It's a combustion.

Combustions are part of what class of chemical reactions?

Combustions are oxidations

This reaction produces heat. It is an ... reaction.

exothermic

The white solid is Phosphorus (V) oxide. Formula?

In theory P₂O₅ ( In reality P₄O₁₀ ).

What is the meaning of the yellow color of bromothymol?

It means that P₄O₁₀ reacts with water to form an acid.

What is the formula of that acid? Describe it!

Phosphoric acid H₃PO₄. It is a colorless and water-soluble substance.

Write down the equations of the two observed reactions!

P₄+5O₂→ P₄O₁₀ P₄O₁₀+6H₂O→ 4H₃PO₄