When sugar is heated for a short time and forms a thick, colorless liquid, it undergoes a physical change.

Here's why:

* No new substances are formed: The sugar molecules themselves are not changing their chemical structure. They are simply rearranging and dissolving in their own water content.

* The change is reversible: If you cool the liquid down, it will solidify back into sugar.

This process is called melting, and it is a physical change because it involves a change in state (solid to liquid) but not a change in chemical composition.