Activity: Surface Tension
Theme: Water
Ideal Group Size
Grade Level
Set Up/Clean Up Time
Activity Time
Small (up to 10)
All
2 minutes / 2 minutes
5 minutes
Math/science questions addressed: What is surface tension?
Overview: Participants explore the concept of surface tension as they observe a paper clip float
in water.
Materials: 2 styrofoam bowls, paperclips, soap, picture of water strider, (optional: 3 aluminum
pans), water.
Set up: Fill 2 bowls almost to the top with water.
Ask: Does metal usually sink? Make a prediction about what you think will happen when I put
this paperclip into a cup of water? Will it float or sink?
To do:
1. In each bowl gently put a paper clip on the surface of the water in the middle of the bowl
(make sure the paperclip is dry). (Be careful – it may not stay up for too long.)
2. After both bowls contain a “floating” paperclip, add a drop or two of soap to one of them
(no need to stir). The paperclip should sink.
Ask: What is holding up the paperclip?
Explain: Molecules (particles) of water stick together really well and they can hold up the
paperclip. This is called surface tension. When we add soap, the water isn’t as “sticky” and it
can’t hold up the paperclip anymore.
Tips to make your paperclip float:
You have to be really careful when you put the paperclip on
the water surface. If you are having trouble, bend open a
paperclip and use it to lower a paperclip onto the surface like
as in the picture at right.
Activity extensions: for older participants
A) You can have participants try to do this on their own. See if they are able to get a
paperclip to float. For this you could pass out paperclips and fill the tin containers with
water for them.
B) If there are other liquids available, you can see if they have surface tension too.
Background: Surface tension is the reason it hurts to do a belly flop on the water. The bonds
that connect water molecules to each other, called hydrogen bonds, can be pretty strong. When
an object is put on top of a calm water surface, if the weight of that object can be distributed
enough, the hydrogen bonds will be able to hold the object up. This is why water striders and
other insects can “walk on water”. They distribute their weight so that no single leg is holding
enough weight to break the surface of the water. However, when the hydrogen bonds between
water molecules are broken, surface tension is reduced. Soap disrupts these bonds on the
surface of water, reducing the surface tension.
In most circumstances, materials made of metal sink, but surface tension enables them to
float. The bonds between the water molecules are strong enough to hold up a paperclip.
These 2014 Summer Learning science activities were developed by Pacific Science Center