making sugar crystal - physical or chemical change?

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making sugar crystal - physical or chemical change?

Postby mom1967 » Mon Oct 15, 2007 7:44 am

we made sugar crystal last week. then this weekend, when I discuss with my son about physical and chemical changes, he asked me this question.

Crystalization is a physical chemistry. So is it physical change from a 8-yo point of view?

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Postby Theodore » Mon Oct 15, 2007 3:23 pm

Physical changes are when the materials don't change, just the state (solid, liquid, gas, plasma). In the case of salt crystals, they form when you take away the water, but if you add the water again, you are right back where you started.

Mixing vinegar and baking soda, on the other hand, is a chemical reaction because carbon dioxide is produced, and the vinegar and baking soda cancel each other out and become neutral.

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Postby Sheepdog » Sat Jan 23, 2010 9:04 am

There are no chemical changes in the following...

a) Add "ordinary" sugar to water
b) Dissolve it
c) Cause the sugar to form large crystals

I went with the "cause..." description because of the various ways that can be done. None of them involve chemical changes.

You'll get into a tricky area....

Physical changes are usaually defined as changing between solid, liquid and gas.

So what did the sugar do when it "melted" "into" the water.

a) It didn't, in any sense a scientist would recognize, "melt".

For an 8 year old, and for quite a few years past that, I'd go with saying that the sugar and water are very, very well mixed.

The fact that the sugar solution would still taste of sugar is one fact to support the fact that sugar is still there. (Of course, no one ever tasted sugar that wasn't dissolved in water... but I digress.)

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