Explain how species can change over time

This is generally called evolution. Species adapt to their surroundings to survive. Sometimes parts are added to help them, other times things are taken away to help them. Features are added to their bodies and minds to help them survive. Over time their bodies change.

hydrogen is a compound or a single element and a element is a pure substence that cannot be separated into simpler subtances

clorine is a compound its a chemical substance that forms when two or more different elements combine

so both hydrogen and clorine are compounds