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Name:  Zack
Status: student
Grade: 6-8
Location:AZ
Country: N/A
Date: 4/11/2005


Question:
When sugar is burned, there is a chemical change. One substance that remains as a result of the chemical change is ash. Ash is pure carbon. What compound do the remaining elements form? What are the elements?


Replies:
Pure sugars are made up of C, H and O. For example sucrose is C12H22O11, while glucose is C6H12O6.

If a complete combustion is done -that is the carbon completely reacts with oxygen to form the lowest energy molecule of C+O then it will form CO2, H will form H2O. So in a complete combustion, you would get nothing remaining, as the sugar will turn to CO2 and H2O (which under the conditions of complete combustion will probably vaporize).

In an incomplete combustion, such as when you use a match or any low heat source, then all sorts of reactions can happen. The ring might polymerized and form a sticky, tarry substance, some charring might occur (depending on the lack of available O2) - over-all, without knowing the specific conditions, it would be hard to predict all the possible substances that would form.

Greg (Roberto Gregorius)


Sugars are carbohydrates. All sugars are composed of carbon (C), hydrogen (H), oxygen (O) in the ratio: C(H2O). But there are many different sugars (for example: glucose, sucrose, lactose, fructose, ..., plus more). In addition, sugar as it is commonly used in cooking is not even the pure carbohydrate. It contains other substances that lend various sugars their unique characteristics. Honey, unrefined sugar, "brown sugars", turbino, ... and more. For a lot more info, I direct you to a very readable book: "On Food and Cooking" by Harold McGee (chapter 12) that covers the chemistry and cooking with sugars.

When sugar is heated it begins to lose water (H2O) and carbon dioxide (CO2) but this occurs in stages of a complex series of oxidation (burning) reactions. This process is called "carmelization" and is used by cooks and chefs to bring out special flavors of the oxidation products of carefully decomposed sugars. Various chemical classes of compounds called aldehydes, ketones, carboxylic acids and others are formed -- again giving the carmelized sugars their distinctive flavor. Further "cooking" produces more CO2 and H2O and leaves a complex mixture of compounds including some elemental carbon. That carbon is not strictly "ash" since if you continue to heat the carbon it will eventually burn to form CO2. So you can see that the cooking chemistry of sugars is both science and art.

Vince Calder



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