What is required for natural selection to occur

Reproduction, heredity, and variation are important for natural selection to occur. Reproduction is the process by which one organism give rise to new organism so the traits that are more suitable for a species survival will be selected over non- suitable traits over many generations.

Heredity also plays an important role in natural selection simultaneously with reproduction because the advantageous trait that evolved during several reproduction is passed to the next generation by parents which confirms the survival of their offspring.

Variation occurs in a gene that can be transferred to the next generation. Variation that is helpful in a population to survive are favored by natural selection.  

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