A pure substance is any single type of material that has not been contaminated by another substance. Water is considered a pure substance if the water contains only hydrogen and oxygen.
Other examples include gold, silver and salt. In chemistry, a pure substance has a definite composition. It can be a compound or a single element. The material is no longer a pure substance if it has been mixed with another pure substance. Two pure substances mixed together is known as a mixture. Scientists often use filtration to separate pure substances from a mixture in order to analyze the materials.