Detecting acid formation. Pipette being used to add phosphorus trichloride to water that contains universal indicator. Universal indicator is used to indicate acidity or alkalinity. In the water it is green, indicating that the water is neutral. It turns red where the phosphorus trichloride enters the water, indicating that an acidic solution has formed. This evidence helps work out the chemistry taking place. Phosphorus trichloride (PCl3) reacts with water (H2O) to form phosphorous acid (H3PO3) and hydrochloric acid (HCl). The reaction equation is: PCl3 + 3 H2O ----> H3PO3 + 3 HCl