Chemical - Essentially the composites of everything.
Just like Atoms are the building blocks of everything, atoms form Chemicals and chemicals form substances.
A few examples
Wood - mostly Cellulose which is a long chain of ethers, really just a carbohydrate that's impossible to digest.
Drugs - A chemical compound consumed for its chemical effect (Caffeine, heroin etc')
Food - A chemical compound (usually a former organic lifeform of sorts and often mostly carbohydrates) consumed for its chemical effect
Air - A gas, of chemicals, inhaled for its (particularly the O2) marvelous chemical effect of oxydation. Unfortunetly also frequently contains trace amounts of benzene and other petrols, carbon monoxide, in certain urban areas Ozone or Methane in significant amounts all of which produce rather unpleasent side effects to breathing.
Water (H2O) - A chemical solvent consumed for a chemical solvent in all the reactions constantly undergoing in our bodies. AKA dihydrogen monoxide. Usually the water you drink will contain trace amounts of Chlorine or Fluorine, Calcium Ions and Sodium Ions. If you drink bottled water probably no Chlorine/Flourine but quite a number of metal ions.
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