How can you tell the difference between a mixture and a pure substance

Mixtures are combinations of materials that can be separated by in most cases time. For instance, oil and water is a mixture. You can tell this by placing the two chemicals in a container and shaking them. They will mix and look like one substance, however, they will separate into the two liquids after some time is given. Assuming that "pure substance" is a solution, a substance is similar to a mixture, but the materials used in the solution are not separable. Over time the materials in the containing will not separate. I believe this is the answer you are looking for. 

The answer is b carbon-14. the radioactive isotope of carbon used in carbon dating has a half-life of 5730 years. potassium-40 is used to date 1.25 billion years