MadSci Network: Chemistry

Re: Can you turn a solid back into a liquid, besides water and ice?

Date: Tue Aug 22 12:00:08 2006
Posted By: Matthew McConeghy, Professor of Science
Area of science: Chemistry
ID: 1150144403.Ch

Hi Addie

I bet you were making breakfast when you thought this question up!

The first answer is, many solids can be changed into liquids and then changed back. For instance, ice to water, as you mentioned. Some other solids that can change would be metals, rocks, wax, some kinds of plastics. etc. You have probably seen some statues made of bronze or iron. The metal probably came from the factory in the form of a metal rod or plate. Then the artist melted it in a very hot furnace at the foundry and poured the melted metal into a mold. When it cooled it was solid again in the shape of the mold. But, if you wanted, you could take the statue and melt it down in the furnace and turn it back into a big lump of metal. You need a really hot furnace to do this, not just an ordinary fire! (a web page with melting points for different metals is) /jewelry/meltingpoints.html

Rocks melt under the earth, come to the surface as molten lava from a volcano, and then solidify on the surface as new rocks, right? And sometimes as the surface of the earth moves, rocks get slowly sunk down into the earth again and remelted. Rocks that are made from lava are called "igneous" rocks (the word 'igneous' means 'fire') Rocks that have been partly melted again may be called 'metamorphic'. And of course, you have seen a wax candle melt and then get solid again when it is cool.

Here is a fun page about melting rocks at the Annenberg Foundation htt p://

So, what about pancakes? That is a different story!

First, pancake batter is a kind of runny dough. That is, a mixture of flour and water. It is not really completely a liquid. If you looked at it with a microscope you could see that it is really tiny pieces of flour mixed together with water. The flour is not really completely liquid. Flour is made from the seeds of wheat plants, ground up in a big grinder. The flour is mostly 'starch' or 'carbohydrate' but it also has some protein in it made by the wheat plant, called 'gluten'.

OK, say you mix up the flour and water and put it on the stove. At first nothing happens but as the batter heats up, the water turns to steam and makes steam bubbles that puff up the pancake. As you know from cooking pancakes, you have to wait for the bubbles to get all through the pancake and start popping out the top. While the hot steam bubbles are in the pancake they actually 'cook' the batter - that means, they are changing the starch and gluten so that it is chemically changed.

(You can try finding more links by searching for "What happens chemically when food cooks")

The starch is broken apart a bit, but the main change is in the gluten which is "denatured" - that means it breaks into a new form which is distinctly different from the way it was before. It is now stuck into a form sort of like a sponge, rather tough and stretchy. The elastic gluten sponge form holds up the pancake in a solid shape. It is puffy because of the bubbles that were formed by the steam. That makes a yummy pancake, but the gluten can not go back to the way it was. Its atoms have been all rearranged.

Wheat flour has a lot of gluten so it makes good pancakes and bread, but some other kinds of flour made from rice or oats do not work as well because the amount of gluten is different - wheat usually has more than 10 times as much gluten as rice. So cooking rice pancakes would work quite differently! You might try looking for a cookbook that has a recipe for rice cakes and try them out. It will be a lot different!

Have fun

Dr. Matt

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