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 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. 

When we are working with solutions, it is better to have a concentration measure in terms of the volume. Molarity is defined as the number of moles of the solute per liter of solution. 

The molality of a solution is defined as the number of moles of the solute per kilogram of solvent. 

Molarity is usually denoted by the letter M, and molality is denoted by the letter m. 

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For this discussion, we will consider a solute molecule with a dipole moment and then generalize the results. Imagine a molecule with a dipole moment at infinite dilution in a nonpolar supercritical fluid. The dipole s field induces a response of the polarizable fluid which results in a net attractive force. The polarizable solute reacts to the fluid s induced field and further increases the attractive force. A simplified mathematical model of these interactions is helpful in understanding the reasons for solubility enhancement and the density effect. 

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