Potassium nitrate has an orthorhombic crystal structure at room temperature. It will be transformed to a trigonal structure at 129 °C (264 °F).
Potassium nitrate is moderately soluble in water, but its solubility increases by temperature. The aqueous solution is almost neutral, exhibiting pH 6.2 at 14 °C (57 °F) for a 10% solution of commercial powder. It is not very hygroscopic, absorbing about 0.03% water in 80% relative humidity in over 50 days. It is insoluble in alcohol and is not poisonous; it can react explosively with reducing agents, but it is not explosive on its own.
Potassium nitrate has a wide variety of uses and is largely used as a source of nitrate. It is used in fertilizers as a source of nitrogen and potassium – two of the macronutrients are for plants. When used by itself, it has an NPK rating of 13-0-44.
The most famous use of potassium nitrate is probably as the oxidizer in black powder.