What is meant by the rule of thumb "like dissolves like"?

Intermolecular forces (IMFs) are the forces that mediate interaction between molecules, including attraction or repulsion. These forces dictate the physical properties of substances such as boiling and melting points, vapor pressures, and, notably, solubility. IMFs are generally categorized into several types:

• London Dispersion Forces: These are the weakest and occur between all molecules, whether polar or non-polar. They arise due to temporary dipoles induced when electrons move around two adjacent atoms.
• Dipole-Dipole Interactions: These act between molecules with permanent dipoles, which means they have regions of partial positive and negative charge due to the uneven distribution of electrons in the molecule.
• Hydrogen Bonds: A type of stronger dipole-dipole interaction that happens when hydrogen is bonded to a highly electronegative atom like nitrogen, oxygen, or fluorine.
• Ion-Dipole Forces: These are significant when ions from an ionic compound interact with the partial charges on a polar molecule.

Intuitively, IMFs play a critical role in 'like dissolves like' because they must be overcome when mixing two substances, and they must be favorable to form a solution. Stronger IMFs between molecules within the same substance can make them less likely to dissolve in a solvent with different IMFs.

Polarity in molecules refers to the distribution of electrical charge over the atoms involved in the molecule. If the charge is unevenly distributed, the molecule is polar, exhibiting a partial positive and negative end. This phenomenon significantly affects solubility, which is the ability of a substance to dissolve in a solvent.
The principle that 'like dissolves like' is rooted in the concept of polarity. Polar solvents, like water, have molecules that can form highly favorable interactions with other polar molecules or ions. This can include hydrogen bonds or strong dipole-dipole interactions, which are conducive to the dissolving process. Conversely, non-polar solvents, like hexane, cannot engage in these types of interactions and instead dissolve solutes through London dispersion forces. Hence, the polarity alignment between solvent and solute is a key factor in determining solubility.

Polar substances are characterized by having a significant difference in electronegativity between the atoms that form a bond, creating dipoles, or partial charges. Water is the quintessential polar molecule, with oxygen being more electronegative than hydrogen, resulting in a bent structure with partial positive charges on the hydrogens and a partial negative charge on the oxygen.

Non-polar substances, on the other hand, are typically formed by atoms with similar electronegativities, which result in an even distribution of electron charge. The bonds in these molecules do not have a significant difference in charge across them. Octane, a component of gasoline, is an example of a non-polar molecule; its carbons and hydrogens share electrons evenly without creating dipoles. In the context of solubility, non-polar solvents dissolve non-polar solutes effectively because their intermolecular forces are of the same nature, usually London dispersion forces, that comfortably interact with each other.

Solubility is a central concept in chemistry, referring to the maximum amount of a solute that can be dissolved in a solvent at a given temperature and pressure. It is determined by the ability of the solvent to solvate or interact with the solute, which depends on the nature of both the solute and solvent molecules and their respective intermolecular forces.

The 'like dissolves like' concept in solubility helps predict whether a substance will dissolve in another and facilitates understanding chemical reactions. Water's solvating power, due to its polarity and ability to form hydrogen bonds, makes it the 'universal solvent.' However, solubility is complex—it can be affected by temperature, pressure, and the presence of other substances. An increase in temperature, for instance, generally increases solubility for solids, but decreases it for gases. Projections of solubility are indispensable in various fields of science and industry, including pharmaceuticals, environmental science, and cooking.