A compound is a pure substance that is made from more than one element. In a compound, elements are chemically bonded together, which makes it very difficult to separate them. Compounds are not ...

Compounds are made from two or more elements that are chemically bonded together. Compounds are a second type of pure substance. Compounds always have the same ratio of elements. For example, Pyrite ...

Earth’s core could contain helium from the early solar system. The noble gas tucks into gaps in iron crystals under high pressure and temperature.