The teacher burns a strip of magnesium ribbon, which leaves a white powder
The teacher burns a strip of magnesium ribbon, which leaves a white powder. She dissolves the powder in a little water and asks the class what it will do to red litmus paper. Juhi predicts it will turn red litmus blue, because the powder comes from a metal. Pranjal says everything made by burning is acidic, like smoke, so the red litmus will stay red. Whose prediction is correct?
Who do you think is right? Pick one.
The answer: Juhi is right, metal oxides are basic
Why
Pranjal has noticed something real: when some things burn — sulphur, for example — the gas formed does dissolve in water to make an acidic solution. But he is stretching one case into a rule for everything. What matters is WHAT burned. When magnesium burns, it combines with oxygen from the air to form magnesium oxide, the white powder. Oxides of metals dissolve in water to form basic solutions; oxides of non-metals such as sulphur form acidic ones. So the magnesium oxide solution turns red litmus blue. This is one of the handy ways the chapter tells metals and non-metals apart: look at what their oxides do to litmus. Test: the same check done on the solution of burnt sulphur's gas (a teacher-only demonstration, because the fumes are harmful) gives the opposite result — blue litmus turns red. Juhi is right: magnesium is a metal, its oxide is basic, and the solution turns red litmus blue.
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