Surface Tension

Liquids have a unique ability called surface tension. Water’s surface tension is stronger than most other liquids which allows it to hold a certain amount of mass without an object falling to the bottom of the container.

All of this is possible due to two phenomena called cohesion and adhesion. Cohesion is the ability for molecules of the same substance to stick together. Water in is able to do this because of hydrogen bonds. Water is made up of 2 hydrogen atoms and 1 oxygen atom. The hydrogen wants to connect with another oxygen, but it is not able to do so without a chemical reaction to change the water into something else. If there was a chemical reaction, there would be creation of light and/or heat or the removal of energy, which would make the product cold. Since the hydrogen cannot be removed from the water molecule, the hydrogen will attract the oxygen in the other water molecules.

Adhesion on the other hand is the ability for molecules of different substances to stick together. This may not be a permeant connection, but it allows for some interesting effects. With water or liquids, this includes capillary action, which is adhesion and cohesion working together. If you pour a liquid into a container you will notice it is not perfectly straight across. There is a curve near the edge that looks like a small hill going up. This is a concave curve that is creating a meniscus. The water wants to stick together but it also wants to stick to the cup. The force to hold cohesion is not as strong as the force needed for adhesion, so it caves inward instead of outward. If you repeat the action with a high enough surface tension liquid (like mercury), you would see a convex meniscus. Since the water molecules want to stay together but also are being pulled by the other compounds in the cup, they can begin to climb the cup. Since the water cannot continuously bind though, it does not move up. In smaller spaces, the water is able to bring more water up and move through the space. This is how trees are able to have roots deep underground and still be able to get water up to the very top leaves.

The force of the cohesion is able to create the surface tension of liquids. The water molecules want to stick together and are pulling each other towards one another. Since there are water molecules all around the other ones the molecule is held in place in the liquid. The molecules on the surface though want to stay with the water below but have no water above to grab on to. The pull of the water on the surface molecules is stronger than the air pulling on the water molecules.

Surface tension though can be broken using other molecules. In particular soap is VERY good at this. Soap molecules have two sides to them. One that is lipophilic and one the is hydrophilic. The hydrophilic ends break off when exposed to the water and pulls the water molecules away from each other just enough to break the surface tension. This is moving the water molecules out of the way and can be harnessed to do work. The lipophilic side comes together with other lipophilic sides to create a bubble that keeps water out and collects oil, dirt, and germs. This is why it is important to sue soap when washing your hands. The water’s surface tension due to cohesion is so great that it does not pull away anything else since it only wants to pull more water.

Key Terms

Surface Tension - the tension of the surface film of a liquid caused by the attraction of the particles in the surface layer by the bulk of the liquid, which tends to minimize surface area.

Cohesion – the sticking together of particles of the same substance

Adhesion – the sticking together of particles of different substances

Capillary Action – he ability of a liquid to flow in narrow spaces without the assistance of, or even in opposition to, external forces like gravity

Lipophilic – tending to combine with or dissolve in lipids or fats

Hydrophilic – having a tendency to mix with, dissolve in, or be wetted by water.

Work - the product of force and displacement

Supplies

Bowl/Pan/Baking Dish
Index Card (or similar size and shape of paper)
Soap
Scissors
Optional materials:
Q-Tip
Coloring Materials*

Step 1:

Cut the index card into the shape of a boat by cutting one end into a triangle and cutting a small rectangle out on the other side. Decorate if desired.

Step 2:

Fill the bowl/pan/baking dish with water.

Step 3:

Put the boat on top of the water. With either your finger or the Q-tip, but a drop of soap in the middle of the cut-out rectangle on the edge touching the water.

Step 4:

Watch as the boat moves around.

Empty and refill the water if the boat has difficulty moving after placing the soap. The soap will remove all surface tension and needs to be replaced.

What is Happening?

The paper boat is floating on the water because it is not as dense as the water. Once the soap is added, the surface tension of the water is broken and pushes the boat as removing water tension creates work.