Al Stahler: Surprising ices
Solid … liquid … gas … it all depends on how tiny bits of matter – tiny bits of water, say – it all depends on how they stick to their neighbors.
If all the bits – the molecules – of water stick to all their neighbor molecules, we’ve got a solid. Solids don’t slosh around. Ice.
If some molecules of water stick to some of their neighbors – grab hold to one, let go, grab onto another – we’ve got a liquid. Liquids slosh around easily. Ordinary water.
If no water molecules stick to any of their neighbors, we’ve got a gas. Gases don’t just slosh – they don’t even stick around. Gases disappear, into the air. Water vapor.
Put a chunk of solid ice into a pot on the stovetop, and, as the water molecules let go of their neighbors, the ice melts: solid ice becomes liquid water. Keep heating, and all the water molecules lets go of their neighbors. Bubbles of unattached water molecules rise up from the water, as the liquid boils to gas: liquid water becomes water vapor.
Same trick backwards: We can see our breath, on a cold winter’s day, as individual water molecules latch onto their neighbors, to form a cloud of liquid water droplets.
Blow onto a mirror, on that cold winter’s day. When cloud droplets hit the cold mirror, the water molecules latch onto all their neighbors, forming tiny spheres of solid ice.
When we say the word “ice,” it’s natural to think, “frozen water.” But get them cold enough, and the molecules in all sorts of liquids and gases will latch onto their neighbors. Gases turn to liquid, liquids freeze solid … and we get other kinds of ice. There are many different kinds of ice – many different ices.
Sometimes, we can skip the liquid step, and go directly from gas to solid ice … or from solid ice, back to gas
Carbon dioxide gas floats in the air all around us, every carbon dioxide molecule floating free of its neighbors. But drop the temperature to a hundred nine degrees below zero – minus a hundred nine degrees Fahrenheit – and carbon dioxide molecules link arms. At -109°F, carbon dioxide freezes – solidifies – into carbon dioxide ice.
Allow carbon dioxide ice to warm, and we’d expect it to melt. But once a lump of carbon dioxide ice rises to its freezing point, it’s already above its boiling point. As carbon dioxide ice warms, it goes directly from solid to gas – no liquid in-between. Carbon dioxide ice never gets wet … which is why it’s also known as “dry ice.” (Some COVID vaccines, now being tested, fall apart unless they’re kept really, really cold. If these vaccines pan out, the demand for dry ice will explode.).
Maybe you’ve found, in your freezer, a tray of ancient, forgotten ice cubes … and they’ve obviously shrunk. In the super-dry atmosphere of the freezer, the ice has gone directly to vapor – the ice cubes have skipped the liquid step. Under cold, dry conditions, water ice can also be dry ice.
Head outside just after dark tonight, and, in the southeastern sky – bright and orangey – is planet Mars … one of Earth’s closest neighbors in the solar system.
Spacecraft orbiting Mars send back photos of a vast desert. But running through the Martian desert, we see river valleys … lake beds … and possibly the floor of what was once a huge ocean.
Martian river valleys and lake beds today are bone dry … but they seem to be telling us that – maybe three-and-a-half, four billion years ago – Mars was sopping wet.
Travel back three-and-a-half, four billion years on Earth, and you’re back to when atoms in rock … and air … and water … somehow linked up with neighboring atoms … to breathe life into our planet – the earliest life on earth, from which everything today has evolved.
Could it be that, right around the same time – when Mars was wet – atoms in Martian rock … and air … and water … somehow linked up with their neighbor atoms, to create Martian life?
As we look toward Mars, we’re also looking toward a much, much smaller object: A spacecraft, launched from Earth last summer, scheduled to make planetfall (landfall on another planet) on Mars next February. Aboard the spacecraft is a robotic rockhound. One of its goals: to collect rocks that might hold atoms and molecules that might once have been part of microscopic Martians.
Mars reflects a lot of sunlight toward us these nights, sunlight reflecting, especially, off the planet’s south polar ice cap.
Mars is too cold, too dry, to go thru a liquid-to-solid step at its south pole. Rather, both carbon dioxide, and water vapor go directly from gas to ice. Both water ice and carbon dioxide ice are, on Mars, dry ices.
We don’t need to look to Mars, nor in our freezers, to see water behaving as a dry ice.
It’s not yet winter in California, and the air temperature hereabouts has yet to drop below freezing. Yet, here and there, there is ice on the ground. Some of the ice is ordinary – frozen puddles. But some of the ice forms patches of fine, feathery crystals.
By day, the ground soaks up sunlight. At night, the ground beams that solar energy back out into space. The ground beams out so much energy, it becomes very, very cold – cold enough to freeze puddles.
The ground is also cold enough to latch onto any stray molecule of water that happens to touch it. Like bricks being added to a wall, water molecules leave the air to build fine, feathery crystals of ice: hoar frost (the same way ice forms at the south pole of Mars).
Tonight, at 5:37 p.m., the International Space Station will rise in the southwest, heading toward the northeast. A couple minutes after 5:37, the ISS, with a crew of seven – two women astronauts, five men – will fly almost directly over the foothills.
And now, I’d like to wish a very happy Thanksgiving season, to ALL my neighbors.
Al Stahler enjoys sharing science and nature with friends and neighbors in The Union and on KVMR-FM. He teaches classes for both kids and grown-ups, and can be reached at email@example.com.
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