Why do non metal ions change from being soluble in solution to insoluble at the surface of the cathode

An experiment is conducted to see if cats preferred skim milk or 2% milk. a cup of skim milkwas put out for 5 kittens and then measured how much the kittens drank over the course of aday. following a cup of 2% milk was purout for the skittens and then masured how much thekittens drank over the course of a day. the same kittens were used and the milk was served atthe same temperature. it was discovered that the cats liked the 2% milk more than the skimmilk. what is the dependent variable in this experiment?