Intermolecular forces are the weak attractions that exist between molecules and play a major role in determining the physical properties of substances, such as their boiling points, melting points, and solubility in water or other solvents. Unlike chemical bonds, which hold atoms together within a molecule, intermolecular forces act between molecules. Understanding these forces is important because they help explain why water is liquid at room temperature, why alcohol evaporates quickly, and why certain substances dissolve in some liquids but not others.
Meaning of Intermolecular Forces
Intermolecular forces are forces of attraction between molecules. They are weaker than the chemical bonds that hold atoms together within a molecule, but they are strong enough to affect how molecules behave in the real world. For example, the reason water molecules stick together in droplets or why ice floats on water is because of intermolecular forces at work.
In simple terms, intermolecular forces are like tiny “sticky hands” that pull molecules toward each other without forming new chemical bonds.
Types of Intermolecular Forces
1. London Dispersion Forces
London dispersion forces are the weakest type of intermolecular forces and occur in all molecules, whether polar or nonpolar. They are caused by temporary fluctuations in electron distribution that create temporary dipoles.
- Example: Helium atoms in a balloon attract each other very weakly due to dispersion forces.
- Larger molecules have stronger London dispersion forces, which is why bigger nonpolar molecules often have higher boiling points than smaller ones.
2. Dipole–Dipole Forces
Dipole–dipole forces occur in polar molecules where one end of the molecule has a slight positive charge and the other end has a slight negative charge. The positive end of one molecule attracts the negative end of another.
- Example: Hydrogen chloride (HCl) molecules attract each other because of dipole–dipole forces.
- Substances with dipole–dipole forces usually have higher boiling points than nonpolar substances of similar size.
3. Hydrogen Bonding
Hydrogen bonding is a special type of dipole–dipole force that happens when hydrogen is attached to very electronegative atoms like oxygen, nitrogen, or fluorine. The hydrogen atom is attracted to a lone pair of electrons on a nearby molecule.
- Example: Water molecules form hydrogen bonds, which explains why water has a high boiling point compared to other small molecules like H₂S.
- Hydrogen bonding also explains why ice floats — the bonds create an open, less dense structure.
4. Ion–Dipole Forces
Ion–dipole forces occur when ions interact with polar molecules. These forces are especially important in solutions, explaining why salts dissolve in water but not in nonpolar liquids.
- Example: Sodium chloride (NaCl) dissolves in water because Na⁺ and Cl⁻ ions are attracted to water molecules.
Comparison of Intermolecular Forces
| Type of Force | Molecules Involved | Relative Strength | Example |
|---|---|---|---|
| London Dispersion | All molecules | Weakest | Helium atoms |
| Dipole–Dipole | Polar molecules | Moderate | HCl molecules |
| Hydrogen Bonding | H with O, N, or F | Strong | Water molecules |
| Ion–Dipole | Ions + polar molecules | Strongest | Na⁺ and water molecules |
Intermolecular Forces vs Intramolecular Forces
Intermolecular forces and intramolecular forces are two types of forces that affect molecules, but they work in very different ways. While intermolecular forces are the weak attractions between molecules that control physical properties like boiling point and solubility, intramolecular forces are the strong bonds within a molecule that hold atoms together and determine the chemical identity of a substance.
| Feature | Intermolecular Forces | Intramolecular Forces |
|---|---|---|
| Act Between | Molecules | Atoms within a molecule |
| Strength | Weak | Strong |
| Examples | Hydrogen bonding | Covalent and ionic bonds |
| Main Role | Control physical properties | Determine chemical identity |
Importance of Intermolecular Forces
Intermolecular forces are important because they explain many physical properties of substances:
- Boiling and Melting Points – Stronger intermolecular forces require more heat to separate molecules.
- States of Matter – Forces determine whether a substance is solid, liquid, or gas at room temperature.
- Solubility – Substances dissolve best in solvents with similar intermolecular forces (“like dissolves like”).
- Viscosity and Surface Tension – Liquids with strong forces flow slower and form droplets more easily.
- Everyday Applications – Intermolecular forces explain why alcohol evaporates faster than water, why mercury stays liquid at room temperature, and why ice floats.
Everyday Examples of Intermolecular Forces
- Water forming droplets on a surface → hydrogen bonding
- Salt dissolving in water → ion–dipole forces
- Alcohol evaporating quickly → weak intermolecular forces
- Gas molecules spreading in a room → London dispersion forces
Frequently Asked Questions (FAQs)
- Are intermolecular forces chemical bonds?
No, they are weaker attractions between molecules, not bonds within molecules. - Which intermolecular force is the strongest?
Ion–dipole forces are the strongest among common intermolecular forces. - Do nonpolar molecules have intermolecular forces?
Yes, all molecules have London dispersion forces. - Why does water have a high boiling point?
Because water molecules form strong hydrogen bonds. - How do intermolecular forces affect solubility?
Molecules dissolve best in solvents with similar types of intermolecular forces. - Can intermolecular forces be broken easily?
Yes, they require less energy to break compared to chemical bonds. - Are intermolecular forces important in real life?
Yes, they explain why liquids behave the way they do, like evaporation, surface tension, and dissolving substances.
Revision Questions and Answers
- Define intermolecular forces.
Weak attractions between molecules. - Name four types of intermolecular forces.
London dispersion, dipole–dipole, hydrogen bonding, ion–dipole. - Which force is present in all molecules?
London dispersion forces. - Why are intermolecular forces important?
They determine physical properties like boiling point, melting point, and solubility. - Give one real-life example of hydrogen bonding.
Water molecules forming droplets on a surface.
STUDY ALSO; What Is Electrochemistry and How Batteries Produce Energy
Intermolecular forces may be weak individually, but together they have a big impact on how substances behave in everyday life. Understanding these forces helps students predict and explain the physical properties of all kinds of chemicals, making it a key topic in high school chemistry.