Question: a. The teacher puts a sugar cube into a container of warm water. Eventually the sugar cube is no longer visible. What type of change is this and why?

Answer:This is the full equation for the reaction if you wanted to know: ... A physical change in a substance doesn't change what the substance is ... when cooled and then can be returned to a liquid form when heated.

Answer: physical change.

Explanation:

A physical change is defined as a change in which there is alteration in shape, size etc. No new substance gets formed in these reactions. Example: melting of ice

A chemical change is defined as a change in which a change in chemical composition takes place. A new substance is formed in these reactions. Example: Combustion

As the sugar cubes dissolves into a container of warm water, it only changes its state from solid to aqueous and no new substances are being formed. Thus it is a physical change.