Sugar is the compound everyone knows for its sweetness. On the molecular level, however, it goes by the formula [[$ \text{C}_6\text{H}_{12}\text{O}_6 $]]​. In other words, a sugar molecule consists of 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms bound together with covalent bondsFrom covalent bonds into molecular compounds.​ You can see the 3D structure model of a sugar molecule with your own eyes here.

Sulphuric acid, on the other hand, is a strong dehydrating agent, so it has the capability to cleave water molecules off of other particles. When added on top of sugar, the colour of the yellowish acid turns slowly into a coke-like brown, and slowly but surely towards black. Once the reaction truly starts, a steaming black structure grows out of the container.

The question is: what does the tower and the gas consist of?

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If you look closely, the composition of [[$ \text{C}_6\text{H}_{12}\text{O}_6 $]] has a part that ressembles water, namely [[$ \text{H}_{12}\text{O}_6 $]], which has all the atoms to make up 6 water molecules [[$ \text{H}_{2}\text{O} $]]. This can be rewritten as [[$ 6\text{H}_{2}\text{O} $]].

Once the acid removes the six [[$ \text{H}_{2}\text{O} $]] molecules from [[$ \text{C}_6\text{H}_{12}\text{O}_6 $]], we only have carbon left. As individual carbon atoms, they are written as multiples: [[$ 6\text{C} $]]. In other words,

[[$$ \text{C}_6\text{H}_{12}\text{O}_6 \rightarrow 6\text{C} + 6\text{H}_{2}\text{O} $$]]

This equation explains that the black tower structure consists of carbon atoms [[$\text{C}$]], and the gas is water vapour [[$\text{H}_2\text{O}$]].