When phosphorus reacts with nitric acid, it undergoes an oxidation-reduction reaction. Nitric acid is a strong oxidising agent, and phosphorus is a reducing agent. The reaction between the two produces phosphoric acid and nitrogen dioxide gas. The overall reaction can be represented as follows:

$$4P + 10HNO_3 -> 4H_3PO_4 + 10NO_2 + H_2O$$

In this reaction, phosphorus is oxidised from a neutral state to a +5 oxidation state, while nitrogen is reduced from a +5 oxidation state to a +4 oxidation state. The reaction is highly exothermic, and the products are typically formed as a dense white smoke.