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Name: Isabella
Status: student
Grade: 4-5
Location: FL
Date: January 2008

Question:
I am doing a science fair project on which crystals will grow faster salt crystals or sugar crystals. With the help of my mom I put one cup of boiling water in each glass and 1 cup of salt in one glass and 1 cup of sugar in another. Added a string with a paper clip attached and waited. By the next day the salt had started to crystallize but the sugar has not. I have come to realized that I probably did not add enough sugar but cannot find out why the same amount of sugar in water was not enough to grow crystals as it was for salt. Can you help me? I will be trying the experiment again with more sugar this time but still wonder why the first one did not work.



Replies:
Isabella,

First, great job doing your own experiment! Science is so much more fun when you do it hands-on! The quick answer to your question is to use 2 cups of sugar... more sugar can dissolve in water than salt can. Keep reading if you're interesting in a little more science.

To explain what's going on here, imagine a giant sack. You can add things into it, and keep adding more until it's full. Once you fill the sack, you can't fit in any more. Now imagine that you shrink the sack. What happens to what was in the sack? It falls out. The sack is a little like what you're doing with the boiling water. When you add sugar or salt to the boiling water, it will keep dissolving until the water is 'full' -- the science term is 'saturated'. Saturated means you can't dissolve any more in it. Then, when the water cools, that's like shrinking the sack. Hot water can hold more sugar or salt than cold water can hold. So when the water cools, it can't hold as much, and so the extra has to fall out. When sugar or salt falls out of solution, it forms crystals you can see.

It just so happens that water can hold more sugar than salt. That's like saying the 'sack' is larger when you work with sugar than when you work with salt. Even when the water cools, if you don't add enough sugar to overflow the sack when it "shrinks", you won't see any crystals form.

Every substance has a different amount that will dissolve in water. A lot of sugar can dissolve in water, a little less salt. Sand won't dissolve practically at all. There are complicated chemical reasons why things dissolve differently, and it's very hard to predict just by what things look like.

It turns out, that one cup of salt can dissolve in hot water (it fits in the sack), but not in cold water (once the sack shrinks, the salt doesn't fit anymore). So that's why you see the crystals. However, one cup of sugar will still fit in the sack even after it shrinks. To "overflow the sack", you would need a little more than two cups of sugar for one cup of water.

To help things along, you can add something to the liquid to help the crystals grow. Crystals will grow more easily on a rough surface than on a smooth one, so if you put the liquids in a smooth glass container, the crystals will take longer to form. Using a rough container (like an old glass jar instead of a new one) or even adding a little bit of undissolved sugar or salt, will give a rough surface for the crystals to grow on.

For even more information, here's a good site:

http://www.exploratorium.edu/cooking/candy/recipe-rockcandy.html

This focuses on sugar, but does a good job of explaining the science too.

Hope this helps,

Burr


Isabella- their solubilities are greatly different. Sugar is _crazy_ soluble in water, especially when hot. Salt is so-so, and heat doesn't even help it much.

If you want to do the same thing to both solutions and still grow crystals, what you do to make a saturated solution is this:

Pour a lot of powder in a container. Add less water than what it takes to dissolve the powder. Heat, stir, cool, stir, then let the powder settle to the bottom. Pour off some or most of the clear solution into a new bottle, carefully leaving the remaining powder on the bottom. There must be some remaining powder, or else the clear solution doesn't have enough stuff dissolved in it.

The clear liquid in the new bottle is your "saturated solution" of salt, or of sugar, and it will grow crystals if you make water evaporate out of it.

The evaporation method is what works best for salt crystals.

Sugar, on the other hand, works better by cooling than by evaporating. The sugar is so sticky that the water sometimes won't leave it, won't evaporate. So, to grow sugar crystals you must: add some more sugar to your saturated solution, heat it and stir it until it all dissolves, then let it cool all the way down, then hang a seed crystal on a string in the solution.

It is harder to grow crystals of sugar than of salt, but sugar makes nicer ones, so it may be worth trying more than once.

Jim Swenson



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