(c) A student uses this apparatus to determine the formula of magnesium oxide. This is the student's method. - find mass of empty crucible and lid - place some magnesium ribbon in crucible - find mass of crucible, lid and magnesium ribbon - heat crucible, lifting lid occasionally - after five minutes, find mass of crucible, lid and magnesium oxide (i) Explain why the student lifts the lid. (c) A student uses this apparatus to determine the formula of magnesium oxide. This is the student's method. - find mass of empty crucible and lid - place some magnesium ribbon in crucible - find mass of crucible, lid and magnesium ribbon - heat crucible, lifting lid occasionally - after five minutes, find mass of crucible, lid and magnesium oxide (i) Explain why the student lifts the lid