Which thing the word "it" refers in this sentence? And can I omit "it"?

"To pollute means to contaminate-to spoil something by introducing impurities that make it unfit or unclean to use."
I don't know which part the word "it" refers.
Which thing the word "it" refers in this sentence? And can I omit "it"?

'It' refers back to 'something'. You can't omit it.