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Trend 1.8 of the Lithium Group (Group IA) Trends is that the solubilities of the hydroxides in water increase down the column: LiOH < NaOH < KOH < RbOH < CsOH

Atomic size[]

Water solubility of the alkali hydroxides is based on ionic radii. Atomic size increases moving down the column from Li to Cs. The hydroxide ion is relatively small. Alkali metals with large atomic sizes produce relatively unstable lattices with the small hydroxide. Unstable alkali hydroxide lattices therefore more easily dissociate in water.

Thus, increasing atomic size down the column leads to increasingly unstable alkali hydroxide lattices. This then lead to increasing solubility in water.

Group I Hydroxide Lattice Energy Water Solubility (20°C)
LiOH -481.0 kJ/mol 12.8 g/100 mL
NaOH -469.6 kJ/mol 109 g/100 mL
KOH -425.0 kJ/mol 112 g/100 mL
RbOH -413.8 kJ/mol 180 g/100 mL
CsOH -416.2 kJ/mol 300 g/100 mL (30°C)

In addition, the solvation energy of the M+ cation decreases as you go down the group.  This also contributes to increasing solubility.