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Is sugar a molecule or a large crystalline structure composed of many molecules? What do we mean when we say sugar?

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    $\begingroup$ Sugar is white powder used for sweetening our tea; the moment you realize it consists of molecules, it ceases to be called "sugar". The related molecule, and also the crystalline structure composed of these molecules are both called "sucrose". $\endgroup$ – Ivan Neretin Nov 26 '18 at 11:09
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    $\begingroup$ You're asking another time about the same misunderstanding! Sugar is compound which can be in multiple states, just like water, oxygen etc. Compound or element doesn't equal atom/molecule/crystal /gas/liquid but can be in one of these forms. Is it clear now? $\endgroup$ – Mithoron Nov 26 '18 at 18:32
  • $\begingroup$ So basically, “element” is a way of referring to a certain type of material made of one type of atom regardless of how much you have? So one atom of sodium is “sodium” and a collection of sodium atoms can still be called “sodium”? $\endgroup$ – user70417 Nov 27 '18 at 4:18
  • $\begingroup$ yes, but generally we use extra words to help remove ambiguity, for example sodium metal, or sodium vapour or sodium ions, or sugar solution or sugar crystals. $\endgroup$ – porphyrin Nov 27 '18 at 8:55
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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.

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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.

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  • $\begingroup$ A slab of aluminum is not one big crystal either. $\endgroup$ – Ivan Neretin Nov 26 '18 at 12:14
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    $\begingroup$ "carbonhydrate" is not a correct spelling $\endgroup$ – mykhal Nov 26 '18 at 12:48