Question

$H_{2}O$ contains hydrogen and oxygen but it is a pure substance. How?

Answer 1

Hint: A pure substance is something that has uniform composition and cannot n=be separated into components by physical or non- chemical methods. And elements have uniform composition and they cannot be separated by any chemical method.

Complete Step by step answer: Pure substances are the one that is composed of only one type of molecule or atom. For example pure gold, it only contains gold atoms. Or pure salt which only contains sodium chloride molecules.
Water is a pure substance because it contains only one type of molecule. Pure water is made by the chemical combination of hydrogen and oxygen. One oxygen is bonded to two hydrogen. And it always has fixed composition. These atoms here are joined by chemical bonds to form molecules, and hence it will have its own chemical as well as physical properties. It is not the same as a mixture of oxygen and hydrogen. For example hydrogen is a highly flammable gas while oxygen makes the fire worse, and both the gases have its own properties. On the other hand water has been used to extinguish fire. Because water is a different molecule and it has its own set of properties, not like the combination of two atoms from which it is made.
So water is not a mixture of hydrogen and oxygen. It is a chemical compound which has its own set of properties.

Note: While water is a pure substance, not all water is pure. Mineral water or water from the tap contains dissolved salts and minerals. That does not mean the water which is not pure completely is not safe. In fact dissolved minerals and salt are beneficial to us.