Question

White phosphorus burns in a limited supply of air to form Phosphorus trioxide. When this soft white solid is burnt in plentiful supply of air, we get phosphorus __.

Answer 1

When white Phosphorus burns in limited supply of air,
$P_4\left(s\right)+3O_2\left(g\right)\to P_4{O}_6\left(s\right)$
$P_4{O}_6$ is a white soft, white solid. When this is further burnt in a good suplly of oxygen,
$P_4{O}_6\left(s\right)+2O_2\left(g\right)\to P_4{O}_{10}\left(s\right)$
The overall reaction is very highly exothermic:
$P_4\left(s\right)+5O_2\left(g\right)\to P_4{O}_{10}\left(s\right)$ $\Delta$ H < 0