Drawing lines between chemistry and physics.

Over at Uncertain Principles, Chad Orzel tries to explain the fundamental difference between physics and chemistry:

My take on this particular question is that there's a whole hierarchy of (sub)fields, based on what level of abstraction you work at. The question really has to do with what you consider the fundamental building block of the systems you study.

Chad's rough breakdown is fine as far as it goes. But it wouldn't (in my experience) be a terribly accurate guide to discerning what (say) a physical chemist actually worked on in his or her research.

Chad describes the corresponding rung on the hierarchy like this:

This is the level of the overlapping fields of molecular physics and physical chemistry, which takes atoms as the essential particles and looks at how they fit together to make simple molecules. They don't worry about the nuclei at all, really, and only a little bit about the electrons. It's a tricky division to make, but if I had to make a stab at defining the essential difference between molecular physics and small-molecule chemistry, I would say that the physics side is mostly concerned with how small molecules are put together and how they stay together, while chemists are more interested in how small molecules react with each other and swap pieces back and forth.

Mmmm ... kind of.

I was a physical chemist who worked on mechanistic studies of oscillating chemical reactions. I'm not sure that I took "atoms as the essential particles" to a greater or lesser extent than did my colleagues working in organic chemistry or in biophysical chemistry. Also, I'm not sure all the molecules and ions in my system would count as "small". Sure, they're small compared to proteins, but big compared to diatomic and triatomic species.

But maybe this difference of opinion is, in itself, illustrating a point of difference between chemistry and physics.

Chad (the physicist) sees the division as a matter of the size and type of the building blocks in your system. My own view is that the differences between chemistry and physics have less to do with the objects of study and more to do with the particular theoretical, methodological, and instrumental "toolbox" one brings to the job of studying those objects.

When I mentioned this to Chad, he replied, "The objects being studied play a large part in determining the appropriate methodology and theoretical approach for dealing with them."

That's true, but at the boundaries, a chemist and a physicist will look at the same stuff using different theoretical and methodological toolboxes.

They may borrow tools from each other, but they won't always use them in ways that the folks they borrowed them from would envision (or completely endorse). Maybe some of this is related to physicists generally using their tools in a scaling-up-from-atoms way and chemists generally using their tools in a scaling-down-from-molecules way. But some of it may have more to do with the sorts of problems people in each field seem themselves as responsible for explaining -- something I think may not always track building-block size perfectly.


More like this

"That's true, but at the boundaries, a chemist and a physicist will look at the same stuff using different theoretical and methodological toolboxes."
Ah paradigms...
And also, there are boundary objects that are the same tool to look at different things (a la Fujimura)
I love when I read your posts and can use them to test myself on my STS readings :)

I remember a lecture by Chris Walsh (a chemist) where he nailed it by saying that it's not so much that people decide what field of research to go into, the field picks them depending on what level of uncertainty they want to deal with.

At one end, pure mathematicians can choose to study problems they can define absolutely: systems with no room for black boxes, systems with absolute proofs. Physicists have to model the real world, which means some uncertainty is introduced, even if it only shows up after six or seven significant figures. Chemists have to deal with a lot more uncertainty than physicists, and then cube n^x that for biologists. At the other end, sociologists basically have to study the emergent properties of groups of Russian dolls built up from many layers of the black boxes that the other sciences haven't figured out how to open yet.

Repost of my comment at Chad's blog.

Just having fun here. ;-)

Offhand, I'd say that, with the exception of fluid mechanics, physics tends to be a dry science. Chemistry is the wet science; biology is the wet and messy science.

In my experience, physics tend to choose the most boring samples to study that they can possibly find. Pick something chemically insteresting for once, please.

I have the impression that Chad is looking more at the objects studied by the respective fields, whereas you are talking about what is going on inside the heads of the people who are doing the studying. Both are valid ways to look at it.

From my point of view, a field of study is defined by the set of traditions within the discipline. Just as cultures can borrow traditions from each other, so can scientific disciplines.

Of course there is much overlap.

Where I come from, anyone with the suffix of -physicist is devoted to ignoring "fundamental building blocks" so that we can use continuum mechanics. We do this on spatial scales covering many orders of magnitude, from atoms to mountains.

Anyone with the suffix of -chemist is devoted to creating building blocks where there are none - especially in the lower mantle.