B2 中高級 美國腔 620 分類 收藏
Chemistry is in everything, including snow.
Crystallography allows us to study the arrangement of atoms
in a snowflake crystal.
Though they all pretty much start the same, once they begin crystallizing,
it's true that no two snow flakes are alike.
And the number of possible shapes is staggering.
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A snowflake starts as a dust grain floating in a cloud. Water vapor in the
air sticks to the dust grain and the resulting droplet turns directly into ice.
Crystal faces appear on the frozen droplet.
Then, a prism forms with six faces and a top and bottom.
A cavity forms in each prism face because ice grows fastest near the edges.
Faster growth on the corners causes six branches to sprout.
The lines in each branch are due to ridges and grooves on the surface.
These six branches form the corners of a hexagon, which occurs because the water
molecules chemically bond into a hexagonal network.
When the temperature cools to -13 degrees C (9F), new
growth at the branch tips narrows.
At -14 degrees (6F), side branches sprout on each branch.
Suddenly, the crystal encounters a quick blast of warmer air followed by cooler
air, and more side branches sprout.
The crystal gradually warms, making the tips long and narrow.
The crystal encounters into even warmer air, which slows
the growth and widens the tips.
Finally, this unique and delicate structure falls to the earth along
with countless other snowflakes.
Cool, right?
Over the years, crystallographers have been classifying snow crystals into
different categories based on their arrangement of atoms
(column, plane, aggregation, rimmed, germs, irregular, other).
In the 1930s, there were 21 different classifications of snowflake could be in,
but by 2013 that number has soared to 121 categories.
To see the snow crystals at the molecular level, scientists send a beam of
x-rays through a samples of snowflakes.
The X-rays bounce off all the atoms in the snowflake and head in all
different directions, sort of like light off all the sides of a disco ball.
By seeing where these beams end up, we can figure out what arrangement
the snowflake's atoms are in, and therefore what it looks like at the atomic level.
Who's to say what new snowflake categories crystallographers will find in 2016.
But one thing's for certain, the ever-changing environment means that snowflakes
can have a mind-boggling array of shapes.
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美麗的雪花如何形成的? (How Do Snowflakes Form?)

620 分類 收藏
綾羅飄起 發佈於 2019 年 1 月 11 日
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