Solubility & Solution Principles – Comprehensive Study Notes

Common Non-Polar Solvents

• Characteristic: do not dissolve appreciably in water → dominated by London (dispersion) forces, very low (or zero) permanent dipole moment.
• Typical laboratory / industrial examples
  • Toluene $(\text{C}<em>7\text{H}</em>8)$  – component of paint thinner
  • Hexane $(\text{C}<em>6\text{H}</em>{14})$  – grease-cutting solvent in organic labs
  • Carbon tetrachloride $(\text{CCl}_4)$  – historical “dry-cleaning agent” (now restricted for toxicity)
  • “Oil” (mixture of long-chain hydrocarbons)
  • Gasoline-range hydrocarbons (general term: non-polar “gas solvent”)
• Practical identification tip: “If it doesn’t dissolve in water, treat it as non-polar.”

Solids Dissolved in Liquids

• Fundamental rule: “Like dissolves like.” Intermolecular forces (IMF) of solute and solvent must be compatible.
  • Polar / ionic solids dissolve in polar solvents (e.g.
  • Table sugar $(\text{C}<em>{12}\text{H}</em>{22}\text{O}<em>{11}) + \text{H}</em>2\text{O}$
  • Sodium chloride $(\text{NaCl}) + \text{H}_2\text{O}$)
  • Non-polar solids dissolve in non-polar solvents (e.g. grease + toluene)
• All three primary IMFs are in play (London, dipole-dipole, H-bonding), but matching polarity is decisive.

Techniques to Speed Up Dissolving (Solid ⟶ Liquid)

• STIRRING / AGITATION
  • “Mechanical mixing” increases collision frequency between solute particles and solvent molecules.
  • Faster distribution of high-concentration boundary layer.
• HEATING
  • Raising $T$ increases average kinetic energy → more frequent & higher-energy collisions.
  • Most solids become more soluble as $T$ rises (endothermic dissolution) – though exceptions exist.
• GRINDING / POWDERING
  • Smaller particle size ⇒ larger total surface area exposed to solvent.
  • Provides “more places” for solvent molecules to attack the lattice.

Fundamental Vocabulary for Solutions

• Solution: Homogeneous mixture at molecular level.
• Solute: Component being dissolved (typically present in smaller amount).
• Solvent: Component doing the dissolving (larger amount; defines the phase).

Common Phase Combinations

• Gas in Gas – air: $\text{O}<em>2$ dissolved in $\text{N}</em>2$
• Gas in Liquid – carbonated water: $\text{CO}<em>2$ in $\text{H}</em>2\text{O}$
• Liquid in Gas – fog: minute $\text{H}_2\text{O}$ droplets in air
• Liquid in Liquid – ethanol in water (miscible)
• Solid in Liquid – seawater: $\text{NaCl}$ in $\text{H}_2\text{O}$
• Solid in Solid – metal alloys: dental amalgam $\text{Hg}$ in $\text{Ag}$ matrix

Gas Solubility in Liquids

• Temperature effect
  • As $T \uparrow$, gas solubility $\downarrow$ (gases escape more readily).
  • Everyday observation: cold soda keeps its fizz longer; warm soda goes flat quickly.
• Pressure effect (Henry’s Law)
  • $P \uparrow \;\Rightarrow\;$ gas solubility $\uparrow$ proportionally.
  • Bottled soda sealed at high $P$; opening reduces $P$, bubbles emerge.
  • Expressed mathematically: $C = k<em>H P$ where $C$ is concentration, $P$ partial pressure, $k</em>H$ Henry constant.

Liquid-in-Liquid Solubility: “Like Dissolves Like”

• Dictated by IMFs:
  • Polar with Polar (capable of dipole-dipole / H-bonding)
  • Water $\text{H}_2\text{O}$
  • Ethanol $\text{CH}<em>3\text{CH}</em>2\text{OH}$ (forms 1:1 H-bonds with water)
  • Acetone $\text{CH}<em>3\text{CO}\text{CH}</em>3$ (polar aprotic, miscible with water)
  • Acetic acid $\text{CH}_3\text{COOH}$ (“vinegar” component)
  • Ammonia $\text{NH}_3$
  • Non-polar with Non-polar (London forces dominate)
  • Alkanes, aromatics, CCl$_4$, etc.
• Miscibility continuum: “completely miscible” (ethanol–water) vs. “partially miscible” (acetone–water beyond certain ratios) vs. “immiscible” (hexane–water).

Intermolecular-Force Connections & Significance

• Dispersion (London) forces present in all molecules; dominant in non-polars.
• Dipole-Dipole interactions require permanent molecular dipole.
• Hydrogen bonding (strong dipole interaction) needs $\text{H}$ directly bonded to $\text{N}, \text{O}, \text{F}$.
• Matching IMFs lowers enthalpy of mixing $\Delta H_{mix}$, providing the energetic payoff for dissolution.

Practical / Real-World Implications

• Choice of solvent in paint, dry-cleaning, extraction, pharmaceuticals hinges on polarity compatibility.
• Environmental & safety aspects: non-polar solvents are often volatile organic compounds (VOCs) → health regulations (e.g.
  $\text{CCl}_4$ largely banned).
• Beverage industry exploits gas-solubility vs. temperature & pressure (carbonation, nitrogenated beers).
• Dentistry relies on solid-in-solid solutions (silver amalgam).