The other night I found myself on ice skates, gliding almost frictionlessly over the ice. The wind whistled through (what’s left of) my hair; my breath left a series of small cumulus clouds behind me. The problem with this vision was that I wasn’t skating like an Olympic skater on a pristine frozen lake high in the mountains. I was in the middle of a shopping mall and skating like a klutz.
Unfortunately, my skating skill and my thoughts were in cahoots against me. I couldn’t stop thinking about the unusual properties of the ice. It forms beautiful frost crystals on windows and snowflakes in the air. Unlike most solids, the solid form of water is less dense than the liquid form, and hence ice floats in water. Applying pressure to ice causes it to melt. In fact, I was gliding along on a thin, lubricating film of water that had been melted by the pressure of my skates. And why wasn’t the ice in the rink clear? It was just made of clear water, after all.
Well, the best way I know of to find out about these things is to roll up your sleeves and dive in. (A chilling thought, especially given the subject.) We’re going to do this in true Exploratorium style by performing some simple experiments.
Ice Crystals, Clear Ice, and Ice Worms
To do & notice
• Boil a kettle of water and fill the other container to the same level with the boiled water. Be sure to pour the hot water into the container carefully to avoid introducing any gas. Put the lids on both containers and place them in the freezer overnight.
• In the morning, carefully remove the lid from the previously hot container.
The steam rising from the hot water should have formed intricate spire-shaped ice crystals on the inside of the lid. Examine these crystals with a magnifying glass, and you’ll find that each subspire meets the main axis of the spire shaft at an angle of 120 degrees. When formed in a free environment, such as a cloud, the spires join in sixes, forming the familiar snowflake. (But more about this later.)
• Now remove the ice blocks from their containers by running warm water over the outside of each container. Smooth the surfaces of the blocks by running a little cold water over them. Is there a difference between the two? The block made from aerated water should be cloudier in the center than the one made from boiled water. If you look closely, you may see bubbles and tunnels that look like worm holes in the cloudy ice. Put the cubes back in your freezer and save them for a later experiment.
What’s going on
Because each water molecule has a positively charged side and a negatively charged side, water molecules stick to each other in a regular pattern. The negatively charged oxygen is attracted to the positively charged hydrogen atom of a neighboring molecule. The molecules gather together to form rough hexagons. Why hexagons? This has to do with the 105-degree bend of the water molecule. If you drew a six-sided hexagon, the corner angles would measure 120 degrees. The 105-degree bend of the water molecule isn’t exactly the 120 degrees necessary for six molecules to form a hexagon, so the six connected water molecules don’t lie exactly flat; the corners of the hexagon go up-down-up-down-up-down, as you can see in the diagram below. This six-fold symmetry of the hexagonal crystalline structure gives us the traditional shape of the snowflake. (As a matter of interest, the word “symmetry” is from the Greek sym, “with,” and metron, “measure,” and the word “crystal” is from the Greek word krystallos, meaning “frost stone.”)
Most of the ice we are familiar with is in the form of cubes in a glass of soda or an absolutely flat sheet covering a pond or lake. Even though what we see is not apparently crystalline, the water molecules are nonetheless arranged the same way in ice as they are in a snowflake.
As the ice crystallizes, water molecule joins neighboring water molecule to create a crystal of pure water. Any impurities in the water won’t fit into the precise structure of the ice, which is defined by the shape of the water molecule. These impurities are excluded from and pushed ahead of the growing crystal.
The ice blocks you made in the activity above formed from the outside inward. As the water froze, the gas molecules you dissolved in the water by shaking or aerating the water didn’t fit into the new crystalline structure. They were rejected from the growing ice and pushed to the center of the cube. If larger bubbles formed, they too were pushed along in front of the forming ice, creating a “worm tunnel” inside the cube. If the bubbles were all very small, they just made the center of the cube cloudy. When you boiled the water, you removed some of this dissolved gas, producing ice with fewer bubbles.
Why Icebergs Float
To do & notice
• Look at the cup in the morning. I’ll bet that the sides have split. The ice expanded when it froze.
• You can figure out exactly how much water expands when it freezes. Either peel the Styrofoam from the ice or use one of the blocks you made in the previous experiment. Get a large bowl, put the block in, and carefully fill the bowl with water until it’s just on the verge of overflowing. Look from the side and observe how much of your mini-iceberg is above the water.
• Intuitively, you might think that the bowl will overflow when the ice melts, and the amount of overflow would give you the volume sticking up above the surface. But sometimes intuition can be misleading, because the bowl will not overflow by a single drop.
What’s going on
Water is one of the few substances where the solid is less dense than the liquid. As water is cooled, it does become denser and denser, but only until it reaches a temperature of four degrees Celsius. Here it’s at its densest, most compact state. Below this temperature, microscopic ice crystals begin to form and the mixture begins to expand.
Look back at the diagram of the arrangement of water molecules in a crystal. These ice crystals have a very open distribution of molecules. The strict structure that the molecules are bound into is even more open than liquid water, and hence takes up more room. If the solid ice takes up more room than the liquid water, it is less dense and hence will float. In this respect, ice is very unlike most substances, where the molecules take up less space when in an orderly arrangement. (Thought experiment: Do dice take up more space when they are jumbled and disorderly or when they are nicely arranged face-to-face?)
Let’s get back to that unintuitive and unspilled bowl of water. Remember that the water expanded when it froze. Well, it shrank when the ice melted by exactly the same amount. This also happened to be the same as the volume of ice sticking up above the water.
With most substances, the cooler stuff keeps sinking and the body freezes from the bottom up. If this were the case with water, fishing in the winter would be as simple as walking out on the ice (with no threat of cracking through) and picking up the fish. All life would perish in the lake during winter. As it is, the layer of ice that forms on top of the water acts to insulate the water beneath, reducing the amount of ice formed. The top layer is also the first to thaw in the spring.
If water acted “normally,” we would probably have amazingly salty oceans. The freezing water would reject the salt the same way it rejects air, leaving saltier water behind. If this weren’t bad enough, the ice would form from the bottom upward, leaving immensely thick layers of ice covering the ocean floors. With an insulating layer of salty water above, there would be little likelihood that the ice would ever melt. In such an environment, life as we know it would not have evolved.
The expansion of ice is also responsible for much of the erosion that makes our soil. During the winter, water that has found its way into cracks in rocks freezes. As the water freezes, it expands, and the cracks are forced open and enlarged by the ice. This turns big rocks into little rocks and makes mountains into molehills. In a more painful vein, frostbite occurs when skin cells freeze, expand, and rupture.
Ice Under Pressure
To do & notice
• This experiment is a little more complicated to set up. Find a thin strip of wood about 1½ inches wide, ½ inch thick, and a couple of feet long. Set it up bridging the backs of two chairs or across your bathtub. Make a loop of thin copper wire about 1 to 1½ feet in diameter by twisting the ends securely (or by soldering it if you have the tools). Place the loop over the wood. Get an ice cube or one of the blocks from the previous activities and put it on top of the wood but under the loop. Hang something heavy from the loop. I tied the loop through the handle of my iron and used the iron as the weight. I also tried tying the loop through the handle of a plastic jug filled with water.
The loop now is applying great pressure to the ice cube. Within ten minutes, you should be able to see the loop beginning to pass through the cube. Eventually, the loop makes it all the way through, but the cube will still be in one piece. The ice refreezes behind the wire when the pressure is released.
• Make some crushed ice in your blender. Pack and squeeze it together and make a snowball. Throw the snowball gently at a loved one. Apologize.
What’s going on
The melting and refreezing of ice due to an increase then decrease in pressure is called regelation. You’ve probably noticed the line on the ice behind an ice skater’s blade. When the blade presses on the ice, the ice melts and the resulting water lubricates the skate on its way. Once the skate has passed, the pressure is released and the water refreezes. It’s easy to walk with shoes on the nice flat ice at the rink, but regelation makes walking on bumpy ice much more hazardous. Your shoes apply more pressure to the bumps than to the uniform flat ice. The bumps melt and lubricate you into a fall.
When you make a snowball, you squeeze the snow together, causing some of the ice to melt. When you let go, the water refreezes, cementing the ball together. If it’s too cold, you can’t make snowballs because the ice can’t melt.
Even though it would be great to have an entire moon to skate on, I think I’ll stick with skating on Ice I. I have a hard enough time skating while thinking about the physics of ice without adding magnificent views of giant gas planets that take up half the sky. Besides all that, even compared to the shopping mall, those moons simply have no atmosphere.
Originally published in the Exploratorium Quarterly, “Ice,” vol. 13, no. 2, Summer 1989, pages 11–15.