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A question about water properties

A question about water properties - Physics Forum

A question about water properties - Physics Forum. Discuss and ask physics questions, kinematics and other physics problems.


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  #1  
Old 11-29-2006, 10:59 AM
Bob
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Default A question about water properties



I am not a scientist, more of a curious observer, but enjoy reading all your
questions and comments, most of which I don't really understand.

If this question is beneath you guys, I apologize for posting.

When I make a mug of coffee in the morning, I boil the water and pour it
into the mug until it is full. By the time the coffee is cool enough to
drink, it no longer fills the mug. I figure this is normal expansion and
contraction due to the heat causing increased atom activity which decreases
as the liquid cools.

I then assume that the liquid, assume water, continues to contract as
cooling takes place.

But, when the water is cooled to the point that it freezes, then it once
again expands. True, we now call the water by a different name, "ice", but
it is still the same material that we boiled.

Question: Does hot water continue to contract as it cools, up to the
instantaneous point that it begins to freeze, and at that instant, cease
contracting and begin expanding?

Sorry for the dumb question, but I am curious.
Bob




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  #2  
Old 11-29-2006, 11:23 AM
tadchem
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Default A question about water properties


Bob wrote:

It is not a dumb question.

The arrangement of H2O molecules in a crystal of ordinary ice has a lot
of empty space in it.

When the ice is heated some of the bonds between adjacent water
molecules break and the structure collapses, taking up less space as it
gets hotter, until there is almost no 'ice-like' structure left.

Then as it continues to heat up us expands as the more agitated
molecules keep jostling each other about, preventing overcrowding.

The process is reversible as the temperature drops.

As an interesting side note, the empty spaces in ice crystals can be
occcupied by small molecules of gas such as methane. This give the ice
a higher density and a little more stroength ('shoring up' the ice
structure). This combination of a gas and water (called a 'gas
hydrate') has a higher melting point and density than regular ice - as
high as about 55 F (13 C), but it doesn't float on water. It is
found on the ocean *floor* in great quantities. Google "gas hydrate
beds".

Tom Davidson
Richmond, VA

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  #3  
Old 11-29-2006, 11:23 AM
tadchem
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Default A question about water properties


Bob wrote:

It is not a dumb question.

The arrangement of H2O molecules in a crystal of ordinary ice has a lot
of empty space in it.

When the ice is heated some of the bonds between adjacent water
molecules break and the structure collapses, taking up less space as it
gets hotter, until there is almost no 'ice-like' structure left.

Then as it continues to heat up us expands as the more agitated
molecules keep jostling each other about, preventing overcrowding.

The process is reversible as the temperature drops.

As an interesting side note, the empty spaces in ice crystals can be
occcupied by small molecules of gas such as methane. This give the ice
a higher density and a little more stroength ('shoring up' the ice
structure). This combination of a gas and water (called a 'gas
hydrate') has a higher melting point and density than regular ice - as
high as about 55 F (13 C), but it doesn't float on water. It is
found on the ocean *floor* in great quantities. Google "gas hydrate
beds".

Tom Davidson
Richmond, VA

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  #4  
Old 11-29-2006, 03:08 PM
Greg Neill
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Default A question about water properties

"Bob" <[Only registered users see links. ]> wrote in message
news:5Kdbh.11537$[Only registered users see links. ]...
your
decreases
but

I would think that the observed drop in water level
can be attributed to three factors:

1. Expansion of the cup volume as it heats up
(absorbs heat from the water).
2. Evaporation of some of the liquid.
3. Contraction of the water volume as it cools.

My guess is that item 1 dominates.


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  #5  
Old 11-29-2006, 03:08 PM
Greg Neill
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Default A question about water properties

"Bob" <[Only registered users see links. ]> wrote in message
news:5Kdbh.11537$[Only registered users see links. ]...
your
decreases
but

I would think that the observed drop in water level
can be attributed to three factors:

1. Expansion of the cup volume as it heats up
(absorbs heat from the water).
2. Evaporation of some of the liquid.
3. Contraction of the water volume as it cools.

My guess is that item 1 dominates.


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  #6  
Old 11-29-2006, 03:47 PM
operator jay
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Default A question about water properties


"Bob" <[Only registered users see links. ]> wrote in message
news:5Kdbh.11537$[Only registered users see links. ]...


From

[Only registered users see links. ]

Fresh water at standard atmospheric pressure is most dense at 3.98 C

j


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  #7  
Old 11-29-2006, 03:47 PM
operator jay
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Default A question about water properties


"Bob" <[Only registered users see links. ]> wrote in message
news:5Kdbh.11537$[Only registered users see links. ]...


From

[Only registered users see links. ]

Fresh water at standard atmospheric pressure is most dense at 3.98 C

j


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  #8  
Old 11-29-2006, 11:47 PM
OG
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Default A question about water properties


"Bob" <[Only registered users see links. ]> wrote in message
news:5Kdbh.11537$[Only registered users see links. ]...

As you know, water is made up of one Oxygen atom and two Hydrogen atoms.
Because of the way that the atoms are joined, the three atoms in the
molecule are not in a straight line, rather they form with an angle of about
104 degrees at the Oxygen atom.

Because of the asymmetric shape, in addition to the strong bonds between
oxygen and hydrogen within the molecule, the hydrogen atoms can also take
part in weaker bonds between different molecules.

These bonds hold liquid water together and without them, water would boil at
a much lower temperature. The forces are significant at all temperatures for
liquid water, but they are particularly significant at low temperatures.
Just before water freezes, these hydrogen bonds begin to change the way the
water molecules fit together - so they are pushed apart by an extra force as
the temperature drops below about 4 degrees Celsius. As the water finally
freezes, the hydrogen bonds give the ice its structure and the unusual
property of floating on liquid water.




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  #9  
Old 11-29-2006, 11:47 PM
OG
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Default A question about water properties


"Bob" <[Only registered users see links. ]> wrote in message
news:5Kdbh.11537$[Only registered users see links. ]...

As you know, water is made up of one Oxygen atom and two Hydrogen atoms.
Because of the way that the atoms are joined, the three atoms in the
molecule are not in a straight line, rather they form with an angle of about
104 degrees at the Oxygen atom.

Because of the asymmetric shape, in addition to the strong bonds between
oxygen and hydrogen within the molecule, the hydrogen atoms can also take
part in weaker bonds between different molecules.

These bonds hold liquid water together and without them, water would boil at
a much lower temperature. The forces are significant at all temperatures for
liquid water, but they are particularly significant at low temperatures.
Just before water freezes, these hydrogen bonds begin to change the way the
water molecules fit together - so they are pushed apart by an extra force as
the temperature drops below about 4 degrees Celsius. As the water finally
freezes, the hydrogen bonds give the ice its structure and the unusual
property of floating on liquid water.




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  #10  
Old 11-30-2006, 12:20 AM
N:dlzc D:aol T:com \(dlzc\)
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Default A question about water properties

Dear OG:

"OG" <[Only registered users see links. ].uk> wrote in message
news:[Only registered users see links. ]...
....

Not all that "unusual". Rock seems to float on magma OK... ;>)

David A. Smith


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