Steam and liquid water are two different forms of the same chemical substance, water.
In chemistry, a chemical substance is a form of matter that has constant chemical composition and characteristic properties. It cannot be separated into components by physical separation methods, i.e. without breaking chemical bonds. It can be solid, liquid, gas, or plasma.
Chemical substances are often called 'pure' to set them apart from mixtures. Such 'pure' substances are called chemical compounds. A common example of a chemical substance is pure water; it has the same properties and the same ratio of hydrogen to oxygen whether it is isolated from a river or made in a laboratory. However, in practice, no substance is entirely pure, and chemical purity is specified according to the intended use of the chemical.
Chemical substances exist as solids, liquids, gases or plasma, and may change between these phases of matter with changes in temperature or pressure. Chemical reactions convert one chemical substance into another.
Forms of energy, such as light and heat, are not considered to be matter, and thus they are not "substances" in this regard.