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phosphorus(noun) and phosphorous(adjective). Can you explain me with some sentences when to use these forms.
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Not much to explain, Guest-- you have said it already: 'phosphorus' is a noun and 'phosphorous' is an adjective. So when you are writing about the chemical element, you use the noun, as in 'Phosphorus has an atomic number of 15 and an atomic weight of 30.973761'; and when you are writing about something which is phosphorus-like, you use the adjective, as in 'Phosphorous acid is handy for making phosphites'. A confusion arises, however, in that 'phosphorous' is also an alternative spelling for 'phosphorus'.

Good luck in all your endeavors.
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And, although both may be pronounced the same, the noun is "PHOSphorus", and the adjective is most often "phosPHORous" (like "phosPHORic"). At least that's how my chemistry teacher said them.
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Comments  
This is not correct. "Phosphorous" is NEVER an alternative spelling for "phosphorus". "Phosphorus" is the spelling for the chemical element and is a noun. "Phosphorous" is a specific chemical description used when phosphorus is in a particular arrangement, known as "oxidation state". When phosphorus is in a compound with an oxidation state of "phosphorous".

So, using sulfur as an analogy, you can have "sulfur", "sulfuric acid" and "sulfurous acid" - all being different chemicals. Similarly you can have "phosphorus", "phosphoric" acid" and "phosphorous acid" - the "phosphorous" description is specific to its chemical state.
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