Question

Which non-metal catches fire if it is exposed to air ?
A.Sodium
B.Phosphorous
C.Calcium
D.Uranium

Answer 1

The non - metal which catches fire at room temperature should be the one which is most reactive or unstable at room temperature .

Complete step by step answer:
Phosphorus is a non metal that catches fire when exposed to air.
It is a highly reactive metal and spontaneously reacts with oxygen to catch fire.
Phosphorus is a non metal , which occurs in three forms white , red and black White phosphorus appears translucent. It is a waxy solid and is poisonous .It is insoluble in water but soluble in $C{S_2}$ , alcohol and ether .It has a garlic odour.
This form of phosphorus is the most reactive among all the three allotropes.
It also shows the property of phosphorescence , that is , it glows in dark .
One of the reasons of white phosphorus being more reactive than red phosphorus is angular strain , that is it has a structure in which it is aligned at an angle of ${60^ \circ }$ .
So when white phosphorus is exposed to air , it catches fire . The reaction is highly exothermic and dense white fumes of ${P_4}{O_{10}}$ are formed.
${P_4} + 5{O_2} \to {P_4}{O_{10}}$ where , $\Delta H = - ve$
Therefore, phosphorus is stored under water to avoid burning on reacting with oxygen.
So option B is correct.

Note:
It can be noted that an element which is unstable is more reactive , here phosphorus is highly reactive because of angle strain and therefore can even react with oxygen at room temperature .