Structure 1.1.2 – The Kinetic Molecular Theory and States of Matter

Kinetic Molecular Theory (KMT):

The kinetic molecular theory explains the physical properties of solids, liquids, and gases based on the motion and arrangement of particles (atoms, ions, or molecules). It helps describe the differences in energy, spacing, and particle behavior across different states.

• All matter is made up of tiny particles.
• These particles are in constant motion.
• Temperature is a measure of the average kinetic energy of the particles.
• Intermolecular forces affect the arrangement and behavior of particles.

1. Solid

• Particle arrangement: Tightly packed in a regular, fixed pattern (lattice).
• Movement: Particles vibrate in fixed positions; they do not move freely.
• Forces: Strong intermolecular forces keep particles in place.
• Compressibility: Not compressible – particles are already tightly packed.
• Shape and volume: Fixed shape and fixed volume.
• Kinetic energy: Lowest among the three states.
• State symbol: (s)

2. Liquid

• Particle arrangement: Close together but not in fixed positions.
• Movement: Particles move/slide past each other randomly.
• Forces: Moderate intermolecular forces (weaker than solids, stronger than gases).
• Compressibility: Not easily compressible – particles still relatively close.
• Shape and volume: No fixed shape (takes shape of container) but has fixed volume.
• Kinetic energy: Moderate, more than solids but less than gases.
• State symbol: (l)

3. Gas

• Particle arrangement: Far apart, no fixed arrangement.
• Movement: Rapid, random motion in all directions.
• Forces: Negligible intermolecular forces (almost none).
• Compressibility: Highly compressible – particles are far apart and easily pushed closer.
• Shape and volume: No fixed shape and no fixed volume – expands to fill container.
• Kinetic energy: Highest among the three states.
• State symbol: (g)

Comparison of the Three Main States of Matter:

State Symbols Used in Chemical Equations:

• (s): Solid – particles are tightly packed (e.g., \( \text{NaCl (s)} \))
• (l): Liquid – particles are close but mobile (e.g., \( \text{H}_2\text{O (l)} \))
• (g): Gas – particles are far apart and move rapidly (e.g., \( \text{CO}_2 (g) \))
• (aq): Aqueous – substance dissolved in water (e.g., \( \text{NaCl (aq)} \))

Changes of State:

These are physical changes that occur when a substance changes from one state to another, involving the addition or removal of energy (usually heat).

Melting:

• Solid → Liquid
• Occurs when heat is added.
• Particles gain energy and overcome some of the attractive forces.

Example:

\( \text{H}_2\text{O (s)} \rightarrow \text{H}_2\text{O (l)} \)

Freezing:

• Liquid → Solid
• Occurs when heat is removed.
• Particles lose energy and become locked in a fixed position.

Example:

\( \text{H}_2\text{O (l)} \rightarrow \text{H}_2\text{O (s)} \)

Vaporization:

• Liquid → Gas
• Includes both evaporation and boiling.
• Particles gain enough energy to completely overcome intermolecular forces.

Example:

\( \text{H}_2\text{O (l)} \rightarrow \text{H}_2\text{O (g)} \)

Evaporation:

• A surface phenomenon at temperatures below boiling point.
• Happens slowly and only at the surface.
• Requires less energy than boiling.

Boiling:

• Happens throughout the liquid at a specific boiling point.
• Requires continuous heat input.

Condensation:

• Gas → Liquid
• Occurs when a gas loses heat energy.
• Particles slow down and come closer together.

Example:

\( \text{H}_2\text{O (g)} \rightarrow \text{H}_2\text{O (l)} \)

Sublimation:

• Solid → Gas (direct transition without passing through the liquid phase)
• Requires addition of heat.

 Example:

\( \text{CO}_2 (s) \rightarrow \text{CO}_2 (g) \)

Deposition:

• Gas → Solid (direct transition)
• Requires removal of energy.

Example: Formation of frost from water vapor:

\( \text{H}_2\text{O (g)} \rightarrow \text{H}_2\text{O (s)} \)