If you have a gallon of water and want to freeze part boil the rest then mix them back together after ice melts it be about 70F what portions would boil?

It is not possible to boil part of a gallon of water and then freeze the rest, and have the mixture be 70°F after the ice melts. When water boils, it turns into steam. When steam cools down, it turns back into water. You cannot freeze steam, and you cannot boil ice.