Is sugar a molecule or a large crystalline structure composed of many molecules? What do we mean when we say sugar?
closed as off-topic by Mithoron, Tyberius, Jon Custer, Todd Minehardt, airhuff Dec 1 '18 at 19:27
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Terminology can be confusing especially when some terms are only vaguely defined.
"Sugar" is one of those vague terms. There is a whole family of sugars recognised by chemists. In casual use what people usually mean is sucrose, a specific sugar molecule made from two other sugars, fructose and glucose, joined together. this is produced from sugarcane or sugar beet and is the dominant form of sugar seen by non-chemists.
But is sugar the molecule or the crystal? Both. Many pure chemicals form crystals, indeed crystallisation is a very common way of producing purer chemicals. Those crystals consist of a regular 3D array of a single molecule (for a pure chemical). So "sugar" describes both the molecule and the crystals of that molecule which consist of a large number of them arranged in a regular structure.
To answer your question I will separate it into two parts, crystal and sugar.
I wills start with the concept of a crystal, for that I will cite wikipedia since I couldn't find the IUPAC definition of a crystal:
A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions.
So basically stating a crystal is a structure that has a unit-cell that repeats in all direction. Every unit-cell is exactly the same as the other ones.
Sugar is a carbonhydrate. It may constitute of one (example glucose), two(example sucrose), or even a chain of sub-units (example starch).
The white sugar that one buys at a store is crystalline sucrose. It is not a big crystal as a slab of Aluminum is. But it is still a crystalline structure made of a repeating unit-cell which is sucrose molecules.