explain why there are more substances on earth than are listed on the periodic table of elements.

Brief Explanations:

Elements can combine in various ways to form compounds. Different combinations and ratios of elements create numerous substances. For example, hydrogen and oxygen can form water ($H_2O$) or hydrogen peroxide ($H_2O_2$). So, the number of possible combinations leads to more substances than just the individual elements on the periodic table.

Answer:

Elements can combine to form compounds, resulting in a greater number of substances than the individual elements listed on the periodic table.