The golden rule of solubility is "like dissolves like." This principle dictates that substances with similar polarities are more likely to dissolve in each other.
Understanding "Like Dissolves Like"
This simple phrase encapsulates a crucial concept in chemistry. It means that:
-
Polar solvents dissolve polar solutes: Polar molecules have an uneven distribution of electrical charge, creating positive and negative ends. Water (H₂O) is a prime example of a polar solvent. Other polar solvents include alcohols (like ethanol) and ketones (like acetone). Polar solutes like sugar (sucrose) and salt (sodium chloride) dissolve well in water because they also have polar characteristics.
-
Non-polar solvents dissolve non-polar solutes: Non-polar molecules have an even distribution of electrical charge. Examples of non-polar solvents include hexane, toluene, and diethyl ether. Non-polar solutes such as fats, oils, and waxes dissolve well in non-polar solvents.
Why Does This Happen?
The driving force behind solubility is the intermolecular forces between the solute and the solvent. When the solute and solvent have similar intermolecular forces (either both polar or both non-polar), they can effectively interact, leading to the solute molecules dispersing evenly throughout the solvent.
-
Polar interactions: Polar solvents and solutes interact through dipole-dipole interactions, hydrogen bonding, and ion-dipole interactions.
-
Non-polar interactions: Non-polar solvents and solutes interact through London dispersion forces (also known as Van der Waals forces).
If the intermolecular forces between the solute and solvent are significantly different (e.g., a polar solute in a non-polar solvent), the solute molecules will tend to stick together, and the solvent molecules will also stick together, resulting in poor solubility.
Examples
| Solute | Solvent | Solubility | Explanation |
|---|---|---|---|
| Salt (NaCl) | Water (H₂O) | Soluble | Both are polar. Strong ion-dipole interactions between Na⁺ and Cl⁻ ions and water molecules overcome the lattice energy of the salt crystal. |
| Oil | Water (H₂O) | Insoluble | Oil is non-polar, and water is polar. They do not mix because the weak London dispersion forces between oil molecules are not strong enough to |
| overcome the strong hydrogen bonding between water molecules. | |||
| Sugar | Water (H₂O) | Soluble | Both are polar. Sugar molecules form hydrogen bonds with water molecules. |
| Grease | Hexane | Soluble | Both are non-polar. Dissolution occurs due to similar London dispersion forces. |
Exceptions and Considerations
While "like dissolves like" is a useful guideline, there are exceptions. Factors like temperature, pressure, and the presence of other solutes can also influence solubility. Some molecules may also have both polar and non-polar regions, allowing them to dissolve to some extent in both types of solvents (these are called amphipathic molecules, like soaps and detergents).
