Single most (what?) substance
---common?
---important?
---fragile?
This question is not formed properly and thus can not be answered
carbon
Carbon.
Water would be the most abundant substance but if you're asking about elements then oxygen would make up the greatest percentage of body mass.
a substance
An element is a single kind of matter that is pure. It consists of atoms with the same number of protons in their nucleus. Examples of elements include oxygen, gold, and carbon.
If a single atom of a hunk of matter is still the same substance as the matter, then that matter is an element. (Gold, sodium, carbon, uranium, and so forth) If the matter can only be broken down to a molecule, but not an atom, then it is not an element. (Salt, ammonia, sugar, alcohol, and so forth)
mixture bobsaysftw YouTube
a pure substance! ;)
A single substance of matter refers to a material that has a uniform and definite composition, characterized by distinct physical and chemical properties. This can include elements, like gold or oxygen, which consist of only one type of atom, or compounds, like water or carbon dioxide, which are made up of two or more types of atoms chemically bonded together. In essence, a single substance is a homogeneous material that cannot be separated into different components by physical means.
H20
substance.
a pure substance. :)