The purest form of mercury is elemental mercury, also known as liquid mercury. It has the chemical symbol Hg and is a silvery-white, dense, liquid metal at room temperature.

Here's why it's considered the purest form:

* Elemental: It consists solely of mercury atoms, with no other elements or compounds present.

* Liquid: It exists in its natural state as a liquid, meaning it doesn't require any additional processing or purification to achieve its purest form.

While elemental mercury can be further purified to remove trace impurities, it's already the purest form of the element itself.

It's important to note that mercury is highly toxic and should be handled with extreme caution.