Why do some compounds adsorb water from air?

1. ionic compounds:
a. coulomb interaction
i. cation: Mⁿ⁺
ii. anion: SO₄^2-, NO₃^-
iii. both cation and anion: CuSO₄‧5H2O
b. deliquescence
hydroxide (OH^-)
2. porous materials: capillary condensation
zeolites (SiO₂)

1. 離子化合物
a. 庫倫作用力
i. 陽離子
ii. 陰離子
iii. 陽離子及陰離子
b. 潮解
2. 孔洞材料

ref: http://antoine.frostburg.edu/chem/senese/101/compounds/faq/why-hygroscopic.shtml