In Free State
Nitric acid is present in small quantities in the atmosphere. But during thundershowers, a large quantity of this acid is produced in the atmosphere. It then comes down as a very dilute solution in rainwater. This nitric acid present in the acid rain reacts with the minerals present in the soil and gets converted into nitrates.
In Combined State
Nitric acid occurs in the combined state as Bengal salt petre (Potassium nitrate) or nitre, Chile salt petre (sodium Nitrate), Norway salt petre (ammonium nitrate), lime salt petre (calcium nitrate) etc.
