EK 1001 Phys 527 Ice and Density

[aln012]

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An ice cube sits in a container that is exactly the same size as the cube. Which of the following is true, if the ice is allowed to melt?

A. Water level remains the same
B. 10% of the water will spill over
C. Water will only occupy 90% of the container.
D. Random answer

Answer is C

So, elsewhere, I learned that when an ice cube is floating in water, and it melts, then the water level doesnt change. So the question I have is, if it's pure ice, and it melts, the water level changes, but if it's floating in water, then the water level doesn't change? Can someone explain why? Thanks!

 

[MT Headed]

So the question I have is, if it's pure ice, and it melts, the water level changes...

If you have pure ice, then you don't really have any water level to change.

Try thinking of ice as a mixture of solid water and air. In this question 527, the ice melted, all the water went to the bottom of the glass, and all the air went to the top, still inside the glass. The new water level is below the old ice level.

In your other scenario, the ice was floating in a sea of water. Some of it was below the waterline, some was above the waterline. It melted, so again all the water went to the bottom (i.e. occupies the space below the air/water line) and the air went to the top (above the air/water line).

This is a gross simplification, but I hope it at least gets the basic concept across.

 

[Catburr]

An ice cube sits in a container that is exactly the same size as the cube

This is the critical part of this question. It's not talking about floating ice cubes, we're talking about an ice sculpture in a mold, essentially.

So quite simply, because water has a higher density than ice (even pure ice that is devoid of air bubbles), the water will take up slightly less room than the ice did. Conversely, if you fill a container with water and stick it in the freezer, you will see that the ice expands as it freezes. Same mass of H2O molecules, more space taken up.

Try thinking of ice as a mixture of solid water and air. In this question 527, the ice melted, all the water went to the bottom of the glass, and all the air went to the top, still inside the glass. The new water level is below the old ice level.

This is a good analysis of a real life ice cube floating in a glass, but I'd disagree that it's the logic behind this particular question.

 

[aln012]

thanks guys! i understand it a lot better now.