Reading time: 4 Minutes

How surface tension helps float a paper clip on water.

BY IKBAL AHMED

---

Activity 1: Paper clip on water

Let’s dive into the fascinating world of surface tension with a simple experiment you can easily try at home!

Materials needed:

1. A bowl or basin filled with water
2. a paper clip
3. a piece of paper (optional)
4. soap (optional)

Procedure:

1. Fill a bowl or basin with water.

2. Carefully place a paper clip flat on the surface of the water. You’ll notice something interesting – the paper clip stays afloat!

3. Now, try placing the paper clip on its side in the water. What happens this time?

4. If you find it tricky, try using a piece of paper under the paper clip to help it float on its side.

5. For an extra twist, add a little soap to the water. Observe any changes in the behaviour of the paper clip.

Understanding the science

Now, let’s unpack the science behind what just happened. Surface tension is what keeps the paper clip afloat when it’s lying flat on the water. It’s like a thin layer of “stretchy” skin on the surface of the water that holds the paper clip up. 

Think of it this way: Have you ever seen insects like water striders gliding effortlessly on the surface of a pond? That’s surface tension in action!

When you place the paper clip on its side, it breaks the surface tension, causing it to sink. But by using a piece of paper underneath, you’re helping the paper clip distribute its weight more evenly, allowing it to float.

Adding soap to the water reduces surface tension, so you’ll notice the paper clip might not float as easily or might sink faster. This happens because soap molecules disrupt the cohesive forces at the water’s surface.

Surface tension is a fascinating property of liquids, especially water, that causes the surface of the liquid to behave like a stretched elastic sheet. Here’s a simple explanation:

What is surface tension?

Surface tension is the force that makes water droplets bead up, allows small insects to walk on water, and makes it possible to slightly overfill a glass without the water spilling. It’s the reason why the surface of water appears to be “tight.”

Why is surface tension only active at the surface?

Inside the liquid (the bulk), every water molecule is surrounded by other molecules. Each molecule experiences equal attractive forces from all directions—up, down, left, right—so these forces cancel each other out. The molecules in the bulk feel balanced and don’t have any special forces acting on them in one direction more than another.

But on the surface, it’s different. The molecules at the surface have fewer neighbours because there’s air above them instead of more water molecules. They experience a stronger inward pull from the molecules below and next to them. This imbalance creates tension along the surface, making it act like a stretched elastic film.

The forces at play: cohesion and adhesion

1. Cohesion: This is the attractive force between molecules of the same substance—in this case, water molecules. Water molecules “like” to stick together due to hydrogen bonding. This cohesive force is what pulls surface molecules inward, creating surface tension.

2. Adhesion: This is the attractive force between molecules of different substances, such as water and air. However, the cohesion between water molecules is stronger than the adhesion between water and air, leading to the tightness at the surface.

Surface tension exists at the interface, not in the bulk

Surface tension is only significant at the interface between two different phases (like liquid and air or two immiscible liquids, like oil and water). That’s because surface tension is all about the imbalance of forces at a boundary. In the bulk of the liquid, molecules are equally surrounded, so there’s no net pulling force in one direction. But at the surface, the imbalance in forces results in this “tension” effect.

Surface tension and liquid interfaces

Surface tension doesn’t exist in a single liquid alone. It only appears at the interface where two different substances meet, such as liquid-air, liquid-liquid (e.g., oil and water), or liquid-solid. The interaction between these substances creates the tension. For example, the water-air interface is what gives rise to water’s surface tension. If you mix two liquids that completely dissolve in each other, like water and alcohol, surface tension reduces significantly because there’s no distinct interface left.

Surface tension is a force that exists because of the imbalance of molecular forces at the surface of a liquid, causing it to behave as if it’s covered by a tight film. It only exists at the boundary between two substances, not within the liquid itself, because it arises from the difference in forces at that boundary.

Try it yourself:

Now it’s your turn to try this experiment at home! All you need is a bowl of water, a paper clip, and, optionally, some soap. Explore the fascinating world of surface tension and see what other objects you can make float on water!