
  I am curious to know under what conditions of the air pressure(atm), temperature, solute density in the water would cause the Niagara fall frozen?


In general, the answer is "a bit lower than 32 Fahrenheit". Here's two things which one might think would come into play, but actually do not to an appreciable extend.


Solute concentation


The major solutes which are present in the Great Lakes are sodium, chloride, and a host of other ions in lesser amounts. According to the government website here, the typical sodium concentration found in the Great Lakes is on the order of 10 mg/L. This is well within the regime of ideal solution behavior, and so we get a freezing point depression of $\Delta T\approx k_fbi=0.0016$ Kelvin (assuming NaCl). This is tiny compared to seasonal temperature variations, so solute concentration can be mostly ignored.


Air pressure


Air pressure is also almost completely negligible, for two reasons. One is that the pressure variation through the water depth of the water is much greater than typical atmospheric pressure variations; for example, atmospheric pressure ranges from 29.5 to 30.5 inHg, which is comparable to the pressure variation in a foot of water (and the river is considerably deeper than one foot at most places). Two is that even accounting for pressure variations of both the atmosphere and within the water itself, the linearized Clausius-Clapeyron relation predicts a negligible temperature dependence. For example, for the water-ice phase transition we obtain
$$\frac{\Delta T}{\Delta P}=\frac{T\Delta v}{L}=0.00025\mbox{K/inHg}$$
which means that seasonal or daily variations in atmospheric pressure will only change the freezing point by something on the order of milliKelvin. Thus atmospheric pressure variation, and most likely water-depth variations in pressure, are negligible compared to seasonal variations in temperature.


Stuff this leaves out


This obviously leaves out a lot of kinetic and rate information. For example, dissipative heating at the foot of the waterfall will heat up the water there by approximately
$$\Delta T=\frac{mgh}{C_p}=0.12\mbox{K}$$
which easily swamps the contribution due to air pressure and solute concentration. This also ignores nucleation effects, and it ignores the rates of coolings from evaporative cooling and air cooling, so it says nothing about how long one would have to wait for the entire waterfall to freeze in so-and-so temperature. However, I think it's safe to say that the temperature needs to be a good bit lower than 32F for the falls to freeze. It may also be worthwhile to look through historical archives and see what the mean temperature was in the years for the photographs you posted.

I'm not quite sure what you mean by Van der Waals force, as that's pretty much going to be constant within a given substance.

How to freeze the Niagara waterfalls?
is it possible to make the background colour of a line of text fit to the edge of the text itself?

Here is a visual example of what I'm trying to accomplish:



The text is a &lt;h3&gt; inside a &lt;div&gt; and at present all I can achieve is a square block background.
