Substances are characterized by their properties and by their composition. Color, melting point and boiling point are physical properties. A physical property can be measured and observed without changing the composition or identity of the substance.
On the other hand, the statement “gaseous hydrogen burns in the presence of gaseous oxygen to form water” describes a chemical property of hydrogen, since to observe this property a chemical change must take place, in this case combustion. After the change, the original gases, hydrogen and oxygen, will have disappeared and a different chemical substance, water, will remain. It is not possible to recover hydrogen from water by a physical change such as boiling or freezing.
