Deliquescent and hygroscopic

A hygroscopic chemical absorbs water from the atmosphere. Silica gel is hygroscopic and is often found in small bags used to keep sensitive equipment (like cameras) dry.

Self-indicating silica gel showing dry blue crystals and wet pink crystals

Deliquescence is the process in which a chemical absorbs sufficient water from the surrounding air so that it forms an aqueous solution. Anhydrous iron(III) chloride is a typical deliquescent chemical:

The black anhydrous iron(III) chloride absorbs moisture from the air becoming an orange solution

It is difficult to weigh out these materials as they gain water and mass as they are being weighed. Consequently, they are not usually used in analytical work - for example, to make a primary standard for titration work.