Solubility

Full Notes

Solubility describes how well a substance (the solute) dissolves in a solvent to form a homogeneous mixture, or solution. Whether or not a solute dissolves depends on the intermolecular forces between particles of solute and solvent.

General Rule: “Like Dissolves Like”

• Polar solutes tend to dissolve in polar solvents.
• Nonpolar solutes tend to dissolve in nonpolar solvents.
• This rule is based on the compatibility of intermolecular interactions.

If the solute–solvent interactions are similar in strength or stronger than solute–solute and solvent–solvent interactions, the solute will likely dissolve.

Solubility is the ability of a substance (solute) to dissolve in a solvent to form a homogeneous solution.

It depends on:

• The types of particles involved (ions, molecules)
• The interactions between solute and solvent particles
• The types of intermolecular forces present

The general rule is: “Like dissolves like” — substances with similar types of bonding or intermolecular forces are likely to be soluble in each other and be miscible (can uniformly mix).

Water as a Solvent

Water is a highly polar solvent and can form hydrogen bonds. This makes it very effective at dissolving:

• Ionic compounds
• Small polar molecules

Dissolving Ionic Compounds

When ionic compounds dissolve:

• Water molecules surround and separate the ions
• Positive ions are attracted to the δ⁻ oxygen in water
• Negative ions are attracted to the δ⁺ hydrogen

This process is called hydration. If the hydration energy is greater than the strength of attraction holding the lattice together (lattice energy), the compound dissolves.

Dissolving Small Polar Molecules

Water also dissolves some molecules that can form hydrogen bonds with H₂O molecules, such as:

• Alcohols (e.g. ethanol)
• Ammonia
• Simple carboxylic acids

The hydrogen bonds formed between these solutes and water make them soluble.

Matt’s exam tip

Whether a substance dissolves in water depends on the relative strength of the interactions between solute and water molecules compared to the interactions within the solute itself and within water.

For example, ethanol is soluble in water because the intermolecular forces between ethanol and water molecules are similar in strength to the forces between only ethanol molecules and only water molecules. These comparable attractions make it energetically favourable for the substances to mix.

However, hexanol is not soluble in water. The hydrogen bonding between water molecules is stronger than the interactions between water and hexanol molecules. As a result, it is more energetically favourable for water molecules to stick together than to interact with hexanol — so the two do not mix well.

Water as a poor solvent

Water is a poor solvent for:

• Large polar molecules that cannot form hydrogen bonds (e.g. halogenoalkanes)
• Non-polar molecules (e.g. iodine, hydrocarbons)

These substances cannot interact favourably with water’s hydrogen bonding network and are therefore insoluble or only slightly soluble. The overall strength of hydrogen bonding between solute molecules and water molecules is weaker than the overall strength of hydrogen bonding between just water molecules. This means its more energetically favourable for water molecules to simply hydrogen bond to each other rather than mix with the solute molecules.

Non-Aqueous Solvents

For substances that don’t dissolve in water, non-aqueous solvents can be used. These solvents do not rely on hydrogen bonding and instead dissolve substances through London forces or dipole–dipole interactions.

Examples:

• Hexane: non-polar solvent, dissolves oils and iodine
• Ethanol: polar, can dissolve both polar and some non-polar compounds
• Diethyl ether, chloroform: used in organic chemistry for extracting non-polar solutes

Solubility in these solvents depends on matching the type of intermolecular force:

• Non-polar solutes dissolve in non-polar solvents (e.g. iodine in hexane)
• Polar solutes dissolve in polar solvents

Summary

• “Like dissolves like” connects solubility to matching intermolecular forces.
• Ionic compounds dissolve in polar solvents via ion–dipole interactions and hydration.
• Small polar molecules that can hydrogen bond (e.g., alcohols, NH₃) are often water-soluble.
• Nonpolar solutes dissolve in nonpolar solvents through London dispersion forces.
• Whether a solute dissolves depends on the relative strengths of solute–solvent vs. solute–solute and solvent–solvent interactions.