Water level rise or fall problems

[axp107]

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These problems give me headaches.. I try to solve them "conceptually" but my intuition is normally wrong. Should I go about trying to perhaps solve them mathematically using variables?

1) A bottle is half filled with water at 4 degrees and sealed shut. The bottle is placed on a scale and put into a freezer. As the water nears 0 degrees, the watere level in the bottle:

rises and reading on scale remains constant

Why does it rise.. how does the ice cause it to rise? The reason I'm asking is that I saw another problem about ice melting.. when ice melts the water level stays constant.



2) A brick sits on a massless piece of Styrofoam floating in a large bucket of water. If the styrofoam is removed and the brick is allowed to sink to the bottom:

the water level will FALL.. why? you'd think it'd rise right.. like dropping stones in water causes the water to rise

 

[jochi1543]

These problems give me headaches.. I try to solve them "conceptually" but my intuition is normally wrong. Should I go about trying to perhaps solve them mathematically using variables?

1) A bottle is half filled with water at 4 degrees and sealed shut. The bottle is placed on a scale and put into a freezer. As the water nears 0 degrees, the watere level in the bottle:

rises and reading on scale remains constant

Why does it rise? I understand why the weight is constant.

The water reaches 0 degrees and starts to form ice, which occupies greater volume than water.

2) A brick sits on a massless piece of Styrofoam floating in a large bucket of water. If the styrofoam is removed and the brick is allowed to sink to the bottom:

the water level will rise.. why?

When the brick sinks, it displaces water from where it was prior, adding to the total volume of what's in the bucket. Prior, it was just water, now it's the volume of all the water AND the volume of the brick. So the water level rises.

 

[axp107]

my bad MISTAKE

for 2).. the water level FALLS

 

[jochi1543]

my bad MISTAKE

for 2).. the water level FALLS

I'm tempted to test-drive this in my tub.

 

[jheredia]

my bad MISTAKE

for 2).. the water level FALLS

this only seems possible if the volume of styrofoam submerged > volume of entire brick

 

[quiksilver87]

this only seems possible if the volume of styrofoam submerged > volume of entire brick

Also, because the brick is actually floating...a buoyancy must be provided that will lift it out of the water...And that buoyancy is equal to an amount of water displaced by the styrofoam. When the brick is thrown into the bucket, it will sink...this is because less buoyancy will be acting on it and a lesser amount of water is therefore displaced.

 

[smeagol]

1) A bottle is half filled with water at 4 degrees and sealed shut. The bottle is placed on a scale and put into a freezer. As the water nears 0 degrees, the watere level in the bottle:

rises and reading on scale remains constant

I'm very uncomfortable with the wording in this question. Where did it come from?

The water NEARS 0 degrees? So it's actually not at 0 degrees yet? Then I'd want to say that no ice has formed.

 

[thisisapickle]

I'm very uncomfortable with the wording in this question. Where did it come from?

The water NEARS 0 degrees? So it's actually not at 0 degrees yet? Then I'd want to say that no ice has formed.

This is probably a legitimate concern because water is actually the most dense at 4 degree, therefore the greater density water tends to settle towards the bottom and displace some of the water (what you see before lakes ice over), so a water that nears 0 degrees is an ambiguous comment. Good luck!!

 

[littlealex]

Oh my god if you guys can't figure this out, go back and study your physics a bit more.

1) Water is at its greatest density at 4 degrees. If it moves away from 4 degrees in EITHER direction, it'll expand. Even if it is at 2 degrees and no ice forms, it'll expand.

2) YES the water level falls. Previously the water was holding up the ENTIRE brick, so it displaces enough water to equal the weight of the brick. After it falls in, the brick no longer displaces its WEIGHT, just its VOLUME. A brick is denser then water, hence the water level falls.



H:SIUHGSLIUDGHSDIUFHS

 

[smeagol]

Oh my god if you guys can't figure this out, go back and study your physics a bit more.

1) Water is at its greatest density at 4 degrees. If it moves away from 4 degrees in EITHER direction, it'll expand. Even if it is at 2 degrees and no ice forms, it'll expand.

Thank you for clearing that up.

I haven't learned this in my physics course yet and I failed to recall this if it was taught to me in General Chemistry.

 

[littlealex]

I actually learned this in my biology class way long ago in High school. It was brought up about why shallow ponds freeze from the top down.

If water is the most dense at close to 0 degrees and expands once it forms ice, then the ponds should freeze from the bottom and the ice would float to the top. Instead of this, what we see is a thin layer freezing at the top, then expanding downwards.

I'm also wondering if this relates to why refridgerators are operated at 4 degrees instead of any other temperature.