This is not a chemistry question; it is an English question.
Both 'burned' and' burnt' are perfectly correct past participles, and the past tense, of the verb 'to burn'. However, you will never see 'burnt' used when the burning relates to emotion rather than chemical conflagration, and 'burned' is more likely to be used by an American.
Thus, I burned with anger; I burnt the toast; I burned my papers.