Liquid water (H_{2}*O) is formed when two gases, hydrogen (H_{2}) and oxygen (O_{2}) , react. Why are the properties of water different than those of the reactant gases?

Answer
When substances chemically combine, products with completely different properties form
Explanation
The reaction between gaseous hydrogen and oxygen to form water is a chemical change. Hence, when substances (like that of gaseous hydrogen and oxygen) chemically combine, products (water) with completely different properties form.