A student has three samples. The first is a white waxy solid that glows in the dark and dissolves…

Step 1. The samples. The white waxy solid is white phosphorus, made of $\text{P}_{4}$ molecules. The red powder is red phosphorus, made of chains. The black solid is black phosphorus, with a layered structure. It is a semiconductor.

Step 2. Storage. White phosphorus catches fire in air, so it is kept under water.

Step 3. The match box. The striking strip uses red phosphorus, which is safe and not poisonous.

Step 4. Why white is reactive. In the $\text{P}_{4}$ tetrahedron each angle is 60°, far from the 90° that p orbitals prefer. The strain makes the bonds break easily. Red phosphorus has less strain.