What is Surface Tension?
Surface tension is the tendency of the surface of a liquid to behave like a stretched elastic membrane.
It occurs because molecules at the surface of a liquid experience different forces from molecules inside the liquid.
Surface tension helps explain phenomena such as water droplets forming nearly spherical shapes, insects walking on water, and liquids rising in narrow tubes.
Why Does Surface Tension Occur?
Molecules inside a liquid are attracted by neighbouring molecules in all directions.
These attractive forces largely balance each other.
However, molecules at the surface do not have liquid molecules above them. They experience a net inward attraction toward the liquid.
This creates tension at the surface.
Cohesive Forces
Cohesion is the attraction between molecules of the same substance.
For example, water molecules attract other water molecules.
Strong cohesive forces contribute to surface tension.
Surface Molecules
Molecules at the surface have a higher potential energy than molecules inside the liquid.
Because liquids tend toward lower-energy states, the surface tends to contract and become as small as possible.
This is why surface tension tends to reduce the surface area of a liquid.
Surface Tension and Surface Area
Surface tension causes a liquid surface to resist an increase in area.
The liquid therefore tends to form shapes with the smallest possible surface area.
This effect is especially noticeable in small droplets.
Why Are Water Droplets Nearly Spherical?
A sphere has the smallest surface area for a given volume.
Because surface tension tends to minimise surface area, small liquid droplets become nearly spherical.
Gravity can distort larger droplets, so they may not remain perfectly spherical.
Surface Tension Force
Surface tension can be described as force acting along the surface of a liquid.
It acts parallel to the surface and tends to pull the surface together.
Surface tension is commonly represented by the symbol γ.
Unit of Surface Tension
The SI unit of surface tension is newton per metre (N/m).
Surface tension may also be described as energy per unit area.
Surface Tension of Water
Water has relatively high surface tension because water molecules attract each other strongly.
This strong attraction is mainly due to hydrogen bonding between water molecules.
As a result, water shows many noticeable surface-tension effects.
Insects Walking on Water
Some small insects, such as water striders, can move across the surface of water.
Their legs spread their weight over a relatively large area and do not easily break the water surface.
Surface tension then helps support them.
Floating of Small Objects
Small objects such as a carefully placed needle may remain on the surface of water even though the material of the object is denser than water.
This is possible when surface tension supports the object without the surface being broken.
This is different from ordinary floating caused mainly by buoyancy.
Surface Tension and Temperature
Surface tension generally decreases as temperature increases.
At higher temperatures, molecules move more rapidly and cohesive forces become less effective at maintaining a tightly bound surface.
Therefore, warm liquids usually have lower surface tension than cold liquids.
Effect of Soap and Detergents
Soap and detergents reduce the surface tension of water.
They weaken the cohesive effect between water molecules at the surface.
This allows water to spread more easily over surfaces and penetrate dirt and fabrics.
Surfactants
A surfactant is a substance that reduces the surface tension of a liquid.
Soap and detergents are common surfactants.
Surfactants are widely used in cleaning products, cosmetics, medicines, paints, and industrial processes.
Adhesion
Adhesion is the attraction between molecules of different substances.
For example, water molecules may be attracted to glass.
Adhesion and cohesion together influence how liquids behave when they come into contact with solid surfaces.
Meniscus
A meniscus is the curved surface of a liquid near the wall of a container.
The shape of the meniscus depends on the balance between adhesive and cohesive forces.
Water forms a concave meniscus in glass because attraction between water and glass is relatively strong.
Mercury forms a convex meniscus because cohesion between mercury atoms is stronger than its attraction to glass.
Capillary Action
Capillary action is the movement of a liquid through a narrow tube or small space.
It occurs because of the combined effects of adhesion, cohesion, and surface tension.
Water can rise in a narrow glass tube because water molecules are attracted to the glass and pull neighbouring water molecules upward.
Factors Affecting Capillary Rise
The height to which a liquid rises in a narrow tube depends on several factors, including:
- surface tension
- density of the liquid
- radius of the tube
- strength of adhesion between the liquid and the tube
Narrower tubes generally produce greater capillary rise.
Surface Tension and Bubbles
Bubbles form because surface tension tends to minimise the surface area of the liquid film.
A soap bubble forms a nearly spherical shape because a sphere gives the smallest surface area for a given volume.
Soap lowers the surface tension enough to allow a thin film to stretch and form a stable bubble.
Surface Tension and Drops
Surface tension causes liquids to form drops.
When a drop grows, gravity eventually becomes strong enough to overcome the surface-tension forces holding it to the surface.
The drop then falls.
Surface Tension in Plants
Capillary action helps water move through very narrow spaces in plants.
However, the transport of water through tall plants involves several effects, including cohesion between water molecules and transpiration from leaves.
Surface tension and cohesion contribute to this process.
Surface Tension in Everyday Life
Surface tension can be observed in many everyday situations.
Examples include:
- water forming droplets on leaves
- soap bubbles
- insects walking on water
- water rising in narrow tubes
- washing with soap and detergents
- drops forming at the end of a tap
- paint and ink spreading over surfaces
Applications of Surface Tension
Surface tension is important in many practical applications.
These include:
- cleaning with soaps and detergents
- painting and coating
- printing and ink technology
- medical sprays and droplets
- laboratory measurements
- manufacture of cosmetics
- control of liquid spreading in industrial processes
Importance of Surface Tension
Surface tension plays an important role in the behaviour of liquids.
It affects droplet formation, wetting, capillary action, bubbles, cleaning, and liquid movement on surfaces.
Understanding surface tension is important in physics, chemistry, biology, engineering, and everyday life.
Key Points
- Surface tension is caused by attractive forces between liquid molecules.
- Molecules at the surface experience a net inward force.
- Surface tension tends to minimise the surface area of a liquid.
- Small droplets are nearly spherical because a sphere has minimum surface area for a given volume.
- Water has relatively high surface tension because of strong intermolecular attraction.
- Surface tension decreases as temperature increases.
- Soap and detergents reduce surface tension.
- Cohesion is attraction between molecules of the same substance.
- Adhesion is attraction between molecules of different substances.
- Capillary action results from adhesion, cohesion, and surface tension.
- Surface tension helps explain bubbles, droplets, meniscus formation, and several everyday phenomena.
Glossary of Key Terms
Glossary of Key Terms in Surface Tension
- Adhesion: The attraction between molecules of different substances.
- Capillary Action: The movement of a liquid through a narrow tube or small space due to adhesion, cohesion, and surface tension.
- Cohesion: The attraction between molecules of the same substance.
- Concave Meniscus: A meniscus that curves downward in the centre, as water does in a glass container.
- Convex Meniscus: A meniscus that curves upward in the centre, as mercury does in a glass container.
- Droplet: A small amount of liquid that forms a rounded shape because of surface tension.
- Hydrogen Bonding: A strong intermolecular attraction between certain molecules, responsible for much of water’s high surface tension.
- Meniscus: The curved surface of a liquid near the wall of a container.
- Molecular Attraction: The force of attraction between molecules.
- Surface Area: The total area of the exposed surface of a liquid.
- Surface Molecule: A molecule located at the surface of a liquid, where the surrounding attractive forces are not balanced equally in all directions.
- Surface Tension: The tendency of a liquid surface to resist an increase in area because of attractive forces between its molecules.
- Surfactant: A substance that reduces the surface tension of a liquid.
- Wetting: The spreading of a liquid over the surface of a solid.
Questions and Answers
Recap the concepts you have learnt. Try to answer the questions. You can find the answer to any question by clicking on the icon.
Surface tension is the tendency of the surface of a liquid to behave like a stretched elastic membrane.
It occurs because molecules at the surface experience unbalanced attractive forces.
They are attracted by neighbouring molecules in all directions, so the forces are largely balanced.
They experience a net inward attraction because there are fewer liquid molecules above them.
Cohesion is the attraction between molecules of the same substance.How does cohesion affect surface tension?
Stronger cohesive forces generally produce greater surface tension.Why do liquids tend to minimise surface area?
Because surface molecules have higher potential energy, and the liquid tends toward a lower-energy state.Why are small water droplets nearly spherical?
Because a sphere has the smallest surface area for a given volume.What happens to large droplets?
Gravity can distort their shape, so they may not remain perfectly spherical.In which direction does surface tension force act?
It acts parallel to the liquid surface.What symbol is commonly used for surface tension?
Surface tension is commonly represented by the symbol γ.What is the SI unit of surface tension?
The SI unit is newton per metre, written as N/m.Why does water have relatively high surface tension?
Because water molecules strongly attract one another, mainly due to hydrogen bonding.How can insects walk on water?
Their legs spread their weight over the surface without easily breaking it, so surface tension helps support them.How can a needle float on water even though it is denser than water?
If placed carefully, surface tension can support the needle without the water surface breaking.Is the floating of a needle the same as ordinary buoyancy?
No. Surface tension plays the main role in supporting the needle.How does temperature affect surface tension?
Surface tension generally decreases as temperature increases.Why does surface tension decrease when temperature rises?
Molecules move more rapidly, making cohesive forces less effective at maintaining a tightly bound surface.How do soap and detergents affect surface tension?
They reduce the surface tension of water.Why is reduced surface tension useful in cleaning?
It allows water to spread more easily and penetrate dirt and fabrics.What is a surfactant?
A surfactant is a substance that reduces the surface tension of a liquid.Give two examples of surfactants.
Soap and detergents are common surfactants.What is adhesion?
Adhesion is the attraction between molecules of different substances.How is adhesion different from cohesion?
Cohesion acts between molecules of the same substance, while adhesion acts between molecules of different substances.What is a meniscus?
A meniscus is the curved surface of a liquid near the wall of a container.Why does water form a concave meniscus in glass?
Because attraction between water and glass is strong enough to pull water upward along the sides.Why does mercury form a convex meniscus in glass?
Because cohesion between mercury atoms is stronger than its adhesion to glass.What is capillary action?
Capillary action is the movement of a liquid through a narrow tube or small space.What causes capillary action?
It results from the combined effects of adhesion, cohesion, and surface tension.Why does water rise in a narrow glass tube?
Water adheres to the glass and pulls neighbouring water molecules upward.How does tube radius affect capillary rise?
A narrower tube generally produces a greater capillary rise.What factors affect capillary rise?
Surface tension, liquid density, tube radius, and adhesion between the liquid and tube affect capillary rise.Why are soap bubbles nearly spherical?
Surface tension tends to minimise the area of the liquid film, producing a nearly spherical shape.Why is soap useful for making bubbles?
Soap lowers surface tension enough to allow a thin liquid film to stretch and remain stable.Why do drops form at the end of a tap?
Surface tension holds the liquid together until gravity becomes strong enough to pull the drop away.How is surface tension involved in plants?
Surface tension, cohesion, and capillary effects contribute to the movement of water through narrow spaces in plants.Give two everyday examples of surface tension.
Water droplets on leaves and soap bubbles are two common examples.Give two applications of surface tension.
Cleaning with detergents and controlling paints or coatings are two applications.Why is surface tension important in printing and painting?
It affects how liquids spread, wet surfaces, and form uniform coatings.Why is surface tension important in science and everyday life?
It helps explain droplets, bubbles, wetting, capillary action, liquid spreading, cleaning, and many natural and industrial processes.