STICKY RABBIT EARS
By Debora Giese

A water strider bug zooms across a puddle. How does the bug skate on water?

An ice cube floats on water in a glass. Why doesn’t the solid ice cube sink?

Ping. Ping. Water drips from a tap into a sink. Why are water drops round?

To answer these questions, it helps to understand that…

Water Sticks Together

Like rabbit ears, one water molecule is made of two hydrogen atoms linked to its single oxygen atom by shared particles, or covalent bonding. And it is hydrogen bonding from opposite charges between a water molecule’s two hydrogen atoms and another water molecule’s oxygen atom that makes water sticky. Hydrogen bonding makes all of the water molecules stick strongly together.

Sticky Water Flows

Although sticky, water flows because its liquid bonds are weak, and slip over each other easily, like linked rabbit ears. The hydrogen bonds of moving liquid water molecules are always breaking and reforming.

Sticky Water is Strong

Inside liquid water, there is an equal tug of war between water molecules. But at the water’s surface, the tug of war is unequal since the surface molecules are pulled down and stretch the water’s surface like an elastic band, with tension as strong as steel from hydrogen bonding.

With water’s surface tension as strong as steel, no wonder the water strider bug skates on water! Its wide body and long, hairy feet, like seaplane pontoons, keep it floating, too.

Sticky Water Floats

As water freezes into solid ice, its molecules form more hydrogen bonds than liquid water. This means water molecules freeze into place with holes between them. Because there are less water molecules in the solid ice cube than in the same amount of liquid water below it, the ice cube floats!

Sticky Water Shapes Up!

A drop holds a very small amount, or volume, of water inside it. Its round bubble shape results from its surface tension and the force of Earth’s gravity.

But if water’s surface tension is as strong as steel, why doesn’t the drop collapse by pulling back on itself, like an elastic band? Just think. Inside the droplet, water molecules push out from the drop’s center, against air pressure and surface tension. These equal forces press the drop into a shape with the smallest surface area, a round ball or sphere.

Sticky Water in Space

Only a tiny water drop is perfectly round like a ball or sphere, because Earth’s gravity does not have much force on such a small amount of water.

But in space at ‘zero gravity’, surface tension can squeeze a bigger water drop into a sphere!

Sticky Water Loosens Up!

Supplies:

• Bowl
• Fork
• Water
• Pepper
• Liquid dish soap

Two simple activities that show the surface tension ‘skin’ of water can be tried.

For the first activity, fill the bowl about half full with water. Sprinkle pepper onto the surface of the water. Most of it should float because of the surface tension skin of the water.

But can you make the pepper swim without touching it, or blowing on it?

For the pepper to swim, try a second activity.

Using the fork’s outside prong, touch its tip into the dish detergent. Now, touch the tip of the fork to the surface of the water.

The pepper particles swim away! The pepper swims because the soap is a sur-fac-tant that lowers the water’s surface tension.

Did you know, surfactant lowers the surface tension in the fluid lining your lungs? This keeps your lungs from sticking together and collapsing, so you can breathe!