Chemistry Versus Physics: Why Does “Heat Rise”? (Part 1 of 2)

I’d like to start my blog with a question posed by a reader, a former chemistry teacher with 30 years of experience who now writes chemistry texts. He and some colleagues in the physics department had a long running dispute over a very basic observation, a dispute that illustrates how students are often given explanations that, upon reflection, make no sense whatsoever and even contradict other principles students are taught.

I currently live on the fourth floor of building with no elevator. There is a clearly discernible change in air temperature as I walk down the stairs. Ask most high school students (or teachers) of chemistry why this should occur, and we will be told “heat rises.” But why?

First, it must be pointed out that, strictly speaking, “heat rises” is a nonsensical statement. “Heat” refers not to “hotness” but to the exchange of thermal energy between two objects, and it can move in any direction. However, sad to say, it is not uncommon for even textbooks to misuse the term, and most people use the word to refer to “hotness,” as in the common expression “it isn’t the heat; it’s the humidity.”

The erroneous diction aside, there is certainly some element of truth to the aphorism “heat rises.” For example, boiling water in a pan with a glass lid will quickly lead to condensation as hot steam rapidly rises from the water’s surface to form droplets on the lid’s undersurface. Similarly, a burning candle produces hot particles (both solid and gaseous) that rise upward. (This is linked to the real reason why a candle goes out when put under a jar, discussed in chapter 1 of Science Myths Unmasked Volume 2.)

But why do hotter particles tend to move upward? We could ask the same question about lighter particles, like helium particles that slowly escape the atmosphere. If the acceleration of gravity is equal for all particles, why do lighter gas particles float upward?

Here, physics teachers will confidently point to buoyancy, giving further examples like hot air balloons and ice cubes (ice is less dense than water, so it floats). And that is where the problems start. The hot particles given off by a burning wick and free helium particles in the atmosphere are fundamentally different from the hot air contained inside a balloon or the particles in a cube of ice, in both cases there is a boundary segregating the space populated by one species from the space populated by the other.

Physics texts like to draw an imaginary, invisible wrapper around hot air and consider it as a “parcel.” They then pretend that the hot particles inside can be treated as a single entity that rises because it is less dense than surrounding, cooler air. But this makes no sense to a chemistry student because basic chemistry says the hot particles all bounce around, colliding millions of times a second with all manner of other particles. There is no coherent “chunk” of hot air that is pushed up by colder air and there is no membrane against which the outside air can smoothly push to buoy the hot air as in a hot air balloon. Textbooks generally do not give satisfactory or clear explanations of buoyancy, and what little explanation they give is critically linked to a fluid being displaced by an object with a well-defined boundary. Hence it has no application to the question of hot or light particles mingling with cooler or heavier particles.

In terms of basic chemistry, all that being hot means is that the particle is moving faster than otherwise; it says nothing about the direction of that motion. Similarly, there is no obvious reason why lighter particles should tend to move upward. The particles are just bouncing around chaotically, each collision randomly sending one upward and the other downward. These collisions do not favor faster (or lighter) molecules over heavier (or slower) ones, and basic physics says that all particles experience the same acceleration from gravity, so they all are pulled down equally.

If gravity does not favor one particle over another and the collisions do not favor one particle over another, and the motion of these particles is completely based on these two mechanisms, why do faster or lighter particles move upward compared to their slower or heavier colleagues?

So we have a real conundrum. We have a genuine physical phenomenon, but the common explanation contradicts both basic chemistry (that says particles just move about randomly) and basic physics (which says the acceleration of gravity is the same for all particles). What is going on?

In Part 2 of this blog post, I will explain the real reason why lighter and hotter particles tend to rise, but I’ll give people a few days to post conjectures or thoughts/comments.

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