States of Matter

Matter exists in three common states -- solid, liquid, and gas. You can see all three just by looking at a glass of ice water: ice (solid), water (liquid), and water vapour above the surface (gas). The behaviour of each state is governed by intermolecular forces and the kinetic energy of particles.

Intermolecular Forces

These are the attractive forces between molecules that determine the physical state of a substance.

Types (in increasing order of strength):

1. London dispersion forces: Weak, temporary forces present in all molecules. Stronger in larger molecules.
2. Dipole-dipole forces: Between polar molecules (e.g., HCl).
3. Hydrogen bonding: A special strong dipole interaction when H is bonded to N, O, or F. Examples: water (H₂O), ammonia (NH₃).

Stronger intermolecular forces = higher boiling point, higher viscosity, lower vapour pressure.

Solid State

• Particles in fixed positions, vibrating in place
• Definite shape and volume
• Incompressible
• Two types: Crystalline (regular arrangement, sharp melting point) and Amorphous (irregular arrangement, no sharp melting point like glass)

Liquid State

• Particles close together but can slide past each other
• Definite volume but no definite shape (takes shape of container)
• Nearly incompressible
• Properties: viscosity, surface tension, capillarity

Surface tension explains why water droplets are spherical and why small insects can walk on water.

Gaseous State

• Particles far apart, moving randomly at high speed
• No definite shape or volume
• Highly compressible

Gas Laws (Chemistry Perspective)

Ideal Gas Equation: PV = nRT (R = 0.0821 L atm / mol K)

At STP (Standard Temperature and Pressure: 0 C, 1 atm): 1 mole of any gas occupies 22.4 litres.

Real gases deviate from ideal behaviour at high pressure and low temperature. The Van der Waals equation accounts for this:

(P + an²/V²)(V - nb) = nRT

Worked Example

Calculate the volume occupied by 2 moles of an ideal gas at STP.

Solution:

• At STP, 1 mol = 22.4 L
• Volume = 2 x 22.4 = 44.8 L

Nepal Connection

At high altitudes in Nepal (like Everest base camp), lower atmospheric pressure means gases expand and water boils at lower temperatures. Understanding states of matter helps explain why cooking takes longer and why climbers need pressurized oxygen.

Key Takeaways

• The three states of matter are determined by intermolecular forces and temperature
• Hydrogen bonding is the strongest common intermolecular force
• At STP, 1 mole of ideal gas = 22.4 L
• Real gases deviate from ideal behaviour at extreme conditions