Question

which of these is an example of a physical change?
sugar dissolving in warm water
iron rusting in the presence of moist air
wood burning in the presence of oxygen
baking soda reacting with acid to form a gas

Explanation:

A physical change is a change in which no new substances are formed. When sugar dissolves in warm water, it is still sugar and water at the molecular level. Rusting of iron forms iron oxide (new substance), wood burning forms ash, carbon dioxide etc. (new substances), and baking soda reacting with acid forms carbon dioxide (new substance).

Answer:

sugar dissolving in warm water