Carbondiooxide does not support in burning but mg burs in it why.?

To keep burning, fires require oxygen. Carbon dioxide is used in certain fire extinguishers to put out flames. Explanation: Because magnesium is higher on the reactivity scale than carbon, it is more reactive and eliminates oxygen from carbon dioxide (to give carbon and magnesium oxide).

we can conclude that the melting point of table salt is above room temperature (it's actually 801 deg c). the melting point of mercury must be below room temperature (- 39 deg c).