This is a strange question I would like to have some explanation accompanied with:
We have an ice block, with in it a lead block. This entire block is put into a glass of water, resulting in a rise of water level until the edge of the glass. The block stays floating on the surface.
What will happen with the water level when the ice melts?
The answer is that it will drop, but how is this even true??
My thoughts are:
– block floats on water, so rho(block) < rho(water)
– Archimedes law, the Fg of block = Fg of water pushed away.
– Since a smaller volume of water has equal mass (and so Fg) to block, and ice is less dense than water, when it melts, no mass is lost, so it will take in less space??? Please help me with this point of thinking.
Best Answer
Let's deal first with a pure ice cube.
A 100 gram ice-cube (pure water) is floating in a glass of pure water, filled to the very top. By buoyancy laws the ice-cube must be displacing exactly 100 grams of pure water from the glass. The volume of the ice-cube that is below the water level is the volume of 100 grams of water. But that's what the ice-cube will become when it melts. The water level will remain unchanged as the ice melts and exactly fills the hole it occupied. No more water will overflow, and the level will stay right at the top of the glass.
Now let's add the lead, but in a particular way:
You now have a largish ice-cube, floating as before in a full-to-the-top container of water.
What happens if you carefully put 100 grams of lead on the ice-cube, which settles a little, but remains floating. How much, if any, water will overflow?
Remove the 100 grams of lead from the ice-cube. What will happen to the water level?
Put the lead, without splashing, into the water beside the ice-cube and let it settle to the bottom. What will happen to the water level? BY how much?
Now let the ice-cube melt. How does the water level change? (See first part of answer)