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Expert: The chemistry of snowflakes, explained


Tue, Jan 16th 2018 03:10 pm
A single snowflake. (Credit: Douglas Levere/snowcrystals.us)
A single snowflake. (Credit: Douglas Levere/snowcrystals.us)
No two snowflakes are exactly alike - but why?
In the midst of a frigid winter, a University at Buffalo expert is available to discuss one of the season's great natural marvels: snowflakes.
Jason Benedict, Ph.D., UB associate professor of chemistry, studies crystals - materials whose atoms and molecules arrange themselves in an orderly pattern when in a solid state.
Snowflakes fall under this category: Each one that tumbles from the sky is an ice crystal, made from frozen water molecules that join together in a lattice according to the laws of chemistry, Benedict said.
Below, Benedict's insights on these winter wonders.
Q: No two snowflakes are exactly alike. Why?
A: Benedict explains snowflakes form through a process called nucleation, in which water molecules come together - typically around a speck of dust or pollen - to create an ice crystal. As the structure falls through the atmosphere, additional water molecules latch on, causing the snowflake to grow.
"The shapes you observe are remarkably intricate," Benedict said. "No two are exactly alike because the local environmental conditions - like the temperature, the humidity and the concentration of water molecules in the air - all affect where and how water molecules are going to attach to the crystal."
"To have two snowflakes that are exactly identical, you'd need to have identical conditions, and in nature, that's basically impossible," Benedict added. "We're talking about air swirling around, about crystals forming under tumultuous circumstances."
Q: How do snowflakes get their limbs?
A: Some of the most ornate snowflakes - those with complex, feathery branches - take shape under conditions of rapid growth, Benedict said.
They arise when the air is full of water vapor molecules that deposit themselves on the snowflake in quick succession, causing limbs to sprout from the crystal's hexagonal base, with spiky offshoots forming on each branch.
The same theme applies to crystal growth in general: Fast growth is typically chaotic growth, said Benedict, who founded and runs the U.S. Crystal Growing Competition, which challenges children and teachers nationwide to grow crystals each year.
Crystals that grow more slowly tend to be more regular in shape, with smooth facets, while crystals that grow more quickly commonly end up with erratic, jagged edges, he said.
Q: Why do snowflakes have six sides or branches?
A: The shape of snowflakes is driven by the crystal structure of ice.
As Benedict explained, the water molecules that form ice crystals (snowflakes) are made from two hydrogen atoms bonded to one oxygen atom. On Earth, when these molecules come together in the sky to create ice, they arrange themselves in a lattice of hexagonal rings.
"This crystal structure means that snowflakes are going to exhibit six-fold symmetry," Benedict said. "Apparently, they can also show 12-fold symmetry, because 12 is a multiple of six."
But those eight-sided snowflakes you may have seen in holiday decorations?
"As a crystallographer (or fan of symmetry), when you see something like that, you laugh," Benedict said. "You just go, 'That's not right.' "
Water molecules in ice will arrange themselves in non-hexagonal structures (such as tetragonal lattices) under super-high pressure and/or in extreme cold, but these shapes do not occur under the regular atmospheric conditions that generate snowfall on Earth, Benedict says.

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