Surface Tension, Capillary Action, Viscosity & Physical Changes

Instructor: Julie Zundel

Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.

There are many mysteries in the universe. This lesson will focus on a few that can be easily explained due to intermolecular forces, mainly surface tension, capillary action, viscosity, and physical changes.


Science is happening all around you, even if it's not completely obvious. Some of these phenomena are due to intermolecular forces, or when particles, like atoms and molecules, are attracted to or repelled from each other.

Let's break down some observations in order to see how intermolecular forces make science happen.

Surface Tension

You see that bug walking on water? How in the world is it not falling in?

Water strider walking on water

It's able to walk on water because of surface tension, which means that water molecules on the top of the lake have formed a 'layer' due to intermolecular forces. Intermolecular forces can happen to a variety of substances. Water is made up of two hydrogen atoms and one oxygen atom. The oxygen end of water is slightly negative and the hydrogen end is slightly positive.

The oxygen end of a water molecule is negative while the hydrogen ends are positive.

Because of the charges, water molecules kind of 'stick' together like a magnet. The negative end of one molecule sticks to the positive end of another molecule and so on. The 'sticking' together of these molecules is called cohesion. Now, on the surface of the lake, there are less water molecules to 'stick' to because, instead of water, there is air above the surface. So the water molecules at the surface cohere more tightly to the water molecules next to them because they don't have anything to stick to above them.

This additional 'stickiness' makes the surface of water appear to have a layer or a film that bugs can run across. In reality, that layer is just the result of water molecules sticking together tightly, which makes breaking through the water difficult.

Water molecules at the surface are more attracted to one another than water molecules below

Capillary Action

Check out that plant over there. How in the world is it able to suck water up through its roots? I mean, it's sucking water up against gravity. This phenomenon is known as capillary action, or when a fluid, like water, can travel up a thin tube, like a root. Again, like surface tension, capillary action doesn't only happen to water, but that'll be our focus so you can see how it works.

Water flowing up a thin tube

So how can water defy gravity in roots or straws or other tubes? So far you learned at water has cohesive properties, meaning that water sticks to other water molecules. But sometimes water molecules are attracted to other substances in what is called adhesion.

Remember, water has a charge so, if other substances have an opposite charge, water will be attracted to them. If you have a thin tube, water molecules will be attracted to the sides of the tube and can actually 'climb' the tube. And capillary action, to some extent, is why water can flow up plant roots. The water in the root is attracted to the sides of the roots and thus 'climbs' up into the plant.


Ever noticed how the syrup for your pancakes flows slowly out of the bottle? Why would it flow so much slower than other liquids? Viscosity measures a liquid's resistance to flowing, so you would say syrup has a high viscosity. In other words, if a substance doesn't flow easily, it has a high viscosity.

The molecules in the syrup want to stick together due to charges (just like water did in the previous two sections). And what is this called? Yep! Intermolecular forces. Because they are 'sticking' together it makes them resist flowing.

Physical Changes

You see some water boiling in a teakettle on the stove and some ice melting in your orange juice, and you wonder how in the world a liquid changes into a gas (steam) or a solid (ice) changes into a liquid. Changing from a solid to a liquid or a liquid to a gas is a type of physical change, meaning the substance's molecules don't change, even if the appearance of the substance changes.

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