Is water boiling a chemical reaction?

No, boiling water is NOT a chemical reaction, but it is a physical change.

When water, H2 O, is in liquid form, it can be heated to a boil. Boiling occurs by the process of convection, in which water closest to the heat source becomes warm and rises. The cooler water sinks, and this continues as a circulation pattern until all the water is hot and begins to boil. As the water is boiling, it is not undergoing any chemical reaction. It is still water, even as it boils. It has simply changed temperature.

As it continues to boil, water molecules will undergo phase changes and will gain enough energy to enter the air as water vapor. This is still a physical change, as water vapor in the air is still water, H2 O. No new products have been created, and this is a reversible reaction.

Molecular Changes

A chemical reaction occurs when substances combine and react with each other, changing their molecular structure and creating new products. Chemical reactions typically give off heat, and are not reversible. A physical change, by contrast, is one that changes only the physical features of a substance.