Surface Tension and Viscosity — Revision Notes

⚡ 30-Second Revision

Surface Tension ($\gamma$) — Force per unit length (N/m) at liquid surface, minimizing area. Caused by unbalanced inward IMFs.
Viscosity ($\eta$) — Resistance to flow (Pa\cdot s), internal friction. Caused by IMFs resisting layer movement.
Temperature Effect

- Liquids: $\gamma \downarrow$, $\eta \downarrow$ as T $\uparrow$. - Gases: $\eta \uparrow$ as T $\uparrow$.

IMFs — Stronger IMFs $\implies$ higher $\gamma$, higher $\eta$.
Surfactants — Decrease $\gamma$ (e.g., detergents).
Capillary Action — Rise if adhesive > cohesive; fall if cohesive > adhesive.

2-Minute Revision

Surface tension ($\gamma$) is the property of a liquid surface to contract and minimize its area, acting like a stretched elastic film. It arises from the net inward pull on surface molecules due to unbalanced intermolecular forces (IMFs).

Stronger IMFs lead to higher surface tension. Temperature increase generally decreases surface tension for liquids. Surfactants like detergents significantly reduce surface tension, aiding in wetting and cleaning.

Viscosity ($\eta$) is a fluid's resistance to flow, essentially its internal friction. It also depends on IMFs; stronger IMFs result in higher viscosity. For liquids, viscosity decreases with increasing temperature, as molecules gain kinetic energy to overcome IMFs.

However, for gases, viscosity increases with temperature due to more frequent and energetic molecular collisions. Key phenomena include spherical drops (due to surface tension minimizing area) and capillary action (interplay of cohesive, adhesive forces, and surface tension).

Remember the units: N/m for surface tension, Pa\cdot s or Poise for viscosity.

5-Minute Revision

Let's quickly review surface tension and viscosity, two critical properties of liquids for NEET. **Surface tension ($\gamma$)** is the force per unit length (N/m) acting on a liquid's surface, causing it to contract and minimize its area.

This occurs because molecules at the surface experience a net inward pull from the bulk liquid, unlike bulk molecules with balanced forces. Stronger intermolecular forces (IMFs), like hydrogen bonding in water, lead to higher surface tension.

For example, water has a high surface tension, allowing insects to walk on it. Increasing temperature generally decreases surface tension because higher kinetic energy weakens IMFs. Adding surfactants (like soap) dramatically reduces surface tension by disrupting IMFs at the surface, which is why detergents help water penetrate fabrics for cleaning.

**Viscosity ($\eta$)** is a fluid's resistance to flow, or its 'internal friction,' measured in Pascal-seconds (Pa\cdot s) or Poise. It arises from the resistance of adjacent fluid layers to slide past each other, again due to IMFs.

Liquids with strong IMFs (e.g., glycerol with extensive hydrogen bonding) are highly viscous. The effect of temperature is crucial: for liquids, viscosity decreases with increasing temperature (e.g.

, warm honey flows faster) because increased kinetic energy helps molecules overcome IMFs. For gases, however, viscosity increases with temperature because faster-moving molecules lead to more frequent and energetic collisions, increasing momentum transfer.

Remember that viscosity also depends on molecular size and shape; larger, more complex molecules tend to increase viscosity.

Key phenomena: Small liquid drops are spherical because surface tension minimizes surface area. Capillary action (liquid rising or falling in a narrow tube) is a balance of cohesive forces (liquid-liquid), adhesive forces (liquid-tube), and surface tension.

If adhesive forces are stronger (e.g., water in glass), the liquid wets the surface and rises. If cohesive forces are stronger (e.g., mercury in glass), it falls. Always link these properties back to the strength and nature of intermolecular forces.

Prelims Revision Notes

Surface Tension ($\gamma$)

Definition — Force per unit length (N/m or dyn/cm) acting perpendicular to a line on the liquid surface, tending to minimize surface area. Also, work done per unit area (J/m$^2$ or erg/cm$^2$).
Origin — Unbalanced net inward attractive intermolecular forces on surface molecules.
Factors Affecting

* Intermolecular Forces: Stronger IMFs $\implies$ higher $\gamma$ (e.g., water due to H-bonding). * Temperature: $\gamma$ decreases as T increases (kinetic energy overcomes IMFs). * Impurities: * Surfactants (detergents): Significantly decrease $\gamma$ by disrupting IMFs at the surface. * Inorganic salts: Often increase $\gamma$.

Phenomena — Spherical drops (minimum surface area for given volume), capillary action, wetting/non-wetting, insect walking on water.

Viscosity ($\eta$)

Definition — Measure of a fluid's resistance to flow; internal friction between adjacent layers moving at different velocities.
Units — SI: Pa\cdot s (or N\cdot s/m$^2$). CGS: Poise (P). $1\,\text{P} = 0.1\,\text{Pa}\cdot\text{s}$. Centipoise (cP) is common.
Origin — Intermolecular forces resisting relative motion between fluid layers.
Factors Affecting

* Intermolecular Forces: Stronger IMFs $\implies$ higher $\eta$ (e.g., glycerol due to extensive H-bonding). * Temperature: * Liquids: $\eta$ decreases as T increases (kinetic energy overcomes IMFs). * Gases: $\eta$ increases as T increases (more frequent/energetic collisions, momentum transfer). * Molecular Size/Shape: Larger, more complex, or elongated molecules $\implies$ higher $\eta$. * Pressure: Slight increase in $\eta$ for liquids with increasing pressure.

Types of Flow — Laminar (smooth), Turbulent (chaotic).

Key Distinctions & Relationships

Both $\gamma$ and $\eta$ are directly related to the strength of IMFs.
Temperature has opposite effects on liquid vs. gas viscosity.
Capillary action involves both surface tension and the balance of cohesive/adhesive forces.

Vyyuha Quick Recall

To remember the effect of temperature on viscosity for liquids and gases: 'Liq-Down, Gas-Up'.

Liq-Down — For Liquids, viscosity goes Down as temperature goes up.
Gas-Up — For Gases, viscosity goes Up as temperature goes up.

This helps recall the opposite trends quickly.