Substances that absorb moisture from the atmosphere to become sticky are said to exhibit hygroscopy. Examples are \(\mathrm{CaO}_2 \mathrm{NaNO}_3\) and \(\mathrm{CuO}\).
Some substances absorb water up to the point that they turn into solutions. They are said to exhibit deliquescence. Examples are \(\mathrm{CaCl}_2\), \(\mathrm{NaOH}, \mathrm{P}_2 \mathrm{O}_5\) etc.
There are some that lose their water of crystallization to the atmosphere. These are said to exhibit efflorescence.xamples are \(\mathrm{Na}_2 \mathrm{CO}_3 .10 \mathrm{H}_2 \mathrm{O}, \mathrm{CuSO}_4 .5 \mathrm{H}_2 \mathrm{O}\) and \(\mathrm{ZnSO}_4 \cdot 7 \mathrm{H}_2 \mathrm{O}\)