Physical & Chemical Changes; Intensive vs. Extensive Properties

Legend

• $S$ = solid, $L$ = liquid, $G$ = gas
• Phase-change shorthand: $S \rightarrow L$ (melting), $S \rightarrow G$ (sublimation), $L \rightarrow G$ (vaporization/boiling), $G \rightarrow L$ (condensation), $L \rightarrow S$ (freezing), $G \rightarrow S$ (deposition)

Physical Changes – General Ideas

• Physical change = transformation that alters state, size, or shape without changing chemical composition.
• Particle picture: identity of atoms/ions/molecules remains the same; only inter-particle arrangement & energy vary.
• Energy flow controls phase changes:
  • Absorption of heat (endothermic) → particles gain kinetic energy, overcome attractive forces, move “farther” (solid → liquid → gas).
  • Release of heat (exothermic) → kinetic energy decreases, particles come closer (gas → liquid → solid).

Solid

• Particles vibrate about fixed lattice positions; lowest kinetic energy.
• Example pictured: ice (solid $\text{H}_2\text{O}$).
• Rigidity ⇒ definite shape & volume.

Liquid

• Particles still close but slide past one another; intermediate kinetic energy.
• Example pictured: water.
• Has definite volume but assumes shape of container.

Melting (Solid → Liquid)

• Definition: Heat absorbed breaks rigid bonds, allowing particles to move more freely.
• Thermodynamic marker: occurs at melting point $T<em>m$ (ice: $T</em>m = 0\,^{\circ}!\text{C}$ at $1\,\text{atm}$).
• Enthalpy of fusion symbol: $\Delta H<em>{fus}$. For water, $\Delta H</em>{fus} \approx 6.02\,\text{kJ mol}^{-1}$.
• Example: $\text{H}<em>2\text{O}(s) \xrightarrow{\;\;0\,^{\circ}!\text{C}\;\;} \text{H}</em>2\text{O}(l)$.

Sublimation (Solid → Gas)

• Definition: Direct transition from solid to gas without passing through liquid phase.
• Occurs when vapor pressure of solid exceeds atmospheric pressure before melting point is reached.
• Requires heat input; enthalpy of sublimation $\Delta H<em>{sub} = \Delta H</em>{fus} + \Delta H_{vap}$ (Hess’s Law).
• Canonical examples:
  • Dry ice: $\text{CO}<em>2(s) \rightarrow \text{CO}</em>2(g)$ at $−78.5\,^{\circ}!\text{C}$.
  • Naphthalene (moth balls) slowly sublimes at room temperature, giving characteristic odor.
• Practical uses: freeze-drying foods, sublimation printers, theatrical fog.

Chemical Changes

• Definition: Process that forms a new substance with different composition & properties; chemical bonds are broken/formed.
• Involves composition, reactivity, and often energy change (heat/light).
• Typically irreversible by simple physical means.
• Examples given:
  • Burning wood: $\text{C}<em>x\text{H}</em>y\ +\ O<em>2 \rightarrow CO</em>2 + H<em>2O + \text{heat/light}$. • Rusting iron: $4\,Fe + 3\,O</em>2 \rightarrow 2\,Fe<em>2O</em>3$ (iron(III) oxide).
  • Cooking an egg: denaturation & cross-linking of proteins yield new solid matrix; cannot be uncooked.

Properties of Matter

Intensive Properties

• Definition: Independent of the amount of substance.
• Listed examples: density, boiling point, melting point, color, flammability, reactivity, temperature, concentration, luster.
• Illustration: Small gold speck vs. 1 kg gold bar – both are yellow, $T_m \approx 1064\,^{\circ}!\text{C}$, $\rho = 19.32\,\text{g cm}^{-3}$.

Extensive Properties

• Definition: Depend on the amount of substance present.
• Examples from transcript: weight (mass), length, volume, entropy.
• Other typical ones: energy, moles, charge.

Why the Distinction Matters

• State functions in thermodynamics: Some (e.g., internal energy) are extensive; some (e.g., temperature) are intensive.
• Combining substances: intensive props remain constant (if identical matter), extensive props add.
• Scaling-up industrial processes: know which variables change with batch size.

Connections & Significance

• Physical vs chemical change duality foundational to conservation of matter and stoichiometry.
• Phase transitions underpin refrigeration, metallurgy, meteorology.
• Sublimation’s bypass of liquid phase enables purification of heat-sensitive materials.
• Intensive/extensive classification is prerequisite for Gibbs free-energy calculations & material selection.
• Ethical/practical: Proper understanding prevents mislabeling hazards (e.g., subliming dry ice in confined spaces may cause $CO_2$ buildup).