Matter (Kinetic Theory)

1. Particulate Structure: All matter is composed of tiny particles called molecules.
   2. Intermolecular Space: Molecules are separated by intermolecular spaces.
   3. Intermolecular Force: Molecules attract each other via cohesive forces.
   4. Continuous Motion: Molecules are in ceaseless, random motion, possessing kinetic energy.
   5. Thermal Dependence: The kinetic energy of molecules increases with an increase in temperature.

• When water reaches its boiling point ($100^{\circ}\text{C}$), the heat supplied is NOT used to increase the kinetic energy (speed) of the molecules.
• Instead, the supplied heat is absorbed as Latent Heat of Vaporisation to perform work against the strong intermolecular forces of attraction, pulling the water molecules apart into the gaseous state.
• Since temperature is a measure of average molecular kinetic energy, and kinetic energy remains constant during phase change, the temperature stays fixed at $100^{\circ}\text{C}$ until all liquid is vaporised.

• Cohesive Force (Cohesion): The force of attraction between molecules of the SAME substance.
Example: Mercury droplets coalesce into a single spherical bead because cohesive forces between mercury atoms are exceptionally strong.
• Adhesive Force (Adhesion): The force of attraction between molecules of DIFFERENT substances.
Example: Water wets glass because the adhesive attraction between water molecules and glass molecules exceeds the cohesive force between water molecules.

1. Temperature: Evaporation occurs spontaneously at any temperature below the boiling point; Boiling occurs strictly at a fixed temperature (Boiling Point).
   2. Location: Evaporation is strictly a surface phenomenon involving only high-energy surface molecules; Boiling is a bulk phenomenon occurring throughout the entire volume.
   3. Thermal Effect: Evaporation causes cooling of the remaining liquid; Boiling maintains a constant temperature without causing localized cooling.

• Spreading Clothes (Surface Area): Evaporation is a surface phenomenon. Spreading clothes increases the surface area exposed to air, allowing more surface water molecules to escape into the atmosphere simultaneously.
• Windy Day (Wind Speed): Wind rapidly blows away the saturated water vapor layer hovering over the damp fabric, reducing local humidity and accelerating the rate of fresh evaporation.

• Sublimation is the physical process in which a solid changes directly into its vapor state on heating without passing through the intermediate liquid state.
• Examples of Sublimable Substances:
1. Ammonium chloride ($\text{NH}_4\text{Cl}$)
2. Camphor
3. Naphthalene (mothballs)
4. Solid carbon dioxide (Dry Ice)
5. Iodine crystals.

• In gases, the intermolecular spaces between molecules are enormous (often more than $10$ times the molecular diameter). Applying external pressure easily pushes the widely separated molecules closer together.
• In solids and liquids, molecules are already packed closely together with minimal intermolecular space. Strong repulsive forces emerge if they are forced any closer, making them practically incompressible.

• An earthen pot has thousands of microscopic pores on its clay walls.
• Water continuously seeps through these pores to the outer surface.
• The water on the exterior surface evaporates into the dry summer air, absorbing the required latent heat of vaporisation directly from the pot and the remaining water inside.
• As heat is continuously extracted, the temperature of the water inside drops, keeping it refreshingly cool.