Density of Frozen Water

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When I freeze water in an ice cube tray in my freezer they expand slightly. Their weight remains the same therefore density has reduced. When I put these ice cubes in a glass of water they float because of the lower density. A small portion of the cube rises above the surface of the water.

Is the volume of the cube which is below the surface equal to the volume of the original thawed water? And if so, why do people say melting antartic ice caps cause sea levels to rise? Wouldn't the ice simply melt and occupy the same volume as the frozen portion which is below the surface therefore sea levels would remain exactly the same?

Cheers
 

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  • #2
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Yes, by Archimedes principle the weight of the volume of water displaced must be equal to the total weight of the floating object.

As far as the melting ice caps go, you have to be aware of the two types of ice under consideration: sea ice and land ice. The former is floating in the arctic and antartic seas and varies considerably up and down over the year. The latter are the ice masses and glaciers that have built up over a long time on land. It is the melting of land ice that contributes to the rise in sea levels.

And as a passing note, it is currently the arctic sea ice melting while the antartic sea ice has been actually increasing. However, the large majority of land ice on the planet has been steadily decreasing.
 
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Ok thanks
 
  • #4
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Also note that thermal expansion of ocean waters may lead to sea level rise even if no ice melts.
 
  • #5
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When I freeze water in an ice cube tray in my freezer they expand slightly. Their weight remains the same therefore density has reduced. When I put these ice cubes in a glass of water they float because of the lower density. A small portion of the cube rises above the surface of the water.

Is the volume of the cube which is below the surface equal to the volume of the original thawed water? And if so, why do people say melting antartic ice caps cause sea levels to rise? Wouldn't the ice simply melt and occupy the same volume as the frozen portion which is below the surface therefore sea levels would remain exactly the same?

Cheers
The volume of the cube which is below the surface is less than the volume of the original thawed water.
The total volume of the cube which includes the ice above the surface is equal to the volume of the original thawed water.
 
  • #6
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Also note that thermal expansion of ocean waters may lead to sea level rise even if no ice melts.
Doubtfull as water is at it's densest at 4 degrees so the oceans will contract before they expand if no ice melts.
 
  • #7
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The volume of the cube which is below the surface is less than the volume of the original thawed water.
The total volume of the cube which includes the ice above the surface is equal to the volume of the original thawed water.

Unless I am misunderstanding you, this is not correct. Ice does indeed expand when it freezes, so the volume of an ice cube will be greater than the same cube when melted.
 
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  • #8
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The volume of the cube which is below the surface is less than the volume of the original thawed water.
The total volume of the cube which includes the ice above the surface is equal to the volume of the original thawed water.
This is just not correct. Water expands something around 10% in volume when it transitions to ice.

Let V1 and V2 be the volumes of water and ice cube respectively.

Let V3 be the portion of the ice cube submerged below the water line.

By definition:

V2 = 1.1*V1

The weight of the ice cube is the same as the water before it froze = gamma*V1

By Archimedes principle, the buoyant force acting on the ice cube = gamma *V3

In order to be floating the buoyant force must be equal to the weight of the ice cube:

gamma*V3 = gamma*V1

which means that V3 = V1
= V2 / 1.1

Therefore, the submerged volume V3 is equal to the volume of water V1 and is less than the volume of the ice cube V2.
 
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  • #9
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Doubtfull as water is at it's densest at 4 degrees so the oceans will contract before they expand if no ice melts.
Not correct. We are talking about sea water whose maximum density is close to its freezing point which in turn depends on the salinity and pressure.
 
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  • #10
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Doubtfull as water is at it's densest at 4 degrees so the oceans will contract before they expand if no ice melts.

The ocean's temperature is mostly above 4 C. Any warming will lead to expansion, not contraction.

Note that we're talking about temperature near the surface (where warming is currently happening, not about deep ocean waters which are indeed below 4 C but are not currently changing.
 
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  • #11
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Unless I am misunderstanding you, this is not correct. Ice does indeed expand when it freezes, so the volume of an ice cube will be greater than the same cube when melted.
Depends if the water talked about is liquid or solid.
I probably misunderstood davies was the cube thawed or not.
 
  • #12
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Depends if the water talked about is liquid or solid.
I probably misunderstood davies was the cube thawed or not.

Yeah, it doesn't really depend on that if we are talking about seawater as the OP was doing. I think you probably misunderstood that he wasn't talking about pure water but rather was talking about sea ice and how its melting would affect sea levels (which it doesn't).
 
  • #13
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Yeah, it doesn't really depend on that if we are talking about seawater as the OP was doing. I think you probably misunderstood that he wasn't talking about pure water but rather was talking about sea ice and how its melting would affect sea levels (which it doesn't).
Actualy I am not sure about the misunderstanding which the OP could clear up.
What he said.
Is the volume of the cube which is below the surface equal to the volume of the original thawed water?
If he had said water instaid of thawed water then there would be no misunderstanding on my part.
By mentioning that the water was thawed could mean that he is talking about volume of the cube when it has melted and the amount of melted water above and below the surface.
 
  • #14
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Not correct. We are talking about sea water whose maximum density is close to its freezing point which in turn depends on the salinity and pressure.
We are also talking about ice which when melts or does not is at it's densest at 4 degrees.
 
  • #15
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Not correct. We are talking about sea water whose maximum density is close to its freezing point which in turn depends on the salinity and pressure.
Are we not also talking about the melting of ice and sea ice which is still at it's maximum density at around 4 degrees.
The freezing point of sea ice is lower than freshwater ice due to salinity and pressure also has an effect.
But salts in the ice can be excluded when it freezes so it becomes fresher won't this fresh water behave like normal ice when it thaws becoming denser at 4 degrees?
http://www.britannica.com/EBchecked/topic/531121/seawater/301668/Density-of-seawater-and-pressure
 
  • #16
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When I freeze water in an ice cube tray in my freezer they expand slightly. Their weight remains the same therefore density has reduced. When I put these ice cubes in a glass of water they float because of the lower density. A small portion of the cube rises above the surface of the water.

Is the volume of the cube which is below the surface equal to the volume of the original thawed water? And if so, why do people say melting antartic ice caps cause sea levels to rise? Wouldn't the ice simply melt and occupy the same volume as the frozen portion which is below the surface therefore sea levels would remain exactly the same?

Cheers

The Antarctic (and Greenland) have land ice, which will contribute to the sea level rise. This ice isn't floating on water.

There's no land under the Arctic ice cap, where your argument is relevant.

A major component of sea level rise will be thermal expansion of water. Incidentally, hot water is also an essential factor in the formation of hurricanes.

One reason that the receding of the ice caps is relevant is that it's a measure of warming.

Another reason is that snow and ice reflect heat much more than water, meaning that the lower the surface area that they cover, the more of the heat from the sun is absorbed by the earth, which contributes to warming, in a positive feedback loop. Positive feedback loops lead to instabilties.

That's just the physics (and a little bit of geography) of it. Sadly Climate Change is a banned topic on these forums so we have to be careful, but I guess it's still ok to talk about the physics.
 
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  • #17
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We are also talking about ice which when melts or does not is at it's densest at 4 degrees.
There are two different issues:

1) The OP was talking about sea ice which has salt in it. The OP just made analogy of an ice cube since the physics of buoyancy is the same for pure water and sea water but the density and melting points are not.

2) You brought up the issue of the density of water being a maximum at 4 degrees and assumed that it was the same as ocean sea water, which it is not.
 
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  • #18
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Are we not also talking about the melting of ice and sea ice which is still at it's maximum density at around 4 degrees.
But salts in the ice can be excluded when it freezes so it becomes fresher won't this fresh water behave like normal ice when it thaws becoming denser at 4 degrees?
http://www.britannica.com/EBchecked/topic/531121/seawater/301668/Density-of-seawater-and-pressure
Not true, again. Look at your own referenced link. It shows the maximum density of sea water with a salinity of 10 or more is actually at 0 degrees, not 4 degrees.

When sea water freezes (around -2 degrees) the salinity remains in the ice, it does not fall out of the solution. But, even if it did, it would be irrelevant to the OP's question and it is also irrelevant to your false statement about the density of sea ice being at 4 degrees.
 
  • #19
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Sadly Climate Change is a banned topic on these forums so we have to be careful, but I guess it's still ok to talk about the physics.
Why is it banned? (I can guess the reasons but just want to know the official reasons for this censorship.)
 
  • #20
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Not true, again. Look at your own referenced link. It shows the maximum density of sea water with a salinity of 10 or more is actually at 0 degrees, not 4 degrees.

When sea water freezes (around -2 degrees) the salinity remains in the ice, it does not fall out of the solution. But, even if it did, it would be irrelevant to the OP's question and it is also irrelevant to your false statement about the density of sea ice being at 4 degrees.


So even if it salt did fall out of the solution and became fresh water ice it would not become densest at 4 degrees when it thaws.
How come? Fresh water is fresh water even if it had salt in it at any time.
Can you drink melted sea ice?
New ice is usually very salty because it contains concentrated droplets called brine that are trapped in pockets between the ice crystals, and so it would not make good drinking water. As ice ages, the brine eventually drains through the ice, and by the time it becomes multiyear ice, nearly all of the brine is gone. Most multiyear ice is fresh enough that someone could drink its melted water. In fact, multiyear ice often supplies the fresh water needed for polar expeditions. See Salinity and Brine in the Characteristics section for more information.
 
  • #21
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Why is it banned? (I can guess the reasons but just want to know the official reasons for this censorship.)

The forum administrators claim they don't have the expertize needed to properly moderate that topic.
 
  • #22
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So even if it salt did fall out of the solution and became fresh water ice it would not become densest at 4 degrees when it thaws.
How come? Fresh water is fresh water even if it had salt in it at any time.

Once it melts it does mix with salty water again, doesn't it?
 
  • #23
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Why is it banned? (I can guess the reasons but just want to know the official reasons for this censorship.)

It's a politically charged area of science regardless of its validity so becomes difficult to moderate. You can find the long and incredibly tedious discussion about this in the Earth Sciences forum.

Allow me to preempt your next question. Politics in science is a banned topic too.
 
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  • #24
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So even if it salt did fall out of the solution and became fresh water ice it would not become densest at 4 degrees when it thaws.
How come? Fresh water is fresh water even if it had salt in it at any time.
We are talking about sea ice the majority of which only lasts for the winter season not multiple years like land ice. So most of it will be salty.

But again it's irrelevant to the OP's original question and it is irrelevant to your false claim about the density of sea water.
 
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  • #25
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It's a politically charged area of science regardless of its validity so becomes difficult to moderate. You can find the long and incredibly tedious discussion about this in the Earth Sciences forum.

Allow me to preempt your next question. Politics in science is a banned topic too.
I don't see it as politically charged anymore than an area like evolution or cold fusion. But someone felt it was I guess.
 

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