Short Answer

Polar molecules dissolve in polar solvents because both have partial positive and negative charges that attract each other. These attractions help the solvent molecules pull the solute molecules apart and surround them, allowing the solute to mix evenly.

This process is summarized by the rule “like dissolves like.” Polar solvents, such as water, can interact strongly with polar solutes through dipole–dipole forces or hydrogen bonding, making the solute dissolve easily.

Detailed Explanation :

Polar Molecules Dissolve in Polar Solvents

The reason polar molecules dissolve in polar solvents is based on the nature of intermolecular forces. A polar molecule has regions of partial positive and partial negative charge due to uneven sharing of electrons. A polar solvent, such as water, also has similar charge separation. When a polar solute is placed in a polar solvent, the opposite charges attract each other, creating strong interactions that allow the solute to separate and disperse throughout the solvent.

This is why substances like sugar, salt, ethanol, and ammonia dissolve well in water, while non-polar substances like oil do not. The key principle behind this behavior is “like dissolves like.” Polar solutes dissolve in polar solvents because the intermolecular forces between them are strong enough to overcome the forces that hold the solute particles together.

1. Nature of Polar Molecules

Polar molecules have a permanent dipole moment. One end of the molecule carries a slight positive charge (δ⁺), and the other carries a slight negative charge (δ⁻). This happens due to differences in electronegativity between atoms.

Examples of polar molecules:

• Water (H₂O)
• Hydrogen chloride (HCl)
• Ethanol (C₂H₅OH)

These molecules interact strongly with other polar molecules because of charge-based attractions.

2. Nature of Polar Solvents

Polar solvents contain molecules with permanent dipoles. Water is the most common polar solvent and has a strong ability to form hydrogen bonds. Other polar solvents include methanol, ethanol, and acetone.

These solvents have molecules that:

• Attract polar solute molecules
• Surround them during dissolution
• Stabilize them in solution

This makes polar solvents excellent at dissolving polar substances.

3. Role of Dipole–Dipole Interactions

Dipole–dipole interactions occur when polar molecules attract each other through their opposite charges.

When a polar solvent interacts with a polar solute:

• The δ⁺ side of the solvent is attracted to the δ⁻ side of the solute.
• The δ⁻ side of the solvent is attracted to the δ⁺ side of the solute.

These attractions help break the solute particles apart and spread them evenly in the solvent.

Because dipole–dipole forces are fairly strong, they allow many polar solutes to dissolve easily.

4. Role of Hydrogen Bonding

Many polar molecules contain hydrogen atoms bonded to electronegative elements like oxygen, nitrogen, or fluorine. Such molecules can form hydrogen bonds. Water forms strong hydrogen bonds with other polar molecules.

When a polar solute is added to a polar solvent:

• Hydrogen bonding occurs between solute and solvent molecules.
• The solvent surrounds the solute particles (solvation).
• The solute becomes stabilized in solution.

This process explains why substances like sugar and alcohol dissolve readily in water.

5. Energetic Considerations

Dissolution occurs when energy released during solute–solvent interactions is enough to overcome:

• The forces holding solute molecules together
• The forces holding solvent molecules together

Polar solvents release a lot of energy when they interact with polar solutes because of strong attractions. This makes the overall dissolution process energetically favorable.

Thus, polar solutes dissolve easily in polar solvents.

6. Solvation and Stabilization

Once the solute molecules are separated, the solvent molecules surround them. This process is called solvation (or hydration in the case of water). Solvation:

• Prevents solute molecules from recombining
• Keeps them evenly dispersed
• Makes the solution stable

The strong interactions between polar solvent and polar solute ensure long-term stability of the solution.

7. Contrast With Non-Polar Solutes

Non-polar molecules lack charge separation. They cannot interact strongly with polar solvents. The polar solvent cannot pull non-polar solute molecules apart, so they do not dissolve.

This is why:

• Oil does not dissolve in water
• Wax does not dissolve in alcohol

Again, the rule “like dissolves like” applies.

Conclusion

Polar molecules dissolve in polar solvents because their opposite charges attract each other. Dipole–dipole interactions, hydrogen bonding, and strong solute–solvent attractions help break apart the solute and stabilize it in solution. This makes the dissolution process easy and energetically favorable. The principle “like dissolves like” explains why polar substances mix well with polar solvents.