Colour
Pure nitric acid is a colourless liquid. But commercial nitric acid may be yellowish brown, due to the presence of dissolved nitrogen dioxide.
Odour
Nitric acid is a fuming, hygroscopic liquid, the fumes of which give it a choking smell.
Taste
It is sour in taste.
Density
Its density is 1.513 g cm-3 at 20oC. Thus pure nitric acid is about 1½ times as dense as water while commercial nitric acid has a lower density.
Boiling point
Pure nitric acid boils at 86oC. However it undergoes partial decomposition at this temperature.
Melting point
When cooled below 0oC nitric acid freezes to a white solid, which melts at - 42oC.
Solubility
It is soluble in water in all proportions.
Concentration
Nitric acid can be concentrated up to 68%. It cannot be concentrated beyond this percentage by simply boiling because the aqueous solution of this concentration of nitric acid forms a constant boiling mixture at 121oC.
However, the acid can be further concentrated by one of the following methods:1) By passing a mixture of nitrogen dioxide and air through the 68% aqueous solution. This solution can be further concentrated by distilling it with concentrated sulphuric acid under reduced pressure. By this method nitric acid of 98% concentration can be obtained.
2) If the 98% acid is cooled to -42oC, then pure nitric acid of 100% concentration crystallises out as colourless crystals.