Surface tension is the tendency of a liquid surface to act like a stretched skin. In water, this happens because water molecules attract each other strongly through hydrogen bonding. Surface tension helps explain why small insects can stand on water, why droplets become round, and why careful measurements in the lab depend on the shape of a liquid surface.
Capillary action occurs when a liquid rises or falls in a narrow tube or porous material because of forces at the surface. Water climbs a glass capillary tube because adhesion between water and glass pulls water up the walls, while cohesion between water molecules pulls the rest of the liquid column upward. The curved surface in the tube is called a meniscus, and its shape reveals whether adhesion or cohesion is stronger.
Understanding Chemistry: Surface Tension and Capillary Action
Molecules deep inside a liquid are pulled in every direction by nearby molecules. These pulls mostly balance. Molecules at the top have no liquid molecules above them, so they experience a net pull toward the liquid below.
Creating extra surface therefore takes energy. A liquid naturally tries to reduce its exposed area, which is why a free droplet tends toward a sphere.
A sphere encloses the greatest volume for the least surface area. Gravity changes this for larger drops, flattening them because their weight becomes more important than the surface effect.
Surface tension is not a fixed property in every situation. Heating a liquid gives its molecules more kinetic energy, making their attractions less able to hold a tight surface. Warm water therefore has lower surface tension than cold water.
Soap and detergents lower it much more strongly. Their molecules collect at the surface and disrupt some water to water attractions.
This is why soapy water spreads across a surface more easily and why it can help remove oily dirt. It also explains why adding detergent can make a floating needle sink, even though the needle itself has not become heavier.
Capillary rise stops when the upward effect at the tube wall is balanced by the weight of the lifted liquid. The rise depends on surface tension, the contact angle, liquid density, gravity, and tube radius. The contact angle describes how the liquid edge meets a solid surface.
A small contact angle means the liquid wets the surface well and rises more readily. In the height equation, height equals two times surface tension times the cosine of the contact angle, divided by density times gravitational acceleration times tube radius.
This relationship shows why a very narrow space matters so much. It also shows why a dense liquid, such as mercury, behaves differently from water in glass.
Students meet these ideas in plant stems, paper towels, fountain pen feeds, chromatography paper, and medical test strips. In soil, tiny gaps between particles can hold water against gravity. In plants, capillary effects help water enter narrow spaces, but they cannot by themselves lift water to the top of a tall tree.
Water loss from leaves creates an additional pulling effect through the plant. In laboratory work, notice the meniscus at eye level to avoid reading errors.
For clear water in a glass measuring cylinder, read the bottom of the curve. Keep glassware clean because grease, soap residue, or dust can change wetting behavior and alter results.
Key Facts
- Surface tension is caused by cohesive forces between molecules at a liquid surface.
- For water at room temperature, surface tension is about 0.072 N/m.
- Capillary action depends on both adhesion between liquid and surface and cohesion within the liquid.
- Capillary rise can be estimated by h = 2γ cosθ / (ρgr), where h is height, γ is surface tension, θ is contact angle, ρ is density, g is gravitational acceleration, and r is tube radius.
- Water in clean glass forms a concave meniscus because adhesion to glass is stronger than cohesion within water at the edge.
- A narrower capillary tube gives a greater rise because h is inversely proportional to r.
Vocabulary
- Surface tension
- Surface tension is the energy or force effect that makes the surface of a liquid resist being stretched or broken.
- Cohesion
- Cohesion is the attraction between molecules of the same substance, such as water molecules attracting other water molecules.
- Adhesion
- Adhesion is the attraction between molecules of different substances, such as water molecules attracting glass.
- Capillary action
- Capillary action is the movement of a liquid through a narrow space due to adhesion, cohesion, and surface tension.
- Meniscus
- A meniscus is the curved surface of a liquid where it touches the wall of a container or tube.
Common Mistakes to Avoid
- Saying capillary action is caused only by surface tension is wrong because the rise also requires adhesion to the surface and cohesion within the liquid.
- Confusing cohesion with adhesion is wrong because cohesion acts between like molecules, while adhesion acts between different materials.
- Thinking wider tubes always make water rise higher is wrong because the capillary rise equation shows h is inversely proportional to tube radius.
- Reading the top of a concave meniscus is wrong in most water measurements because the correct volume reading is taken at the bottom of the meniscus at eye level.
Practice Questions
- 1 A clean glass capillary tube has radius 0.50 mm. Estimate the height water rises using γ = 0.072 N/m, θ = 0 degrees, ρ = 1000 kg/m^3, and g = 9.8 m/s^2.
- 2 Two capillary tubes are placed in water. Tube A has radius 0.25 mm and tube B has radius 1.00 mm. If water rises 5.9 cm in tube A, how high should it rise in tube B, assuming all other factors are the same?
- 3 Water forms a concave meniscus in glass, but mercury forms a convex meniscus in glass. Explain what this shows about the relative strengths of adhesion and cohesion in each case.