The freezing point of water may seem like a simple fact, but the story of how water freezes is a little more complicated. In water at the freezing temperature, ice crystals usually form around a dust particle in the water. Without dust particles, the temperature can get even lower before the water turns to ice. In the laboratory, for example, researchers have shown that it’s possible to cool water down to -40º C — without producing a single ice cube. This “supercooled” water has many uses, such as playing an important part in helping frogs and fish survive low temperatures.

In a more recent study, scientists showed how the temperature at which water freezes can be changed using electric charges. In this experiment, water exposed to a positive charge froze at higher temperatures than water exposed to a negative charge.

“We are very, very surprised by this result,” Igor Lubomirsky told Science News. Lubomirsky, who worked on the experiment, works at the Weizmann Institute of Science in Rehovot, Israel.

ThomFoto/iStock

Charge depends on tiny particles called electrons and protons. These particles, together with particles called neutrons, make up atoms, which are the building blocks of all matter. An electron is a negative charge and a proton is a positive charge. In atoms with the same number of protons as electrons, the positive and negative charges cancel each other out and make the atom act like it has no charge.

Water already has its own kind of charge. A water molecule is made of one oxygen atom and two hydrogen atoms, and when these atoms get together they make a shape like Mickey Mouse’s head, with the two hydrogen atoms being the ears. The atoms bond together by sharing their electrons. But the oxygen atom tends to hog the electrons, pulling them more toward itself. As a result, the side with the oxygen atom has a bit more negative charge. On the side with two hydrogen atoms, the protons aren’t balanced out as well by electrons, so that side has a bit of positive charge.

Because of this imbalance, scientists have long suspected that forces due to electric charges could change the freezing point of water. But this idea has been hard to test and harder to verify. Earlier experiments looked at water freezing on metal, which is a good material to use because it holds electric charges, but water can freeze on metal with or without a charge. Lubomirsky and his colleagues got around this problem by separating the water and the charged metal with a special type of crystal that could generate electric fields when heated or cooled.

In the experiment, the scientists placed four crystal discs inside four copper cylinders, then lowered the temperature of the room. As the temperature dropped, water droplets formed on the crystals. One disc was designed to give the water a positive charge; one a negative charge; and two gave no charge at all to the water.

The water droplets on the crystal with no electric charge froze at -12.5º C on average. Those on a crystal with a positive charge froze at a higher temperature of -7º C. And on the crystal with a negative charge, the water froze at -18º C — the coldest of all.

Lubomirsky told Science News he was “delighted” with his experiment, but the hard work is only beginning. They’ve taken the first step — observation — but now they have to explore the deep science of what is causing what they observed. These scientists have managed to show that electric charges affect the freezing temperature of water. But they don’t yet know why.

POWER WORDS

electron A stable, negatively charged subatomic particle.

proton A stable, positively charged subatomic particle

electric charge The property of matter responsible for all electric phenomena, in particular for the force of the electromagnetic interaction, occurring in two forms arbitrarily designated negative and positive.

electric field A region of space characterized by the existence of a force generated by electric charge.

Going Deeper:

Now, a scientist named Stephan Gekle has found that you can make air move faster than the speed of sound by doing a simple little trick: throw a rock in a pond.

View video | As a disc representing a stone plunges into still water, it plows out a column of air. The column collapses in an hourglass shape, and the escaping air (in the video, the air is filled with smoke for visibility) shoots thro

Stephan Gekle/Physical Review Letters 2010

Gekle is a scientist at the University of Twente in the Netherlands who studies the physics of fluids. Physics is the study of forces and motion, and Geckle investigates how forces act on liquids, like water. In a recent study, he and his colleagues showed that after a rock drops into a body of water, a tiny jet of air shoots upward faster than the speed of sound.

This isn’t the first time Gekle has explored what happens when a rock sinks through water. In an earlier study, he and his team showed that as a rock falls into a flat surface of water, like a pond, it carves out a tiny tube of air. This tube connects the sinking rock to the air above the pond. The tube doesn’t exist for very long, though — almost immediately, the surrounding water pushes on the sides. This pressure is stronger in the middle than at the ends. As a result, the tube looks like an hourglass, where the middle gets smaller and smaller as the water forces the air out.

There’s not room in the hourglass for water and air, so as the water comes in the air escapes upward — and fast. These tiny jets of air can blast faster than the speed of sound, Gekle found.

To measure these air jets is trickier than it may seem. Gekle and his colleagues had to do more than stand at the edge of a pond with stopwatches. A careful science experiment requires a scientist to take multiple measurements of the exact same thing, to check and double-check the results. In this case, it would have been almost impossible for Gekle and his colleagues to throw a rock in a pond in the same way over and over again.

Instead, the scientists created a lab experiment that acted like a rock falling through water: They dragged a circular disc down through water at the same speed, over and over again, and watched what happened.

But there was another difficulty: It’s hard to see and measure air. To solve that problem, the scientists filled the air above the water with smoke and illuminated the smoke with a laser, which made the moving air easier to see. (To make the smoke, Gekle said, they used a smoke machine like the ones that provide the dramatic effects seen onstage at theaters.)

Finally, because everything happens so fast when the rock moves through water, the scientists had to find a way to slow down time. As the disc moved through the water, the scientists took pictures with a camera that captured 15,000 frames every second. (That’s faster than most movie cameras.) After the experiment, the researchers could slow down the movie and, aided by computer simulations, calculate the speed of air as it blew out of the hourglass-shaped tube.

But there’s one aspect of supersonic air that Gekle and his team didn’t observe. When a jet exceeds the speed of sound, the air around it produces a noise like thunder, called a sonic boom. So far, however, Gekle says the tiny air jets aren’t making even a teeny, tiny boom — but the researchers will keep listening.

POWER WORDS (adapted from the Yahoo! Kids Dictionary)

speed of sound About 760 miles per hour, through air at sea level.

supersonic Faster than the speed of sound.

physics The science of matter and energy and of interactions between the two, grouped in traditional fields such as acoustics, optics, mechanics, thermodynamics and electromagnetism, as well as in modern fields including atomic and nuclear physics, solid-state physics, particle physics and plasma physics.

force The capacity to do work or cause physical change.

pressure Force applied uniformly over a surface, measured as force per unit of area.

Supersonic flows in action from Science News on Vimeo.

As a stone plows into still water, it plows out a column of air. The column collapses in an hourglass shape, and the escaping air (in this video, the air is filled with smoke for visibility) shoots through the shrinking opening at supersonic speeds.

Credit: Stephan Gekle/Physical Review Letters 2010

Going Deeper:

The larvae of an African fly known as Polypedilum vanderplanki live in the bottom of rain puddles in the African desert. When the dry season hits, and their habitats dry up, they can endure an almost complete loss of body water. They can persist in this dormant state, which is similar to a very deep sleep, for up to 17 years.

Just add water, and before long the dead-looking critters are moving and feeding again.

Curled up into a 4 millimeter–long mummy, this fly larva can suspend its life for years, withstanding severe drought and extreme temperatures.

Daisuke Tanaka/National Institute of Agrobiological Sciences in Japan

Most other animals that have been known to freeze-dry—sea monkeys (brine shrimp) and water bears (tardigrades), for example—are microscopic. Biologists have known for years that a sugar called trehalose plays a role in the drought-survival tactics of these species.

In water bears and sea monkeys, small command centers of cells remain hydrated during dormancy. In cells throughout the rest of the body, however, water is replaced with the sugar trehalose.

How does this special sugar allow cells, let alone small creatures, to survive? Scientists have long thought trehalose keeps the cells from falling apart by turning into a glassy state. This state is much like melted table sugar that has solidified into hard candy drops. A series of new studies show that’s exactly what happens in the fly P. vanderplanki.

Takashi Okuda of the National Institute of Agrobiological Sciences in Tsukuba, Japan, and his collaborators collected P. vanderplanki—which look more like mosquitoes than flies—and got them to breed in the lab. This provided a steady supply of young larvae for the scientists to study.

The researchers then used a technology called infrared imaging to visualize and measure the amount of water and heat in the larvae’s tissues. The study showed that the trehalose was uniformly distributed throughout P. vanderplanki‘s bodies.

When the researchers turned up the heat, they found the larvae’s absorption of heat peaked at around 70° Celsius (158° Fahrenheit). That’s the same temperature at which solid table sugar begins to melt. The findings showed that the trehalose sugar had, indeed, been in a glassy state.

A second experiment showed that trehalose sugars had bonded with the cells’ outer layer or membrane. This protected the cells’ insides from extreme distortion during dehydration.

Okuda says he and others would like to steal P. vanderplanki‘s secret to learn, for example, how to store people’s blood in a dried form until it is needed for medical uses such as transfusions. The main challenge, he says, is to get trehalose to penetrate the membranes of red and white blood cells. If this could be done, the dehydration technique could eventually be used to preserve entire organs.

Power Words

From The American Heritage® Student Science Dictionary, The American Heritage® Children’s Science Dictionary, and other sources.

dehydration The process of losing or removing water or moisture.

dormant state An inactive state in which growth stops and metabolism is slowed.

habitat The area or natural environment in which an animal or plant normally lives, such as a desert, coral reef or freshwater lake.

heat absorption The process of taking in heat and holding it. During the day the Earth takes in and stores heat from the sun through absorption.

infrared Relating to the visible part of the electromagnetic spectrum with wavelengths longer than those of visible red light but shorter than those of microwaves.

larva An animal in an early stage of development that differs greatly in appearance from its adult stage. Larvae are adapted to different environments and ways of life than adults and go through a process of metamorphosis to change into adults.

transfusion The transfer of blood from one person to another, often to replace blood lost due to injury or surgery.

Copyright © 2002, 2003 Houghton-Mifflin Company. All rights reserved. Used with permission.

Going Deeper:

Castelvecchi, Davide. 2008. Live another day: African insect survives drought in glassy state. Science News 173(March 29):197. Available at http://www.sciencenews.org/articles/20080329/fob5.asp .

Sohn, Emily. 2005. Putting a mouse on pause. Science News for Kids (April 27). Available at http://www.sciencenewsforkids.org/articles/20050427/Note2.asp .

______. 2004. A dire shortage of water. Science News for Kids (Aug. 25). Available at http://www.sciencenewsforkids.org/articles/20040825/Feature1.asp .

______. 2004. Prime time for cicadas. Science News for Kids (May 19). Available at http://www.sciencenewsforkids.org/articles/20040519/Feature1.asp .