About Caramels and Burning Sugar

On this page, you’ll find some experiments concerning the carbohydrates sugar and starch. When you try them, make sure an adult is present! For the iodine-starch reaction you will need chemicals, you normally don’t have at home. So maybe try this experiment in a chemistry lab in school.

Molten Sugar

Common sugar (sucrose) as we know it, is a solid in the form of a crystal or a white powder. Like most solids it can be melted and turns into a liquid above a certain temperature.

When you heat it slowly, it will turn to a clear liquid first, which then changes its colour from yellow to orange to an increasing dark brown. This brown sugar, molten sugar is called caramel. It has a different taste than sugar and is also used as a brown colour, in Coca-Cola, for example.

If you continue to heat the sugar, eventually a black residue will remain: carbon. The sugar molecules have decomposed. Liquid sugar can’t be brought to a boil or be evaporated since it decomposes beforehand.

You can make caramels from molten sugar by yourself:

Grease a piece of parchment paper with some cooking oil and place it on a plate. Put a big spoon of granulated sugar in a small pot and heat it slowly and carefully over low heat. Stir continuously with a spoon. The sugar should become liquid and orange-coloured, but must not be heated too much or else it will char. As soon as all crystals have melted, remove the pot from the burner and pour the liquid sugar onto the paper. Be careful! The caramel hardens quickly, but still remains very hot! Let the caramels cool down properly and voilà, your homemade sweets are ready!

Burning Sugar to Ashes

Place a lump of sugar on a fireproof support, a porcelain dish for example, and try to light it by holding a long match to its side. You will notice that this doesn’t work. The sugar caramelises, but doesn’t burn.

Now rub ashes all over the lump so that it sticks to the surface and try to light the sugar again. The sugar will catch fire and burn producing foam!

The reason for this is substances contained in the ash, which enable the burning of the sugar. Such substances, which facilitate a reaction, but don’t take part in it themselves, are called catalysts.

The Demonstration of Starch

In order to determine whether certain foods contain starch, the iodine-starch reaction can be exploited: In the presence of starch, a brownish iodine solution will turn an intense blue or almost black.

To understand what happens in that experiment, we have to take a closer look at the structure of the starch molecules.

Starch is a polysaccharide consisting of numerous linked glucose molecules. Two components can be distinguished: the water-soluble amylose (approx. 20%) consisting of chains of up to 300 glucose units, and the insoluble amylopectin (approx. 80%) made of branched molecules of up to 1000 glucose units.

The amylose is responsible for the iodine-starch reaction. Its molecule chains are spiral. If iodine is added, the iodine molecules are incorporated into the hollow spaces within the spiral. The resulting compound has an intense blue colour, which can be exploited to detect starch or iodine, respectively.

For the demonstration of starch, an iodine-potassium iodide solution is required. For that purpose, dissolve 1 g iodine and 2 g potassium iodide in 300 ml distilled water.

Pour the ground samples of various foods (flour, bread, rice, vegetables, fruits, sausage...) into test tubes, pour hot water on top of them, and shake well. After cooling down, add a few drops of the iodine-potassium iodide solution. If the solution turns blue, starch is present.