02 Jan 07
1 edit
Originally posted by AcemasterNot true. I explained it further up the page though.
You are a bit backwards there. Water expands when it freezes because ice catches a bit of air in the middle. When it melts, there won't be near as much water as there was ice.
"Dr. Arnold G. Gill
Astrophysician, Malaspina University-College, British Columbia, Canada
The expansion has to do with the peculiar molecular structure of water. Within the limits imposed by this keyboard, a water molecule looks like
O
/ \
H H
Each of the hydrogen atoms weakly attracts nearby oxygen atoms, so water binds together. When liquid, the binding is weak, but when solid, the bonds become quite strong, and constrain how the molecules may arrange themselves as a crystal. In its liquid state, the molecules are free to roam around wherever they wish. However, in its frozen state, the water molecules form a hexagon (six-sided figure), giving the beautiful six-sided snowflakes that we are all familiar with. In this state, the molecules line up approximately like the sketch below
H2O
/ \
H2O H2O
| |
H2O H2O
\ /
H2O
with the lines showing the molecular bonds holding the molecules in place. Notice that the center is empty! In its liquid form, with the molecules moving about freely, you will find an H2O molecule in the center of such a structure, but not in its solid state. So, a given amount of water will use up more room as ice than as a liquid, because of the empty space that appears as it freezes. Thus, ice is less dense and it floats. (And in case you are wondering, the center is EMPTY; it is NOT filled with air. Air bubbles are extra effects that help to float ice, but air bubbles are not needed to get ice to float.)
You've probably heard that 90% of an iceberg is under water. This suggests that the density of ice is about 10% (1/10) less than that of water. Yet, from the simple diagram above, there are 6 water molecules where before there were 7 - a difference of about 14% (1/7). Why the discrepancy? While frozen water has a unique crystal structure, it also acts like every other substance in the world - it contracts when it cools. That is, the average distance between adjacent water molecules still decreases when water freezes (in the diagram, the lines connecting the molecules get shorter). The net result is that the group of molecules shrinks, but the center remains empty - or 1/7 shrinks to about 1/10.
As far as I know, water is unique. However, there may be more exotic liquids that exhibit similar properties. I doubt that you would find any of them outside of a lab."
02 Jan 07
Originally posted by scottishinnzHydrogen bonds, not van der waals forces.
Water is at its densest at 4C. Below that temperature the van der waal's forces (the weak electrical attractions between the slightly negatively charged oxygen atoms, and the slightly positively charged hydrogen atoms) hold the molecules apart. At 4C the energy possessed by the molecules (in terms of movement - i.e. heat) is roughly equal to the stren ...[text shortened]... to be balanced by something (in this case gravity) the water level rises, as the water expands.
02 Jan 07
Originally posted by DoctorDaraSimilar, but different...
I thought hydrogen bonding was actually one of the three Van Der Waals forces? Granted chem was always sludge for me.
"A hydrogen bond is a special type of attractive interaction that exists between certain chemical groups of opposite polarity. Although stronger than van der Waals forces, the typical hydrogen bond is much weaker than both the ionic bond and the covalent bond. Within macromolecules such as proteins and nucleic acids, it can exist between two parts of the same molecule, and figures as an important constraint on such molecules' overall shape."
02 Jan 07
OK last year on this day it was the hottest recorded temperature.
Today it is raining and half the temperature on the same day yesterday.
Does not sound like global warming to me.
For all you people out there that believe in global warming ,just look at the unreliability in the weather since records were taken .
02 Jan 07
3 edits
Originally posted by boarmanhttp://www.grida.no/climate/ipcc_tar/wg1/049.htm
For all you people out there that believe in global warming ,just look at the unreliability in the weather since records were taken .
The global average temperature has gone up by 0.6 degrees Celsius in the
20th century. Still don't believe in global warming?
The question, again, is not whether global warming takes place, but if it will
truly have the catastrophic consequences you often hear about if we keep
releasing greenhouse gases into our atmosphere.
02 Jan 07
Originally posted by jammerYeah, why don't you listen to the teacher, jammer?!! He said "Consider me convinced!." meaning, "Oh, look everybody! Duuuuuh! 😛 I made a fool of myself in front of the whole class! Now I'm going to go back and shoot myself from humiliation!"
Yeah .. listen to the teacher, just like science, whatever he says must be true.
He pretty much taught himself everything he knows .. no one else was qualified.
02 Jan 07
Originally posted by scottishinnzLooked it up to get rid of my confusion, originally it was apparently used to describe all intermolecular forces, however over time it's come to mean more just the London effect. So I guess we're both right in a sense. ;-)
Similar, but different...
"A hydrogen bond is a special type of attractive interaction that exists between certain chemical groups of opposite polarity. Although stronger than van der Waals forces, the typical hydrogen bond is much weaker than both the ionic bond and the covalent bond. Within macromolecules such as proteins and nucleic acids, it can ...[text shortened]... the same molecule, and figures as an important constraint on such molecules' overall shape."
"In chemistry, van der Waals' forces (sometimes called London dispersion forces) is a class of intermolecular forces which arise from the polarization of molecules into dipoles. This includes forces that arise from fixed or angle-averaged dipoles (Keesom forces) and free or rotation dipoles (Debye forces) as well as shifts in electron cloud distribution (London forces). The term originally referred to all intermolecular forces, and is still sometimes used in this way, but is now more commonly used in the more specific sense. The name refers to the Dutch physicist and chemist Johannes Diderik van der Waals, who first documented these types of forces. The Lennard-Jones potential is often used as an approximate model for the Van der Waals force as a function of distance."