In practice, however, it is very difficult to achieve perfect dryness; anhydrous compounds gradually absorb water from the atmosphere so they must be stored carefully.
Molecular sieves are far more effective than most common methods for drying solvents and are safer and require no special equipment for handling.
[4] Typically anhydrous solvents will contain approximately 10 ppm of water and will increase in wetness if they are not properly stored.
Several substances that exist as gases at standard conditions of temperature and pressure are commonly used as concentrated aqueous solutions.
To clarify that it is the gaseous form that is being referred to, the term anhydrous is prefixed to the name of the substance: Reactions which produce water can be kept dry using a Dean–Stark apparatus.