Why is the freezing point of water 32 degrees? Why not 33 or 46 degrees?

Answer 1

You'd have to ask Daniel Gabriel Fahrenheit himself how he came up with his scale. But as mentioned above, water freezes at 0 and boils at 100 in Celsius.

Answer 2

Phase transitions are a very interesting part of statistical physics. A phase transition typically takes place because of interactions between different members of a substance, in this case the individual water molecules. The interaction between water molecules is dominated by the fact that water is polar; that is, one side of the water molecule has a net positive charge, while the other side has a net negative charge.

If you got a bunch of water molecules, and gave them zero energy, they would want to form a regular array of molecules; with the negative side of one water molecule close to the positive side of another. This array would be rigid; we call this ice! Now imagine giving the water molecules more and more kinetic energy; this is equivalent to raising the temperature. At some point, the water molecules will move fast enough to break out of their positions; this is melting, and we now have liquid water. A similar process occurs for boiling to produce water vapor.

Why then does melting occur at 32 degrees farenheit? First of all... it's important to recognize that this temperature isn't the same for all pressures; do a google search for 'phase diagram of water.' Also... the farenheit scale is just one measure of temperature: I could create a new temperature scale called 'zoots' where water freezes at 54, 12098 or .0003 zoots, depending on the scale. That is to say, there is nothing fundamental about the number 32.

But it does occur at a specific temperature, whatever the particular number may be. Ultimately, this temperature (which can be related to an energy) is determined by the strength of the attraction between the water molecules. If the water molecules really want to be lined up, then it will take a lot of energy (or, equivalently, a high temperature) to break up the structure. If there is no attraction, then it will obviously take no energy to melt; and you could say the melting point is at absolute zero.

N.B. I tried my best to give a qualitative, understandable picture of a phase transition. To understand why phase transitions can happen all at once, and not gradually and over a range of temperature, you pretty much have to take some higher level physics classes. I hope this helped, though.

*Response to Steve*
Yes Steve... the reason why the freezing point of water is specifically 32 degrees farenheit is due to historical reasons. That's a silly answer though; that's like saying the sky is blue because we happen to call the color blue. There is nothing fundamental about the choice of temperature scale!! The fundamental reason why water freezes at a thermal energy level corresponding to 32 degrees farenheit (or 0 degrees celsius, or 273 Kelvin.....) has everything to do with statistical physics, chemical bonds, and if you want to go into more depth, entropy. And yes... before people knew about statistical physics, water still froze at the same temperature. Just like things still fell before the law of universal gravitation was formalized.....

*End rant*

Answer 3

Wow... nobody seems to have a real answer. Let me start you on the right track. I'm not sure how much chemistry background you have, so I won't get too technical.

There is a physical quantity of all systems called "entropy". It is logarithmically proportional to the number of possible microstates that can describe the macrostate the system is in.

[Example]
Macrostate: 3 balls under 4 cups total
Possible Microstate 1: 3 balls under cup A
Possible Microstate 2: 2 balls under cup A, 1 ball under cup B
Possible Microstate 3: 1 ball under cup A, 1 ball under cup B, 1 ball under cup C
...etc...

For a process to be spotaneous (that is, for it to occur without help), the entropy of the universe must increase. That is, delta_Entropy > 0.

When water melts, there are two fundamental physical processes that change the entropy of the universe. 1) The ice absorbs energy from the surroundings to supply the energy required for the phase change (energy is required because you are essentially "weakening" intermolecular bonds). This causes a decrease in entropy of the surroundings. 2) A change in volume of the ice, when it changes into water. This increases the entropy of the system. The total change in entropy of the universe, is therefore the sum of the changes in entropy of both the systems and the surroundings. When the increase in entropy due to 2) is greater than the decrease in entropy due to 1) (that is, when the entropy of the universe increases), melting takes place.

So we understand why the change occurs... it has to do with the entropy of the system and the surroundings... but what does this have to do with temperature? The change in entropy due to the energy transferred into the system is dependent on the temperature of the system when this energy transfer occurs. Therefore the spontaneity of the melting/freezing process depends only upon the temperature that creates a situation where the entropy increase due to volume change is greater than the entropy decrease due to the endothermicity of the phase change process. It just so happens that the temperature where this occurs is 32 degrees Fahrenheit or 0 degrees Celsius for water.

Hope this helps...

If you have any further questions, email me at [e-mail]... I love thermodynamics, so if you need help, send it my way.

Answer 4

The thermometer was just being developed. People knew that if you made one and stuck it in certain hot or cold substances it would go to the same point each time. But you needed two standard points and a size of marking to make a scale.
At the time Fahrenheit was doing his research the coldest temperature that could be reliably attained was a mixture of ice, water and salt (and alcohol, I think). He took this to be 0 on his scale. He wanted the other fixed point to be human body temperature. His first choice was 12 degrees for body temperature. That made for a clunky size, so he first went to 48 then 96 degrees. (We now consider 98.6F to be normal body temp.) As different and longer thermometers were developed, the boiling point of pure water at sea level was used and when his scale was extended to there when it showed 212F. On his scale, the point of pure water and ice was about 32 and his scale was adjusted to make this exact. When Celsius came along a little bit later, he chose the freezing of water and the boiling of water as his points and under the influence of the metric system chose a range of 100 degrees between them.

Answer 5

Well, when measured in degrees Celsius, the freezing point of water is 0 degrees. 0 is a nice round number. And 0 degrees Celsius = 32 degrees Fahrenheit. Likewise, the boiling point of water is 100 degrees Celsius - another nice round number. But don't ask me who decided the relationship between Celsius and Fahrenheit, chemistry was not my favourite subject!

Answer 6

Notwithstanding that the freezing point of water depends upon pressure, the standard is 32 degrees. The very structure of water makes it very unique. H2O is not planar (flat) but rather distorted by an electron pair that deflects the hydrogen atoms.
Generally when compounds go from liquid to solid, the solid is more dense. But water is unique. When it goes from liquid to solid, the solid is less dense. This implies that the distance between water molecules is farther apart in the solid than the liquid. This is due to weak covalent bonds with hydrogens to other oxygens in the liquid state.
All these unique properties cause water to possess the basic property of freezing at 32 rather than any other temperature.

Answer 7

it is all of the above. 32 degrees water at sea level. 33 at 800 foot below sea level. do the math to give you depth at which air pressure would aid freezing at 46 degrees.

Answer 8

FP is dependent on chemical composition, which is why pure water freezes at higher temps than sea water. If you live in cold states/countries, then this also explains why snow is 'salted'...decreases freezing point so that the salted ice becomes liquid at 0 Celsius and will only become solid if temp. decreases to -6 to -9 degrees celsius (depending on concentration of salt/ice).

http://chemistry.about.com/cs/howthingswork/a/aa120703a.htm

Answer 9

Study chemistry. Water freezes at 32 degrees and boils at 212 degrees - just the physical properties of water. Ask God.

Answer 10

freezing point of water is 32 degrees F because at this temperature the kinetic energy of molecules in liquid state of water reduces to such an extent that they acquire a solid state.

and this is an experimentally determined temperature