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 does not result in a new substance. When sugar dissolves in warm water, it is just dispersed in the water and no new chemical substance is formed. Iron rusting, wood burning, and baking soda - acid reaction all result in new substances (rust, carbon - dioxide and other products, and carbon - dioxide gas respectively), which are chemical changes.

Answer:

sugar dissolving in warm water