It is a white solid. It is a weak reducing agent in air. It oxidizes to mercury(II) oxide and mercury(II) nitrate. It reacts slowly with water when it is dissolved in water. This makes a basic mercury(I) nitrate and some nitric acid. The basic mercury(I) nitrate is yellow. Mercury(I) nitrate can kill you if you eat or inhale it.
It is made by reacting mercury with a dilute solution of nitric acid. A concentrated solution of nitric acid makes mercury(II) nitrate.