Boiling water is not considered a chemical reaction because it involves only a physical change, not a change in the chemical composition of the substance.
When water is heated to its boiling point, the molecules gain energy and move faster, transitioning from the liquid phase to the gas phase (steam). However, the chemical structure of water (H2O) remains the same throughout this process. No new substances are formed, and the changes are reversible; if you cool the steam, it will condense back into liquid water.
In contrast, a chemical reaction involves the formation of new substances through the breaking and forming of chemical bonds. For example, when hydrogen gas reacts with oxygen gas to form water, the atoms are rearranged and a new substance is created. This is what distinguishes a chemical reaction from a physical change like boiling.