A change in the chemical nature (the chemical characteristics) of elements when they unite to form a chemical compound. Hydrogen is a lighter-than-air gas, and oxygen is also a gas. But when two atoms of hydrogen unite with one atom of oxygen, one molecule of water (H2O), a liquid, is formed.

The compound water has different chemical characteristics from those of either of the elements oxygen or hydrogen.

When hydrogen and oxygen combine to form water, a chemical reaction takes place.