Material

Balloons
salt
food coloring
a flashlight
A shallow tray to collect the melt water.

Assembly

Fill the balloons with water and put them in a freezer two days before you plan to use them.

To Do and Notice

Take the ice balloon from the freezer.

Peel off the balloon.

Put it in a tray to collect the melt water.

Examine the ice. What do you see?

Sprinkle salt on top of the ice. What happens.

Add drops of food coloring to the salted places.

Notice tyhe air bubbles in the ice.

Are their fractures?

Notice that the salt causes the ice to melt.
The melt water flows down the sides of the ice carving channels as it flows.
The channels wind back-and-forth, they meander, as they flow.

The food coloring makes the flow more visible.

Meanders of salt water streams along an ice balloon.

What's Going On?

When water freezes it expels impurities.
In particular, air dissolved in the ice comes out of the ice as air bubbles.

The ice freezes from the outside toward the center.
At first the air expelled by the freezing ice can dissolve in the remaining water.
But eventually, the water becomes saturated with air. Then further freezing will cause air bubbles to form. This is why the air bubbles are inside the ice.

The ice expands as it freezes. After the first ice freezes to make a solid casing, the inner ice freezes and expands making cracks in the outer ice.

Adding salt to ice lowers the freezing point, some of the ice melts. The melt water flows down the side of the ice. As it flows it meanders.

The meanders allow the flowing water to lose energy at a constant rate, the water flows at a constant speed through the meanders. (Just like a skier controls speed by making turns down the fall-line of the slope.). If the water flowed straight down the side of the ice it would accelerate and lose energy in a nonlinear way.

Scientific Explorations with Paul Doherty

© 2005

23 September 2005

From an activity developed by the Institute for Inquiry at the Exploratorium.