Liquids: Surface Tension, Viscosity, Capillary Action, Vapor Pressure, Boiling Point

Learning Objectives

Describe, measure, and explain the macroscopic properties of liquids:
- \text{viscosity}
- \text{surface tension}
- \text{capillary action}
- \text{vapor pressure}
- \text{boiling point}
- \Delta H_{vap} (molar heat of vaporization)
Perform guided-inquiry experiments that reveal each property (paper-clip flotation, “vanishing water,” etc.).
Relate observable behavior to the type & strength of intermolecular forces (IMF): hydrogen bonding, dipole–dipole, London dispersion.
Appreciate real-life significance in biology, medicine, environmental science, and industry.

Surface Tension – energy cost to create new surface; resistance of the surface to external force.
Viscosity – internal resistance to flow (not illustrated in transcript, but part of objective).
Capillary Action – spontaneous rise or fall of a liquid in a narrow tube (implied link to surface tension & adhesion).
Vapor Pressure – pressure exerted by vapor in equilibrium with its liquid/solid.
Boiling Point – temperature where P{vapor} = P{external}.
Heat of Vaporization – enthalpy required to vaporize 1\,\text{mol} at the boiling point.

Definition: \gamma = \dfrac{\text{energy}}{\text{area}} needed to expand surface; measured in \text{J m}^{-2} or \text{N m}^{-1}.
Cohesive forces pull molecules inward; surface behaves like a stretched elastic membrane.
Water’s high surface tension due to an extensive hydrogen-bonding network.

Bulk interior: molecules experience symmetric IMF in all directions ⇒ net force =0.
Surface layer: molecules lack neighbors above; net inward force shortens area.
Diagrams (Pages 3–5) show surface layer (label “1”) vs. interior; arrows denote stronger inward bonds, especially hydrogen bonds.

Untouched water: paper clip can rest on the surface because \gamma{\text{H}2\text{O}} is high.
Adding dishwashing liquid (surfactant) lowers \gamma ⇒ paper clip sinks.
- Conclusion: surfactants disrupt hydrogen bonding, reduce cohesive tension.

Amphiphilic structure (Fig 3):
- Hydrophilic ionic head (often \text{–SO}_3^-\,\text{Na}^+)
- Hydrophobic tail (long hydrocarbon chain)
In water, tails orient away from H₂O, heads stay in contact ⇒ adsorb at interface, lower \gamma.
Practical outcome: creates “low surface tension” water enabling wetting & cleaning (Page 8 comparison diagram).

Intermolecular force strength ↑ ⇒ \gamma ↑.
Temperature ↑ ⇒ molecules gain KE, more escape to vapor, hydrogen bonds break ⇒ \gamma ↓ (Page 10).

Vaporization (liquid → gas) occurs when surface molecules escape.
At equilibrium: \text{rate}{\text{evap}} = \text{rate}{\text{cond}} ⇒ constant P_{vapor}.
Stronger IMF ⇒ fewer molecules escape ⇒ lower P_{vapor}.
“Vanishing Water” station: Different containers lose water at different rates.
1. Container with larger exposed area or higher T loses more water.
2. Difference caused by faster evaporation (higher vapor pressure).
3. Higher P_{vapor} accelerates mass loss.

Boiling when P{vapor} = P{external}.
Normal Boiling Point: value measured at P_{external}=760\,\text{mm Hg} (1 atm).
Lower external pressure ⇒ liquid boils at lower T (e.g., high altitude cooking); higher pressure ⇒ higher B.P. (pressure cookers).

Definition: heat required to vaporize one mole at its B.P.
Units: \text{kJ mol}^{-1}.
Reflects IMF strength; larger \Delta H_{vap} ⇒ stronger forces.
Data table (Page 15):
- Argon: \Delta H_{vap}=6.3\,\text{kJ mol}^{-1}, BP=-186^\circ\text{C} (weak dispersion).
- Pentane \left(\text{C}5\text{H}{12}\right): 26.5\,\text{kJ mol}^{-1}, BP=36.1^\circ\text{C}.
- Acetone \left(\text{CH}3\text{COCH}3\right): 30.3\,\text{kJ mol}^{-1}, BP=56.5^\circ\text{C} (dipole-dipole).
- Ethanol \left(\text{C}2\text{H}5\text{OH}\right): 39.3\,\text{kJ mol}^{-1}, BP=78.3^\circ\text{C} (hydrogen bonding).
Trend: BP \uparrow as \Delta H_{vap} \uparrow since more energy needed to overcome IMF.

Biology: surface tension enables insects to walk on water; capillary action drives water movement in xylem of plants.
Medicine: pulmonary surfactants reduce alveolar surface tension, preventing lung collapse.
Environment: volatility (linked to vapor pressure) influences pollutant dispersion and atmospheric chemistry.
Industry: detergents, paints, ink-jet printing rely on controlled surface tension; pressure cooking leverages boiling-point elevation.

Paper-clip float test demonstrates cohesive force vs. surfactant disruption.
“Vanishing Water” compares evaporation rates, linking to vapor pressure & temperature.
Suggested extensions: measure capillary rise of various liquids; time viscosities with a falling-ball viscometer.

P{vapor} \propto e^{-\frac{\Delta H{vap}}{RT}} (Clausius–Clapeyron, optional advanced link).
Surface energy change: \Delta E = \gamma \Delta A.
Boiling criterion: P{vapor}(T{boil}) = P_{atm}.

Surface tension, vapor pressure, boiling point, and \Delta H_{vap} are macroscopic manifestations of molecular-level IMF.
Manipulating temperature, pressure, or chemical additives (surfactants) allows control of liquid behavior in technology and nature.
Mastery of these concepts provides predictive power for laboratory work, engineering design, and understanding everyday phenomena.