What aldehydes and ketones are water soluble?
The general rule of thumb is that aldehydes and ketones with fewer than six carbon atoms are water soluble.
The solubility of an aldehyde or ketone is a competition between its polar "head" and its nonpolar "tail".
Small aldehydes and ketones are miscible with water in all proportions, because they can form hydrogen bonds with water.
However, solubility decreases with chain length, because the hydrocarbon "tails" of the molecules have to force themselves between water molecules.
They have to break the strong hydrogen bonds between water molecules without replacing them with anything as good.
This makes the process energetically less favourable, so solubility decreases.
If you define "soluble" as "greater than 1 g/100 mL", the dividing line is between five and six carbon atoms.
Hexanal and hexan-2-one are insoluble, but pentanal and pentan-2-one are soluble.