the lattice energy of an ionic solid is, the lesser its solubility.

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