Hot air rises, does hot liquid do the same?

I noticed when I pour cream into my hot coffee, no matter how slowly I do it, the cream always seams to spread very quickly. So is the hot liquid in the coffee constantly rising making the cream molecules spread everywhere quickly?

Yes, it does. I remember doing an experiment back when I was in elementary school. My teacher added food-colouring to an ink-bottle full of hot water, and dropped it into a larger container of clear cold water.

Hot air rises because it is less dense than cold air, the relation between density and temp. genrally holds for liquid, though I imagine the cream is more dense than the coffee anyway.

What makes the cream spread out is diffusion, which is how the random movement of molecules tends to make substances mix together.

Actually cream is less dense than coffee (which is mostly water).

Is it? I suppose it must be, as otherwise they wouldn’t be able to skim it from the top of milk.

In general, the colder something gets, the denser it becomes. Temperature is a measure of kinetic energy of a substance’s constituent molecules. The more kinetic energy, the more they bounce around and therefore take up more space. As the substance cools, the molecules slow down and take up less space. The same amount of the substance by weight fits into a smaller volume; similarly, the same volume requires more of the substance to fill it, meaning that a litre of cold water will have more water in it than a litre of warm water and will thus be heavier.

Actually, water itself is a major exception to the rule. Water is very polar. That is, due to the geometry of how a water molecule forms, it has one end that’s very positively charged and one end that’s very negatively charged, even though the molecule as a whole is of neutral charge. As water cools, the molecules slow and the water contracts, just like most other substances, until they hit about 4 degrees Celsius. At this point the molecules are close enough together that the polarity comes in to play, and the molecules begin to repel each other and set up in a lattice which takes up somewhat more space. Therefore, ice is actually lighter than water, which is why it forms on the top of lakes in winter instead of from the bottom up.


There are some nuclear power plant designs which place the steam generators on a level higher than the reactor vessel. Some background info:

In general, in a pressurized water reactor, the primary coolant travels in a loop, from the reactor to the steam generator, through a big effin’ pump, and then back into the reactor.

The coolant comes out of the reactor much hotter than when it went in. When it gets to the steam generator, it goes through tubes that are surrounded by boiler water. As the boiler water turns to steam, the primary coolant cools off.

What advantage is there to placing the steam generator higher than the reactor? If you design the plant carefully, natural convection is sufficient to cause the coolant to flow enough to handle lower power levels. When the water comes out of the reactor, it is much hotter, and therefore less dense. The water on the far side of the steam generator is cooler, and therefore more dense. The hot water rises and the cool water falls, causing the coolant to flow in the closed loop. The flow is a mere trickle in comparison to what the pumps can produce, so at high power, the pump must be turned on, but you can shut it off at low power.

Why is this so cool? Those pumps are noisy, and some power plants are in places that are better off quiet (run silent, run deep :))
In addition, if you have a loss of electrical power or damage to a pump, you can still cool the core and avoid a nasty event.

If you have a hot water heating system, the fact that the furnace is in the basement and the cooled water upstairs means you get considerable circulation even without a pump. In fact, the house I grew up in had a coal furnace and certainly no pump. This did mean the second floor was not quite as warm as the first, but it certainly circulated, all from the fact that the hot water was less dense than the cold.

I still remember the pain of putting out ashes every week for the city to collect. Wonder when they stopped.

Here’s a cite for my post above:

While it is true the mechanism of “diffusion” is always present, it contributes very little to dispersion of the cream when added to coffee. Currents due to initial conditions of forced movement and convection will be the overriding contributors.

Just to add…

There are confounding variables in regards to the OP.
Salt content
Water density
oxygen level

While it is true that warm water is at the surface this is mostly due to the sun heating it.

The thermocline has to do with the separation of temperatures but is determined by oxygen content and thus density. The denser/colder/water molecules contain less oxygen. The oxygen gives rise :a-hem: to the warm water above the thermocline.

Surface water can be colder than water just below for obvious reasons.

of course when salt is added to the situation things change…ie: fresh water currents and “pools” in the oceans.

Truly amazing, to me at least, that sailors report discovering fresh drinking water at sea and scientist locate fresh water pockets beneath the surface.

BTW some ice floats, some sinks… :wink:

More important than polarity for water are the hydrogen bonding interactions.

I’m gonna make a hot water balloon and get back to you with the results.