Ice vs. Liq water

[ilovemedi]

Two questions

1)I'm confused about the whole water trivia paragraph in TBR page 74. I understand that liq water is more dense than solid form. But I still don't understand the ice skating scenario. It says that warm water is at the BOTTOM of the lake. I read up online that warm water is LESS dense than cold. So, shouldn't the warm water (which is less dense) rise above the cold water (which is denser)?? Also, WHY is cold water more dense? You would think that cold water is closer to being a solid, which is less dense than liquid..

Also
2) It says that isothermal increase in pressure results in liquid being denser than solid. Well, if I increased the pressure (and kept pressure constant) for another substance asides from water, doesn't an increase in Pressure decrease the Volume, and also make the liquid denser (see phase diagram)??

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[ljc]

Two questions

1)I'm confused about the whole water trivia paragraph in TBR page 74. I understand that liq water is more dense than solid form. But I still don't understand the ice skating scenario. It says that warm water is at the BOTTOM of the lake. I read up online that warm water is LESS dense than cold. So, shouldn't the warm water (which is less dense) rise above the cold water (which is denser)?? Also, WHY is cold water more dense? You would think that cold water is closer to being a solid, which is less dense than liquid..

The warm water is at the bottom because the water near the top is closer to the ice, which is cold. The water near the bottom can remain warm longer because it is not in direct contact with ice. Warm water is indeed less dense than cold water, this is due to the kinetic theory of temperature, which states that as temperature increases, molecular motion increases. Thus, warm water is moving more and can't be as tightly packed. And cold water is more dense, because the molecules are not vibrating as much, so they can be tightly packed.

And water has its maximum density at just slightly above freezing. I think it's around like 3 degrees? Warm water increases in density down to 3 degrees, then decreases in density as the water forms into a solid.

Also
2) It says that isothermal increase in pressure results in liquid being denser than solid. Well, if I increased the pressure (and kept pressure constant) for another substance asides from water, doesn't an increase in Pressure decrease the Volume, and also make the liquid denser (see phase diagram).

This is true. This is why on a water phase diagram, the water/solid dividing line is sloped up and to the left, instead of up and to the right. The leftward slope indicates that the solid is less dense than the liquid. On other substances (which is the majority of them), the slope is up and to the right, indicating just as you say, that the solid is more dense than the liquid.

If you trace a point on the water phase diagram from solid, straight up, you see pressure increases, and you move from solid to liquid. This proves exactly what you're saying (unless I misunderstood)

 

[ilovemedi]

So for question 1) the warm water never rises, since it is less dense? It just stays there on the bottom? I always thought less dense things rise up?
and for 2) so what exactly does the negative slope mean? I know it essentially is that solid water is less dense than liquid (which is why it floats) but in technical terms?

 

[ljc]

So for question 1) the warm water never rises, since it is less dense? It just stays there on the bottom? I always thought less dense things rise up?
and for 2) so what exactly does the negative slope mean? I know it essentially is that solid water is less dense than liquid (which is why it floats) but does a negative slope mean that

1) It does, but when it does, it is cooled rapidly, and is no longer warm. You have a circular flow going, but the water touching the ice will always be colder, because it is conducting some its heat away to the ice which it is touching.

2)

In the top image, see how the left-upper most line is facing up and left, while in the lower image, the line in the same spot is facing right? The top one is water, and the direction of that line indicates that liquid is more dense than solid. To prove this, pick a point just slightly to the left of "A", and move upward. You are increasing pressure, which means increased density, but notice you move from solid to liquid? Thus, as you are increasing density, you go from solid to liquid, meaning liquid is more dense. Now try that on the bottom graph - it doesn't work, because the upper-left line is facing in such a way that it is impossible to move straight up and go from solid to liquid. This means that the solid is more dense. This is how most substances will appear. In this case, it is ethanol, but it could have been almost anything. Water is one of only a handful of substances on Earth that becomes less dense as you go from liquid to solid. This is what makes it unique, and what makes it such an important part of life on Earth.

 

[ilovemedi]

Wow thank you so much! Makes sense now. But this is for isothermal conditions, correct? Is isothermal conditions used as opposed to constant pressure just because the slope has more changes in the Y direction (pressure)?

 

[ljc]

Isothermal just means same temperature. Since temperature is on the horizontal axis, all they're saying is "increase pressure (move up on the graph), but stay at the same temperature (don't move left or right)". This is because it's harder to predict what will happen to density when you change two variables, so they're just trying to show that an increase in pressure, which increases density, will result in a solid to liquid phase change.

If you wanted constant pressure, you would move left or right, but not up or down.

 

[ilovemedi]

Yes, but can you keep pressure constant this time (isobaric conditions) instead of isothermal, and get the same results? Because I can go horizontally from solid to liquid on both graphs, whereas I can't cross solid to liquid vertically in the non-water graph (like you pointed out).

edit Nvm, a few paragraphs later, it says phase changes is isothermal!

 

[ljc]

Yes, but can you keep pressure constant this time (isobaric conditions) instead of isothermal, and get the same results? Because I can go horizontally from solid to liquid on both graphs, whereas I can't cross solid to liquid vertically in the non-water graph (like you pointed out).

edit Nvm, a few paragraphs later, it says phase changes is isothermal!

You can get the same results, but that's not what's important. This is merely an illustration of the fact that solid water is less dense than liquid. For this example to be true, you have to move vertically on the axis. Moving horizontal doesn't really tell you anything other than "higher temperature water goes from solid to liquid," which is true of all materials, so we don't gain any new knowledge this way.