but the effective resistance will be lower right? So, less work done to flow from one point to another larhh..... Voltage should drop lah....

Wrong concept. First, Charles's Law does NOT apply here. When Ice turns into liquid there's a decrease in volume.

You cannot use charles's law when there involve a change of state in between like liquid > ice.

General Knowledge:
1) When water turn into ice, there's an increase in volume.
2) Hence, a decrease in density. (P.S. refer Bio form4 textbook chapter 4)

Haven't you all done a question about a floating ice cube on water(with part of it above water surface)? when the ice cube melts, water level will still remains the same. This is because liquid have higher density +  lower volume than the same mass of ice. 

Refering to the question above, I have no idea why this happens unless youre heating the water well above 0 degree (say like heating it until 80). Then the answer is possible.

Btw, did I ever mentioned? I am a SPM student as well