Intermediate Topic 260 of 1102 States of Matter & Solutions
Surface Tension
The energy required to increase a liquid's surface area, caused by cohesive forces.
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In-depth explanation of Surface Tension
Surface tension is the energy needed to increase a liquid's surface area by one unit, or equivalently the force per unit length acting along the surface. It arises because molecules in the interior are pulled equally in all directions by their neighbours, while molecules at the surface are pulled inward, so the liquid tends to minimise its surface area. Surface tension is measured in newtons per metre, and it is the reason drops are spherical and why some insects can walk on water. Soaps and detergents reduce surface tension by accumulating at the surface. Surface tension is strongest in liquids with strong cohesive forces.
Going deeper: Surface Tension
Key ideas: Units: N/m or J/m2. Water is 72 mN/m at 25 C, and decreases with temperature, reaching zero at the critical point. Pressure inside a small spherical droplet is higher by 2 gamma/r (Young-Laplace). Surfactants lower gamma, in water to about 30 mN/m or less.
Common mistakes and connections: Students confuse surface tension with viscosity, or suppose that surface tension is a skin made of different material. Also, a dense steel needle floating on water relies on this effect and not on buoyancy alone. Greasy surfaces and soap break it. Links to capillary action, cohesion, droplet shapes and detergents.
Visualization of Surface Tension
Surface tension is higher when cohesive forces are stronger; water at 72 mN/m and mercury at 486 mN/m resist surface stretching most. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Water has a surface tension of about 72 mN/m at 25°C, whereas mercury has about 485 mN/m.
Frequently asked questions about Surface Tension
What is Surface Tension?
The energy required to increase a liquid's surface area, caused by cohesive forces.
Is Surface Tension a beginner, intermediate or advanced topic?
Surface Tension is a intermediate topic in States of Matter & Solutions. It builds on the vocabulary of beginner topics in this area.
What should I learn before Surface Tension?
Study these earlier topics in States of Matter & Solutions first: Normal Boiling Point, Clausius-Clapeyron Equation, Viscosity.
What comes after Surface Tension?
The next topics in States of Matter & Solutions are Capillary Action and Cohesion and Adhesion.
Can I practise Surface Tension with quizzes?
Yes. The interactive lesson includes a self-check question on Surface Tension, and the States of Matter & Solutions quiz draws on every topic in this area.
Where Surface Tension fits in States of Matter & Solutions
Surface Tension is topic 41 of 109 in States of Matter & Solutions.