Have you ever filled a cup so full of water you thought it was going to overflow, but instead, it hovered just over the surface of the cup? Try it out! If you fill a glass with water slowly enough, you will see the dome-like shape of the water on top of the glass.
In the image to the left, you can see that water molecules inside a droplet are attracted to each other in various directions, but water molecules on the surface are attracted to other molecules sideways and inwards. |
Put the definition into your own words! What is surface tension?
Below, check out a water strider using surface tension to "walk on water." The surface of the water is like a thin membrane that resists the force of objects such as the water strider.
- What are some other examples of times you have seen surface tension in action?
- Why do water molecules at the surface create stronger bonds?
- What is the surface tension of water? Does it ever change, and why?
- What is the difference between cohesion and adhesion?
- When you add soap to water, what happens to the surface tension? Hypothesize why this is.
Follow up question:
- Why does the addition of soap cause the spring to sink?
Let’s try out a similar experiment!
YOU WILL NEED:
- Cup
- Water
- Paperclip
- A square of toilet paper
Extension Materials
- Soap
- Baby powder
YOU WILL DO:
- Fill the cup with water.
- Hypothesize: Does a paperclip float or sink?
- Place the paperclip on the surface of the water. What happens? Record.
- Remove the paperclip from the cup.
- Gently place the square (or small piece of the square) of toilet paper on the surface of the water.
- Gently place the paperclip on the surface of the toilet paper.
- Watch what happens and record observations. How is this different than the first time you put the paperclip in the cup? Why is this?
- You should see the toilet paper absorb water and sink, leaving the paperclip “floating” at the surface!
- What happens when you bend the paperclip? Do different shapes make any difference?
Extension
- Try what Physics Girl did in her experiment! Put a bit of hand soap on your finger and touch the surface of the water. What happens? Why is this?
- Next, hypothesize how many paperclips the surface of your cup of water can hold!
- We know that soap makes the surface tension weaker, but can anything make the surface tension stronger? Take a guess!
- Sprinkle baby powder on the surface of your water, and try the paperclip experiment again. How many paperclips “floated” this time?
- Try out other liquids such as soda, milk, vegetable oil, vinegar... do you obtain the same results, or are they different?
Resources
http://en.wikipedia.org/wiki/Surface_tension
http://hyperphysics.phy-astr.gsu.edu/hbase/surten.html
http://www.sciencebob.com/experiments/paperclip.php
http://www.exploratorium.edu/ronh/bubbles/soap.html
Image and video credits in order of appearance:
Apel, 2006. Water beading on a leaf. Uploaded from Wikimedia Commons on 8/8/2016. https://en.wikipedia.org/wiki/Surface_tension#/media/File:Dew_2.jpg File used in accordance with the Creative Commons Attribution 2.5 Generic license. No changes were made.
Booyabazooka, 2008. Diagram of the forces on molecules of a liquid. Uploaded from Wikimedia Commons on 8/8/2016. https://upload.wikimedia.org/wikipedia/commons/thumb/f/f9/Wassermolek%C3%BCleInTr%C3%B6pfchen.svg/300px-Wassermolek%C3%BCleInTr%C3%B6pfchen.svg.png
File released into the Public Domain.
Vickers, 2008. Water strider. Uploaded from Wikimedia Commons on 8/8/2016.
https://upload.wikimedia.org/wikipedia/commons/thumb/2/2c/Water_strider.jpg/1280px-Water_strider.jpg File released into the Public Domain.
Billerbeck, 2010. Water strider or water bug.
Uploaded from Youtube on 8/8/2016. https://youtu.be/b9nxUtoH7to
Physicsgirl, 2012. Amusing surface tension experiment. Uploaded from Youtube on 8/8/2016.
https://youtu.be/2F64qh9qPAI
Armin Kubelbeck, 2007. A metal paperclip floats on water. Uploaded from Wikimedia Commons on 8/8/2016.
https://upload.wikimedia.org/wikipedia/commons/thumb/9/9c/Surface_Tension_01.jpg/1024px-Surface_Tension_01.jpg File used in accordance with the Creative Commons Attribution-Share Alike 3.0 Unported license. No changes were made.