The passage that you are looking for is in chapter five. Judge Thatcher knows what a bum Pap is; he knows he's a lousy drunk, father and example to Huck. So, when he petitions the new judge to see if Miss Watson can't adopt Huck for her own, he knows it is for the best because Pap is beyond reform. However, the new judge "said he'd druther not take a child away from its father" and didn't want to separate families if he could help it. So, Huck is stuck with Pap. The new judge decides to attempt to reform Pap, so he brings him to his house one evening, feeds him supper, cleans him up, gives him new clothes, and lectures him about temperance (staying sober) and about being a good example. Well, Pap appears to be mighty touched by all of this, and swears that he is a changed man. He, the new judge, and the new judge's wife all cry and hug and get all blubbery; Pap signs a pledge indicating his change of heart. The new judge is naive enough to put Pap up in the spare room for the night. And of course, Pap
"got powerful thirsty and...traded his new coat for a jug of forty-rod...and rolled off the porch and broke his left arm...and was most froze to death when somebody found him after sun-up."
So, the "reform" lasted all of 30 minutes or so, before Pap was up to his old ways. After that incident, the new judge
"said he reckoned a body could reform the old man with a shotgun, maybe, but he didn't know no other way."
What is ironic about this entire incident is that the very man who blocked Huck's adoption by a nice old lady so that he could stay with Pap, ends up admitting that Pap was worthless. I hope that those thoughts help a bit; good luck!