Matter and its Composition

1. Matter is composed of extremely tiny discrete particles called atoms or molecules.
   2. The constituent particles are in continuous, random motion in all directions, possessing kinetic energy that increases with heating.
   3. There exist empty spaces between particles known as intermolecular spaces.
   4. Particles exert mutual attractive forces called intermolecular forces of attraction (cohesive forces between identical molecules, adhesive forces between different molecules).

• Gases: The constituent molecules are separated by colossal intermolecular spaces. When external pressure is applied, the vast empty spaces allow the molecules to be forced much closer together without structural resistance, making gases highly compressible.
• Solids: The constituent particles are tightly packed in a rigid geometric lattice with virtually zero intermolecular space. Any attempt to compress them encounters powerful electrostatic repulsion between atomic electron clouds, making solids incompressible.

• Definition: Sublimation is the physical process in which a solid, upon heating, transitions directly into the gaseous state without melting into an intermediate liquid phase, and on cooling, deposits directly back as a solid.
• Examples:
1. Camphor
2. Ammonium Chloride ($NH_4Cl$)
3. Iodine ($I_2$)
4. Naphthalene (Mothballs) (also Dry Ice / solid $CO_2$).

• The aroma of food consists of gaseous aromatic molecules released by food ingredients that mix with air via diffusion.
• Diffusion rate is directly proportional to temperature ($KE \propto T$). In hot food, the volatile molecules possess high thermal kinetic energy, moving at blistering velocities of hundreds of meters per second and diffusing rapidly across large distances.
• In cold food, the kinetic energy of particles is low, vapor emission is negligible, and diffusion is very slow.

• The thermal energy supplied to melting ice does NOT increase the average kinetic energy of the water molecules (which would cause a rise in temperature).
• Instead, the entire heat energy supplied—called the Latent Heat of Fusion—is exclusively consumed in doing work against the powerful intermolecular bonds of the crystalline ice lattice, breaking the fixed structure to transition into liquid water.
• Once every crystal of ice has melted into water, further heating begins to raise the temperature above $0^\circ\text{C}$.

• Cohesive Force: The intermolecular force of attraction acting between molecules of the same substance.
Example: Strong cohesive attraction between water molecules causing water droplets to form spherical beads on a waxed surface.
• Adhesive Force: The intermolecular force of attraction acting between molecules of different substances.
Example: Adhesive attraction between water molecules and glass causing water to wet the glass and form a concave meniscus.

• Definite Volume: The intermolecular forces of attraction in liquids are sufficiently strong to keep the molecules bound close together within a fixed overall volume, preventing them from flying off into space.
• No Definite Shape: The intermolecular forces are not strong enough to lock the molecules into fixed positions. The molecules can freely slip and slide past one another, allowing the liquid to flow and take the exact shape of whichever container it occupies.

1. Take a graduated $100\text{ mL}$ measuring cylinder and fill it with exactly $50\text{ mL}$ of pure water.
   2. Add $20\text{ g}$ of granulated sugar (or fine salt) to the water.
   3. Stir the mixture gently with a glass rod until all sugar crystals completely dissolve.
   4. Observe the final volume level: surprisingly, the water level remains practically unchanged at $50\text{ mL}$ instead of rising to $60-70\text{ mL}$!
   • Conclusion: The dissolved sugar molecules broke down into tiny particles that slipped into the empty intermolecular spaces between the water molecules without increasing the overall volume.