4.5
Surface Tension
Surface tension is defined as the force per unit length (γ) acting along the surface of a liquid. It arises due to strong intermolecular forces of att…
Surface tension is the tendency of a liquid surface to contract and minimize its surface area.
Inside the bulk of a liquid, molecules are surrounded by other molecules. They experience attractive forces in all directions.
However, at the surface, molecules have fewer neighbors. As a result, they experience unbalanced attractive forces that pull them toward the interior of the liquid.
Because of these unbalanced forces, surface molecules are less stabilized than molecules in the bulk. So, they possess higher energy, known as surface energy.
To reduce this energy, the liquid tends to minimize its surface area and form spherical droplets.
However, the curved surface exerts pressure on the liquid inside. This pressure increases the vapor pressure at the droplet surface, enabling more molecules to escape into the vapor phase.
This effect is described by the Kelvin equation, which relates the vapor pressure of a droplet p to that of the bulk liquid p0. As the radius r decreases, p/p0 increases, causing smaller droplets to evaporate easily.
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Q1: Why do liquid molecules at the surface have higher energy than molecules in the bulk?
Surface molecules experience unbalanced attractive forces because they have fewer neighboring molecules above them compared to below. Bulk molecules are surrounded equally in all directions, so they experience balanced forces. This imbalance leaves surface molecules less stabilized and with excess energy called surface energy, which the liquid minimizes by contracting its surface area.
Q2: How does surface tension cause liquids to form spherical droplets?
Surface tension arises from unbalanced intermolecular forces that pull surface molecules inward. To minimize surface energy, the liquid contracts its surface area into the shape with the smallest possible surface-to-volume ratio: a sphere. This spherical geometry represents the most energetically favorable configuration for the liquid.
Q3: What is the relationship between droplet size and evaporation rate?
The curved surface of a droplet exerts inward pressure that increases vapor pressure at the surface, allowing more molecules to escape. The Kelvin equation shows that as droplet radius decreases, the vapor pressure ratio increases significantly. Consequently, smaller droplets evaporate much more readily than larger ones or bulk liquid.
Q4: How does temperature affect surface tension?
Increasing temperature enhances molecular motion and weakens intermolecular forces of attraction. Since surface tension depends on these attractive forces, higher temperatures cause surface tension to decrease. At the critical temperature, surface tension becomes zero because the distinction between liquid and vapor phases disappears entirely.
Q5: What is the difference between surface tension and interfacial tension?
Surface tension occurs at a liquid-vapor interface, while interfacial tension occurs between two immiscible liquids. Interfacial tension is generally lower because attractive forces between the two liquids partially balance the forces at the interface. Both phenomena arise from unbalanced intermolecular forces at phase boundaries.
Q6: Why does excess pressure develop inside liquid droplets?
The curved surface of a droplet creates a net inward force due to surface tension. This curvature-induced pressure increases with decreasing radius, contributing to droplet instability. The excess pressure inside the droplet is directly related to the surface curvature and the strength of intermolecular forces.
Q7: What role do intermolecular forces play in determining surface tension?
Surface tension arises directly from strong intermolecular forces of attraction, which are fundamental properties studied in thermodynamic properties of ideal solutions. These forces create unbalanced pulling forces on surface molecules, causing them to possess higher energy. The strength of these attractive forces determines how much the liquid surface contracts.