Question

Iodine, \(\mathrm{I}_{2}\), is very soluble in hexane, \(\mathrm{C}_{6} \mathrm{H}_{14}\), but not in water. Explain this using the rule "like dissolves like."

Short answer

The rule 'like dissolves like' refers to the fact that substances of similar nature (polar or non-polar) tend to dissolve well in each other. Since Iodine (\(\mathrm{I}_{2}\)) and Hexane (\(\mathrm{C}_{6}\mathrm{H}_{14}\)) are both non-polar, they dissolve well in each other. Conversely, Iodine is not soluble in water (\(\mathrm{H}_2\mathrm{O}\)) because water is polar and Iodine is non-polar.

Step-by-step solution

Identifying the Nature of the Substances

First define the nature of iodine (\(\mathrm{I}_{2}\)), hexane (\(\mathrm{C}_{6}\mathrm{H}_{14}\)), and water (\(\mathrm{H}_2\mathrm{O}\)). Iodine being a non-polar molecule and hexane being a non-polar solvent share identical characteristics - lack of formation of electric dipoles. Water, on the other hand is a polar solvent.

Apply the 'Like Dissolves Like' Rule

Because both Iodine and Hexane are non-polar, they will dissolve well with each other, endorsing the principle 'like dissolves like'.

Reason Out the Insolubility of Iodine in Water

Water, in contrast, is a polar solvent. As its intermolecular forces differ from those of Iodine a non-polar substance, the two do not mix very well, leading to the conclusion that iodine is not soluble in water.

Key concepts

Solubility

Solubility is the property of a substance to dissolve in a solvent, forming a homogeneous mixture, or solution. The degree to which a substance can dissolve depends on a variety of factors, including the nature of the solute (the substance being dissolved), the solvent (the substance in which the solute is dissolved), temperature, and pressure.

When it comes to solubility, the rule 'like dissolves like' is a helpful guideline. It suggests that substances with similar chemical properties are more likely to be soluble in each other. For example, polar solutes tend to dissolve well in polar solvents, while non-polar solutes mix better with non-polar solvents. This is because the molecular characteristics, particularly the types of intermolecular forces present, are alike, allowing them to interact and mix at a molecular level.

Polar and Non-Polar Molecules

The distinction between polar and non-polar molecules is fundamental in understanding solubility. Polar molecules have an uneven distribution of electron density, leading to the presence of partial positive and negative charges within the molecule. These charges create permanent dipoles that can interact with other polar substances through dipole-dipole interactions.

Non-polar molecules, on the other hand, have a uniform distribution of electron density and thus lack permanent charge separation. They are typically symmetrical, which means that no one region of the molecule is significantly more positive or negative than another. When non-polar molecules are placed in a polar solvent, they do not mix well because there is a lack of compatible intermolecular forces to form solvation, the process by which the solvent surrounds and interacts with the solute particles.

Intermolecular Forces

Intermolecular forces are the attractive and repulsive forces between molecules that dictate phase, solubility, boiling point, and other physical properties. The main types of intermolecular forces include London dispersion forces, dipole-dipole interactions, and hydrogen bonds.

London dispersion forces, often the weakest, are present in all molecules but play a significant role in non-polar molecules. Dipole-dipole interactions occur between polar molecules requiring the presence of a permanent dipole. Hydrogen bonds are a particularly strong type of dipole-dipole interaction that occurs between a hydrogen atom, which is covalently bonded to a highly electronegative atom like oxygen, nitrogen, or fluorine, and another highly electronegative atom. In the case of iodine dissolving in hexane, the dominant intermolecular force would be the London dispersion forces, which are sufficient to allow these non-polar substances to mix.