So, you can't believe everything you read on the Internet? Go figure.

As you may know, Irish Coffee is supposed to be layered, coffee on
bottom and cream on top. The coffee is supposed to be dark. The
cream is not supposed to dissolve in the coffee, if it is done right.
And if the cream does not float on the coffee then it will dissolve in
the coffee. So, what makes it float?

My understanding is that it took a lot of experimentation to create
Irish Coffee, the properties of solutions can be complicated and water
is the exception to many chemical/physical rules.

So, what makes it float?

My understanding is that the factors at work are liquid density,
surface tension, and a spoon.

Plus if you whip or partially whip the cream, not that you would
actually do that, just hypothetically, then you would put air in the
cream making it less dense. And if you completely whip the cream then
the whipping will help cause the cream to stick together, like a
solid, which will help keep it from dissolving in the coffee.

Some of the ingredients in Irish Coffee make it harder to make the
cream float, so some tricks, some compensating factors, are needed.

But basically the sugar makes the coffee denser and increases surface
tension.


Density

The cream is less dense that the coffee so it has a tendency to float
because of that, just like cream floats on milk.

The sugar makes the coffee denser. There is some proof at
http://biology.clc.uc.edu/Courses/bio111/pop.htm.

The whiskey will make the coffee a little less dense, since whiskey is
less dense than water. Please see http://www.henriettesherbal.com/ecle...1/spirits.html.

(If specific gravity that is less than one then it is less dense than
water. If it is greater than one then it is denser than water.)

My understanding is that coffee has about the same density as water,
but the stronger the coffee, the denser it is.

Hot coffee (hot liquid in general) is less dense and cold coffee. In
convection, hot liquid particles rise and cool liquid particles sink.


Surface Tension

I think the whiskey decreases the surface tension of the coffee.
Liquor decreases the surface tension of water.

I think the sugar increases the surface tension of the coffee. I
realize that this might contradict the Wiki article on surface tension
http://en.wikipedia.org/wiki/Surface_tension. But please see:
http://acs.confex.com/acs/csss06/techprogram/P30854.HTM
http://www.bio.net/bionet/mm/neur-sc...ry/027102.html


Spoon

Pouring the cream onto the spoon helps keep the cream from plunging
into the coffee. It redirects the force of the poured cream sidewise
instead of down, so it is less inclined to break the surface of the
coffee.


Thank you.