Saturday, January 2, 2010

A Cube of Ice


You pour yourself a glass of water, drop a cube of ice and see that water edge is only a few millimeters away from the top of the glass. Will it overflow when the ice melts?

Here is a relevant story about golden wreath, Archimedes and Eureka moments of our households.

Enter your answer on our Family Puzzle Marathon page. Solve three and get a prize!

6 comments:

Alin Grin said...
This comment has been removed by the author.
Alin Grin said...
This comment has been removed by the author.
Alin Grin said...

The glass will not overflow. The volume of water will decrease. One of the water anomalies is that it expands when cooled to 4°C or below. When the ice melts, its temperature gradually rises from 0°C or below towards the ambiance temperature. The ice will decrease in volume once it gets warmer than 4°C.

Maria said...

The glass will not overflow, but will the water level decrease? Let's try it out with our next glass of water. A picture is added to this puzzle to help you in explanations.

Maria said...

Well, it is indeed a very tricky puzzle!
There is a complex answer, and a more complex one :)
If we ignore changes in the temperature of the water, then it turns out that when the above water part A and below water part B of the ice cube will melt, in a liquid state they together will fill the same volume as currently is taken by B. Therefore water will not rise or descend.

Why volume of A+B in ice state is equal to the volume B in water state? This comes from Archimedes' Principle:
Since the ice cube is floating, it is displacing a volume of water (equal to the volume of B) that has a weight equal to that of the ice cube. So, we know that water of volume equal B weights the same as (A+B) in ice. After the ice has melted, it still weights the same = A+B. And A+B in water state gives us volume B. The level stays the same for all practical purposes.

Reading more about this puzzle and knowing Alin's expertise, I saw that there is actually more to it if we consider water temperature changes. Water contracts as it cools until it reaches 4 degrees C at which it reaches its maximum density. As it cools below 4 degrees it expands until it freezes. Our water is likely above 4 degrees C and therefore will contract a bit. So, Alin was right!

aron said...

The compound H2O is one of the few coumpounds that are bigger when turned into a solid

Post a Comment

Note: Only a member of this blog may post a comment.