What is a pure substance? element, compound, mixture

Write the symbol for every chemical element that has atomic number greater than 3 and atomic mass less than 12.0 u.

Which of the following is always a reactant in a combustion reaction? oxygen nitrogen hydrogen carbon

Which statement correctly identifies the scientific question and describes why the question is scientific? question 1 refers to the supernatural.question 2 reflects a moral or social value.question 3 refers to something that can be measured.question 4 reflects a question that can’t be observed.