Nitrogen oxyanions and oxyacids
Nitrate, NO3-, and nitrite, NO2-, are the two most common nitrogen oxyanions. Nitrate is more stable than nitrite. This stability difference is the reason why nitrates are less reactive than nitrites. Nitrate and nitrite are, therefore, used for very different applications.
Nitrate, NO3-, is a stable oxyanion with a high solubility in water. The high solubility of nitrates makes them an important part of fertilizers, as plants can easily absorb dissolved species. The acid of nitrate is called nitric acid, HNO3. The high solubility of nitrate makes nitric acid a strong acid. Therefore, nitric acid used to be known as aqua fortis - strong water - and can dissolve silver but not gold. As a result, aqua fortis can separate these two precious metals from each other, just like the alchemists tried to achieve with the philosopher's stone.
Nitrite, NO2-, is more reactive than nitrate. It tends to react in ways that inhibit bacterial growth - without forming very harmful substances in the process. This combination means it is useful in food preservation, e.g. when curing meat. The nitrite gives the meat a nice pink color along with a savory flavor. The corresponding acid of nitrite is nitrous acid, HNO2. In the same way that nitrite will give a pink color to meat, nitrous acid will produce various colors when mixed with organic molecules. Therefore, nitrous acid is part of the Liebermann reagent, which we use for detecting drugs.