DAT General Chemistry: Viscosity, Surface Tension, and Capillary Action

DAT High-Yield Concept

Viscosity increases with stronger intermolecular forces (IMFs) and structural complexity, but decreases as temperature increases.
Surface Tension results from unbalanced cohesive forces pulling bulk liquid molecules inward to minimize surface area; it decreases at higher temperatures or upon adding surfactants.
Cohesion vs. Adhesion: Cohesion is attraction between identical liquid molecules; adhesion is attraction between liquid molecules and a solid surface.
Meniscus & Capillary Action: Stronger adhesion than cohesion creates a concave meniscus and upward capillary rise (e.g., $\ce{H2O}$ in glass). Stronger cohesion produces a convex meniscus and capillary depression (e.g., $\ce{Hg}$ in glass).

Viscosity

Viscosity ( $\eta$ ) is a liquid's internal resistance to flow. Fluid movement requires liquid molecules to slide past one another. The magnitude of viscosity depends on three key factors:

Intermolecular Forces (IMFs): Stronger cohesive IMFs hinder molecules from sliding past each other, increasing viscosity. (Relative IMF strengths are covered in depth in their own lesson.)
Molecular Size and Geometry: Elongated, highly branched, or flexible molecules entangle easily, causing high viscosity compared to small, compact molecules.
Temperature: Higher temperatures increase kinetic energy ( $E_k$ ), disrupting IMFs and causing viscosity to decrease dramatically.

Key Trend: Viscosity increases with stronger IMFs and higher structural entanglement, but decreases with increasing temperature.

Worked Example: Viscosity Ranking and Assignment

Problem: Rank the following liquids in order of increasing viscosity at $\pu{25 degC}$ : hexane ( $\ce{C6H14}$ ), ethanol ( $\ce{CH3CH2OH}$ ), and glycerol ( $\ce{HOCH2CH(OH)CH2OH}$ ). Predict what happens to glycerol's viscosity when heated to $\pu{80 degC}$ .

Step-by-Step Solution:

Identify IMFs: $\ce{C6H14}$ exhibits only weak London dispersion forces. $\ce{CH3CH2OH}$ has one $\ce{-OH}$ group capable of hydrogen bonding. Glycerol ( $\ce{HOCH2CH(OH)CH2OH}$ ) contains three $\ce{-OH}$ groups, forming an extensive hydrogen-bonding network with significant molecular entanglement.
Rank viscosity at $\pu{25 degC}$ : $\ce{C6H14}$ < $\ce{CH3CH2OH}$ < $\ce{HOCH2CH(OH)CH2OH}$ .
Temperature effect: Heating glycerol to $\pu{80 degC}$ increases molecular kinetic energy, breaking IMFs and decreasing viscosity.

DAT Exam Trap: Do not confuse viscosity with density! Dense liquids are not automatically viscous; for instance, heavy motor oil floats on water ( $\ce{H2O}$ ) despite being far more viscous.

Surface Tension

Surface tension ( $\gamma$ ) is the energy required to increase a liquid's surface area by a unit amount (units of $\pu{J/m^2}$ or $\pu{N/m}$ ). Molecules inside the bulk liquid experience symmetric cohesive forces from surrounding neighbors. Surface molecules, however, lack liquid neighbors above them, creating a net inward cohesive pull toward the bulk. This inward force shrinks the liquid surface area to its minimum possible state, causing the surface to behave like a stretched membrane.

Surface tension varies based on:

IMF Strength: Liquids with strong cohesive forces—such as hydrogen bonding in $\ce{H2O}$ or metallic bonding in liquid mercury ( $\ce{Hg}$ )—display high surface tension.
Temperature: Thermal energy disrupts surface cohesive interactions, causing surface tension to drop as temperature rises.
Surfactants: Amphiphilic substances (soaps and detergents) disrupt hydrogen bonds at the liquid-air interface, significantly lowering surface tension.

Worked Example: Surface Tension & Surfactant Action

Problem: A steel needle floats on pure $\ce{H2O}$ at $\pu{20 degC}$ . Explain why it floats despite steel having a higher density than water, and predict what happens when dish soap is added.

Step-by-Step Solution:

Analyze float mechanism: The needle is supported by the elastic surface skin created by strong cohesive hydrogen bonding among surface $\ce{H2O}$ molecules (high surface tension), not by buoyant force.
Predict surfactant effect: Adding dish soap introduces surfactant molecules that disrupt the surface hydrogen-bonding network.
Outcome: Surface tension drops instantly, breaking the surface film and causing the steel needle to sink.

DAT Exam Trap: Objects supported by surface tension are not floating due to density/buoyancy. Buoyancy depends on displaced fluid mass, whereas surface tension relies on cohesive surface forces supporting an un-submerged object.

Cohesion and Adhesion

Interactions between liquids and solid containers depend on the balance of two opposing forces:

Cohesive Forces: Intermolecular attractions between identical molecules within the liquid.
Adhesive Forces: Intermolecular attractions between liquid molecules and the solid container surface (e.g., glass composed of silicon dioxide, $\ce{SiO2}$ ).

The relative magnitude of cohesion versus adhesion determines surface curvature (meniscus shape) and wetting behavior:

Concave Meniscus (Adhesion > Cohesion): Water ( $\ce{H2O}$ ) forms strong adhesive hydrogen bonds with polar $\ce{-OH}$ groups on glass ( $\ce{SiO2}$ ). Because adhesion pulls water up the glass walls faster than cohesion pulls it inward, the surface curves downward in the center (concave meniscus).
Convex Meniscus (Cohesion > Adhesion): Liquid mercury ( $\ce{Hg}$ ) exhibits strong metallic cohesive bonding that far exceeds weak dispersion adhesive forces with glass. To minimize contact with the walls, $\ce{Hg}$ pulls inward, forming an upward central dome (convex meniscus).

Property / Behavior	Water ( $\ce{H2O}$ ) in Glass	Mercury ( $\ce{Hg}$ ) in Glass
Dominant Force	Adhesion > Cohesion	Cohesion > Adhesion
Meniscus Shape	Concave (curved downward)	Convex (curved upward)
Contact Angle ( $\theta$ )	$\theta < 90^\circ$ (wets glass)	$\theta > 90^\circ$ (non-wetting)
Capillary Response	Capillary Rise	Capillary Depression

Capillary Action

Capillary action is the spontaneous movement of a liquid into a narrow tube or porous material without external assistance. It is driven by adhesive forces drawing the liquid along container walls combined with cohesive forces pulling the bulk fluid upward behind it.

When adhesive forces exceed cohesive forces, fluid climbs the tube wall. Cohesion flattens the liquid surface, lifting the bulk column until upward adhesive forces balance the downward force of gravity. The height of capillary rise ( $h$ ) is inversely proportional to the tube radius ( $r$ ): narrower tubes yield higher liquid columns.

Capillary rise: adhesion against cohesion, and the meniscus each produces — A liquid climbs a narrow tube when its attraction to the tube wall, called adhesion, outweighs its attraction to itself, called cohesion; water in glass does this and forms a concave meniscus, rising higher as the bore gets narrower. Mercury behaves the other way: cohesion between mercury atoms is much stronger than adhesion to glass, so the surface bulges upward and the level in the tube sits below the reservoir. The same competition explains why water wets a clean glass surface while mercury beads up on it.

DAT Mnemonic for Meniscus & Capillary Behavior:
ADHESION = ATTACH to walls (creates a Concave curve and Climbs up).
COHESION = CLUMP together (creates a Convex dome and Coils/Depresses down).

Worked Example: Capillary Rise Assignment Skill

Problem: Two glass capillary tubes are placed in a container of liquid water ( $\ce{H2O}$ ): Tube A has an inner radius of $\pu{0.25 mm}$ and Tube B has an inner radius of $\pu{1.0 mm}$ . Which tube exhibits a higher capillary rise, and why?

Step-by-Step Solution:

Identify forces: $\ce{H2O}$ forms strong adhesive hydrogen bonds with glass ( $\ce{SiO2}$ ), so both tubes exhibit capillary rise.
Apply radius relationship: Capillary rise height is inversely proportional to radius ( $h \propto 1/r$ ).
Conclusion: Tube A ( $r = \pu{0.25 mm}$ ) has a smaller radius than Tube B ( $r = \pu{1.0 mm}$ ), so Tube A yields a significantly higher fluid column.

DAT Exam Trap: When reading liquid volume for a concave meniscus, always measure at eye level at the bottom center of the meniscus curve, not at the upper adhesive edges!

DAT Critical Thinking Questions

Biological Application: Plants move water ( $\ce{H2O}$ ) upwards through xylem tubes from roots to leaves without a mechanical pump. How do cohesion, adhesion, and transpiration work together to achieve this capillary rise?
Temperature Effects: Why does warm water ( $\pu{60 degC}$ ) clean oily surfaces more effectively than cold water ( $\pu{10 degC}$ ) when evaluated in terms of surface tension and viscosity?
Container Material Switch: Liquid water ( $\ce{H2O}$ ) forms a concave meniscus in a glass tube ( $\ce{SiO2}$ ). In a nonpolar plastic tube, water forms a convex meniscus. Explain this transition in terms of cohesive vs. adhesive forces.

References

Sources returned by Google Search grounding during lesson verification.

10.2 Properties of Liquids - Chemistry 2e | OpenStax — openstax.org
11.4: Intermolecular Forces in Action- Surface Tension, Viscosity, and Capillary Action - Chemistry LibreTexts — chem.libretexts.org

Viscosity, Surface Tension, and Capillary Action

Summary

Practice Questions

AI Tutor