Polarity significantly affects water solubility following the principle of "like dissolves like"; polar substances tend to dissolve in water, while nonpolar substances do not.
Water (H₂O) is a polar solvent. This means that the water molecule has a slightly positive charge on one side (the hydrogen atoms) and a slightly negative charge on the other side (the oxygen atom) due to the unequal sharing of electrons. These partial charges allow water molecules to interact strongly with other molecules that also have charges or partial charges.
The "Like Dissolves Like" Principle
The fundamental rule governing solubility based on polarity is known as "like dissolves like". This principle states that substances with similar intermolecular forces and polarity will be soluble in one another.
As the reference states, "Polar species are soluble in water, while nonpolar species are soluble in oils and fats. Covalent solubility uses the like-dissolves-like rule. This means that substances with the same type of polarity will be soluble in one another. Moreover, compounds with differing polarities will be insoluble in one another."
Why Polar Substances Dissolve in Water
Because water is polar, it is an excellent solvent for other polar substances and ionic compounds. Polar solute molecules or ions have charges that can form attractive interactions with the partial positive and negative charges of the water molecules. These interactions, such as hydrogen bonds and ion-dipole forces, are strong enough to overcome the forces holding the solute particles together and the forces holding the water molecules together, allowing the solute to disperse throughout the water.
- Examples:
- Table salt (sodium chloride, NaCl) is an ionic compound that dissolves readily in water because the positively charged sodium ions (Na⁺) and negatively charged chloride ions (Cl⁻) are strongly attracted to the polar water molecules.
- Sugar (sucrose) is a covalent compound but has many polar O-H bonds, making the molecule polar overall. These polar parts of the sugar molecule can form hydrogen bonds with water molecules, allowing sugar to dissolve.
- Ethanol (alcohol) is another polar molecule that mixes completely with water due to hydrogen bonding.
Why Nonpolar Substances Do Not Dissolve in Water
Nonpolar substances, such as oils or fats, do not have significant charges or partial charges. They primarily interact through weaker forces like London dispersion forces. When a nonpolar substance is mixed with water, the water molecules' strong attractions to each other are much greater than their weak attractions to the nonpolar molecules. The water molecules effectively push the nonpolar molecules away, causing the nonpolar substance to separate from the water and remain undissolved.
- Examples:
- Cooking oil is nonpolar and does not mix with water; it forms a separate layer on top because the water molecules prefer to interact with each other.
- Gasoline (petrol) is a mixture of nonpolar hydrocarbons and is insoluble in water.
- Fats and waxes are nonpolar and repel water, which is why water beads up on waxy surfaces.
Solubility Summary
| Substance Type | Polarity | Solubility in Water | Reason |
|---|---|---|---|
| Polar | Polar | Soluble | Like dissolves like; favorable interactions |
| Ionic | Highly Polar | Soluble | Strong ion-dipole interactions |
| Nonpolar | Nonpolar | Insoluble (or poorly soluble) | Water molecules prefer self-interaction |
In summary, the polarity of a substance dictates its ability to interact with the polar water molecules. Polar and ionic substances can form strong attractions with water, leading to solubility, while nonpolar substances cannot, resulting in insolubility.
