Melting sugar is a physical change because the substance is still sugar. However, burning a sugar cube is a chemical change.
Let me explain…
Fire activates a chemical reaction between oxygen and sugar. The oxygen in the air then reacts with the cube of sugar and the chemical bonds get broken.
Remember, Sugar starts melting at around 140∘C and after that, decomposition into a brown substance begins.
This browning of sugar is what is referred to as caramelization. At this period, the sweet taste of sugar changes to bitter. If further heated, it begins to turn black and give off bad fumes as the sugar transforms to carbon, while oxygen and hydrogen evaporate as organic fragments. At this point, the change becomes chemical.
A word of caution:
The caramelization process is highly temperature-dependent. So, certain sugars undergo the process readily at different points.
Also, impurities in the melting sugar, such as molasses remaining in brown sugar, greatly speed up the reactions.
It is a type of non-enzymatic browning reaction. As the process happens, volatile chemicals are released forming the characteristic caramel flavor you might remember.
However, we would advise great caution while performing this experiment as melted sugar is very hot and is very adhesive. It will cause some serious burns if it falls on the skin. Plus, sugar heating very fast can ignite to form carbon dioxide and water vapor.
ALSO SEE: Is Sugar Burning A Chemical Change or a Physical Change?
Sugar Melting Point and Boiling Point – Melting Sugar: Chemical Reactions
The solubility of Sucrose is very high: up to 2000 grams of it can be dissolved in one litre of water! However, it is mainly because of the way it reacts that it has become the protagonist of our sweetest recipes.
The chemical reaction most familiar to us is melting, which is when sugar decomposes at a temperature between 184 and 186°C. This recent discovery exists thanks to a team of researchers in Illinois. Basically, when we heat sucrose gently, we get what is known as “apparent melting”. In simple terms, sugar crystals don’t actually melt, but they go through a process called “inversion”. What really happens is that the two molecular components of sugar – glucose, and fructose – decompose. In their turn, they give way to “caramelization”, which happens in two phases.
In the first phase, the structure of sugar changes as the heat increases. We can easily observe this change when we see sugar begin to “melt”. At this point, the second phase kicks in – the extra increase in heat leads to the elimination of the water molecule. This in turn produces a reaction called “beta-elimination” which leads to the formation of hydroxymethylfurfural. The substance darkens in color and tastes more and more like caramel. If too much heat is applied, the sugar will break down into carbon, which means that our caramel is well and truly burnt!
In short, heating sucrose can lead to some interesting reactions, like “inversion.” When sugars are heated with an acid, they become “inverted” sugars, which are very hygroscopic. This means they can absorb a lot of water, making them ideal for preparing soft sweets and desserts, or for any recipe we wish to keep moist when exposed to the air.