Boiling Point of Water and Atmospheric Pressure

Liquid-Vapour Equilibrium

• Water and water vapour reach equilibrium at 1.013 bar and 100°C in a closed vessel.
• The boiling point of water at 1.013 bar is 100°C.
• For pure liquids, the temperature at which the liquid and vapour coexist at equilibrium under 1.013 bar pressure is known as the normal boiling point.
• Boiling point varies with atmospheric pressure; it decreases at higher altitudes due to lower pressure.

Solid-Vapour Equilibrium

• When solids sublime, they transition directly to the vapour phase.
• Placing solid iodine in a closed vessel produces violet vapour, the intensity of which increases over time until it stabilizes, indicating equilibrium.
• The sublimation and condensation of iodine can be represented as:
  I₂(solid) ⇌ I₂(vapour)
• Other substances that exhibit solid-vapour equilibrium include:
  • Camphor: Camphor (solid) ⇌ Camphor (vapour)
  • Ammonium chloride (NH₄Cl): NH₄Cl (solid) ⇌ NH₄Cl (vapour)

Equilibrium in Dissolution

Solids in Liquids

• Solubility of substances like salt or sugar in water is limited and varies with temperature.
• A saturated solution occurs when no more solute can dissolve at a specific temperature.
• Dynamic equilibrium exists between dissolved solute and undissolved solid, represented as:
  • Sugar (solution) ⇌ Sugar (solid)
• In saturated solutions, the rate of dissolution equals the rate of crystallization.
• Radioactive sugar experiments demonstrate dynamic equilibrium; radioactivity transfers between solution and solid, reaching a constant ratio of molecules over time.

Gases in Liquids

• The release of carbon dioxide from opened soda water illustrates changes in solubility due to pressure.
• Equilibrium between gaseous CO2 and dissolved CO2 can be expressed as:
  • CO2 (gas) ⇌ CO2 (in solution)
• Henry's law dictates that the mass of gas dissolved is proportional to the gas pressure above the solvent, with higher temperatures decreasing solubility.
• Sealed soda bottles maintain high gas solubility under pressure; opening the bottle allows gas to escape, transitioning to a lower pressure equilibrium, leading to flat soda.

General Observations on Physical Equilibria

• Solid-Liquid Equilibrium: A single melting point exists at 1 atm pressure where solid and liquid phases coexist without heat exchange.
• Liquid-Vapor Equilibrium: Vapor pressure remains constant at a given temperature.
• Dissolution of Solids: Solubility remains constant at specific temperatures.
• Dissolution of Gases: Gas concentration in liquid is directly proportional to the partial pressure of the gas over the liquid.

Summary of Key Processes

• Liquid-Vapor: H2O (l) ⇌ H2O (g), vapor pressure constant at given temperature.
• Solid-Liquid: H2O (s) ⇌ H2O (l), melting point fixed at constant pressure.
• Solute in Solution: Sugar(s) ⇌ Sugar(solution), concentration constant at given temperature for solute in a solution.
• Gas in Liquid: CO2(g) ⇌ CO2(aq), concentration ratios ([gas(aq)]/[gas(g)]) remain constant at given temperature.

This quiz explores the concepts of boiling points, specifically focusing on water and water vapor equilibrium at atmospheric pressure. It highlights how altitude affects the boiling point of liquids and the significance of the normal boiling point. Test your understanding of these fundamental principles in thermodynamics!