Properties of Liquids

Core Concept

The macroscopic behavior of liquids — how thick they are, how they bead or spread, how they climb a thin tube — is a direct readout of intermolecular forces at the molecular scale. Strong IMFs hold molecules tightly to one another, making a liquid resist flow (high viscosity) and resist having its surface stretched (high surface tension). Liquids also interact with the walls of containers (adhesion) as well as with themselves (cohesion), and the balance of these two determines capillary rise and the shape of the meniscus.

Key Ideas

Viscosity

• Resistance of a liquid to flow. High viscosity = thick, slow (honey, glycerol); low viscosity = runny (water, gasoline).
• Increases with stronger IMFs and with larger, more entangled molecules; decreases as temperature rises (added kinetic energy lets molecules slip past each other).

Surface tension

• Energy required to increase a liquid's surface area (units: J/m² or N/m).
• Interior molecules are pulled equally in all directions; surface molecules feel a net inward pull, which tightens the surface into a minimal-area film.
• Increases with IMF strength. Water: ~72 mN/m at 25 °C; mercury: ~486 mN/m.

Cohesion vs. adhesion

• Cohesion: attraction between like molecules (water–water).
• Adhesion: attraction between unlike molecules (water–glass).
• Capillary action: a liquid rises (or falls) in a narrow tube when adhesion (or its absence) competes with cohesion and gravity.

Meniscus

• Concave meniscus (water in glass): adhesion to glass > cohesion, so liquid climbs the walls.
• Convex meniscus (mercury in glass): cohesion > adhesion, so the liquid beads away from the walls.

Equations and Variables

• Surface tension (definitional): γ = F/L, where γ = surface tension, F = force acting along a line of length L on the surface. Units: N/m (= J/m²).
• Capillary rise (Jurin's law): h = (2γ cosθ)/(ρ g r), where h = height of rise, γ = surface tension, θ = contact angle, ρ = liquid density, g = gravitational acceleration, r = tube radius. A smaller r gives a higher rise.
• Viscosity has no single simple equation at this level; it is defined through Newton's law η = τ/(dv/dy) (η = viscosity, τ = shear stress, dv/dy = velocity gradient), but qualitative IMF reasoning is what is tested.

How It Works

1. Molecules pull on each other through IMFs in all directions.
2. At the surface, the pull is unbalanced — there are no liquid molecules above — so surface molecules are drawn inward, creating a "skin" (surface tension).
3. When a liquid must flow, layers slide past one another; strong IMFs (or big tangled molecules) resist this sliding → high viscosity.
4. At a container wall, liquid molecules are attracted both to each other (cohesion) and to the wall (adhesion).
5. If adhesion wins (water/glass), the liquid climbs the wall → concave meniscus and capillary rise.
6. If cohesion wins (mercury/glass), the liquid pulls into itself → convex meniscus and capillary depression.

Worked Example

Water wets clean glass and climbs a capillary, while mercury beads and dips. Explain both observations in terms of IMFs.

1. Water: water is polar with strong H-bonding, so it hydrogen-bonds to the polar Si–O surface of glass (adhesion) about as strongly as to itself (cohesion). Because adhesion to glass exceeds cohesion, water spreads and rises in a capillary, forming a concave meniscus.
2. Mercury: mercury atoms are held together by strong metallic cohesion, but they interact only weakly with the polar glass surface (poor adhesion). Cohesion dominates, so mercury minimizes its contact with glass, beads up, and is depressed in a capillary, forming a convex meniscus.
3. Ranking of surface tension follows IMF strength: mercury (metallic, very strong cohesion) ≈ 486 mN/m > water (H-bonding) ≈ 72 mN/m. This is why a steel needle can be floated on water but not on most organic solvents.

How it works

1. Molecules pull on each other through IMFs in all directions.
2. At the surface, the pull is unbalanced — there are no liquid molecules above — so surface molecules are drawn inward, creating a "skin" (surface tension).
3. When a liquid must flow, layers slide past one another; strong IMFs (or big tangled molecules) resist this sliding → high viscosity.
4. At a container wall, liquid molecules are attracted both to each other (cohesion) and to the wall (adhesion).
5. If adhesion wins (water/glass), the liquid climbs the wall → concave meniscus and capillary rise.
6. If cohesion wins (mercury/glass), the liquid pulls into itself → convex meniscus and capillary depression.

Common confusions

• "Surface tension is the same thing as viscosity." — Wrong. Viscosity is resistance to flow (a bulk property); surface tension is an energy per area at the surface. A liquid can have high viscosity but low surface tension and vice versa.
• "Hotter liquids are more viscous." — Wrong. Viscosity decreases with temperature because molecules gain energy to overcome IMFs.
• "Capillary action only moves water upward." — Wrong. Liquids that do not wet the tube (mercury) are depressed below the surrounding level.
• "A convex meniscus means the liquid has no IMFs." — Wrong. Mercury's convex meniscus reflects stronger cohesion than adhesion, not weak IMFs overall.
• "Only polar liquids have surface tension." — Wrong. All liquids have surface tension (even nonpolar ones, from dispersion forces); polar/strong-IMF liquids just have more of it.

Quick review

• Viscosity = resistance to flow; surface tension = energy per area at the surface.
• Both rise with IMF strength and fall with temperature.
• Cohesion (like–like) vs. adhesion (unlike–unlike) set the meniscus and capillary behavior.
• Water/glass = concave + capillary rise; mercury/glass = convex + depression.
• Water ≈ 72 mN/m; mercury ≈ 486 mN/m.