Herb A is a dopamine reuptake inhibitor. Will smoking Herb A have comparable physiological effects as drinking an aqueous solution of the same herb?
I would imagine there must be some observable difference when you digest it as opposed to simply inhaling. It's the case for most.. substances.
I think it mostly depends on the speed of uptake. I wouldn't say the effect would be much different when considering one chemical, other than the "rush" from smoking making the chemicals entering the bloodstream much quicker and making it feel more intense. What's important to note however, is that herbs often contain more than just one psychoactive chemical. This means I think it really depends on how efficient the chemicals are digested via stomach lining/esophagus/intestines compared to the lungs being pretty efficient at letting things through. Digestion involves chemicals that often are capable of destroying valuable substance, or even altering the chemical structure of the originally swallowed solution in certain instances.
Some chemicals are heat sensitive and certain unmentionables are impossible to smoke because of their heat sensitivity, while others require higher than normal temperatures to fully combust. Is there a way to know what chemicals are smokeable and which aren't?
Experimentation. I like to vape chemicals with very low boiling points.. but even cannabis is best in vapor imo because the heat isn't as destructive. Your post makes a great point though! This heat I suppose could be looked at similar to how digestion can alter a molecular structure. Burning/vaping THC usually undergoes a process of decarboxylation that transforms THC-A into delta-9 THC; the much more potent psychoactive form.
smoking is a just a direct way into the blood. if you eat said herb, it will still end up in the blood. though it must pass the stomach and liver, and thus can be damaged depending on the herb. or amplified.
It all depends. Like you said, many chemicals are destroyed when exposed to direct flame. However, a lot of these chemicals can still sorta be smoked despite their low boiling points, if you're careful about it and heat the herb more than you burn it. Also, as someone else pointed out, anything taken orally must pass through both the digestive tract and the liver before entering the bloodstream, often completely changing its chemical structure a number of times, whereas anything inhaled can go directly into systemic circulation. All in all, smoking is usually not the way to go, but it's pretty easy to figure out when it is or isn't if you just look to the traditional use of the herb. Did people generally make decoctions out of it or smoke it? People have had all this shit figured out for a really long time. Assuming it is herbs we're talking about.