Can hydrochloric acid and digestive enzymes cause a fire?

Question

I am writing a book and have a species of invasive plants which secretes acid to turn plant and animal matter into a kind of fertilizer. Is this acid capable of causing a fire? Or are the countless other chemicals in plants and animals be enough to prevent a fire from starting?

Obviously the latter is the case when it comes to stomach acids, otherwise we would all combust into flames. But that would be an aspect of natural evolution. This is an invasive species and I have never known of a plant species to act in the matter that I am attempting to accomplish.

Answer 1

The chemical term you are looking for is "hypergolic" and it means spontaneously causes ignition. There are all sorts of chemicals that are hypergolic with other specific chemicals but very few chemicals are hypergolic with most other chemicals.

I'm going to answer your specific question but follow it with a more general description and interesting articles (interesting if you like the topic that is).

Short answer

I didn't find much that was directly (meaning without other chemicals present) hypergolic with hydrochloric $ HCl $, however, there are some chemicals that are hypergolic with sulfuric acid ($ H_2SO_4 $).

This includes:

• Ether (burns vigorously)
• Nitric Acid (vigorous reaction but no flame)
• Potassium Perchlorate (explodes)
• Potassium Perchlorate + sugar/starch
• etc.

Here's some discussion on a science board about what chemists have found react hypergolically or explosively.

Here's a list of videos which show the same.

General discussion
The problem with a plant carrying a digestive chemical which reacts so strongly with the chemistry of other living things is this: creatures evolved on the same planet, share the same chemistry. Meaning that plant contains chemicals which can cause itself to spontaneously burst into the flame if its reservoir of that chemical is ruptured.

Imagine an explore hacking his way through a jungle with a machette. He accidentally hits this "flame bush" and ruptures its digestive sack - immediately causing the plant to burst into flame. If any of that fluid sprayed out of the sack, say onto another plant or the explorer, then adjacent plants and even the explorer also burst into flame.

Fun stuff
There are chemicals which are hypergolic with many other chemicals. You might find the discussion in Sand won't save you this time discussion amusing or interesting.

The chemical is called chlorine trifluoride ($ ClF_3 $ ).

There’s a report from the early 1950s (in this PDF) of a one-ton spill of the stuff. It burned its way through a foot of concrete floor and chewed up another meter of sand and gravel beneath, completing a day that I'm sure no one involved ever forgot. That process, I should add, would necessarily have been accompanied by copious amounts of horribly toxic and corrosive by-products: it’s bad enough when your reagent ignites wet sand, but the clouds of hot hydrofluoric acid are your special door prize if you’re foolhardy enough to hang around and watch the fireworks.

”It is, of course, extremely toxic, but that's the least of the problem. It is hypergolic with every known fuel, and so rapidly hypergolic that no ignition delay has ever been measured. It is also hypergolic with such things as cloth, wood, and test engineers, not to mention asbestos, sand, and water-with which it reacts explosively. It can be kept in some of the ordinary structural metals-steel, copper, aluminium, etc.-because of the formation of a thin film of insoluble metal fluoride which protects the bulk of the metal, just as the invisible coat of oxide on aluminium keeps it from burning up in the atmosphere. If, however, this coat is melted or scrubbed off, and has no chance to reform, the operator is confronted with the problem of coping with a metal-fluorine fire. For dealing with this situation, I have always recommended a good pair of running shoes.”

Put chemicals like these under the title "if you discover someone messing with these, run away from them as fast as you can".

Answer 2

Plants which secrete acids or digestive chemicals actually do exist, in the form of Venus Flytraps, Pitcherplants and other sorts of plants which trap and digest insects. This is probably a bit slow for what you are looking for.

Other plants have various toxic chemicals circulating or stored in their various leaves, stems or fruit in order to deter herbivores from eating them. Advancing a bit from Jim2B's answer, some plants on your world may have evolved a way of storing a hypergolic chemical as a means of deterring predation by even the most aggressive and well armoured herbivores. One bite into the "flame pod" and the problem of being eaten is rapidly solved. As a bonus, the herbivore may be mortally wounded and die close by, providing some much needed bio-matter to the soil. The burst of fire might also clear out competing plants, providing more space for growth, clearings for new seedlings to sprout and a clear space for sunlight to filter in through the canopy.

Since most animals might eventually learn to avoid the flame plant, there would also be some sort of mechanism for the plant to open the flame pods so the plant can get the benefits of fire without waiting to be eaten. It would also be interesting to think through the implications on the rest of the ecological system if such a plant actually existed (what do the other plants use as countermeasures, for example?)

Answer 3

What of the organism was purposfully engineered for that? It could be like Nivin's stage trees. In a civilisation where all types of industrial chemestry is done this way, the only feedstocks produced would be things that can no longer be used in biological enzyme systems, so in addition to growing synthetic fiber, ordinary fuel, fertilizer, etc. they would grow truely nasty stuff that must then be put in a non-living vat for subsequent processing.

Consider that this is somehow approchable to them with a low tech level, due to the nature of life on their world and their species innate capabilities. Directed invented substances would emerge somewhat after agraculture, and maybe became signigicant in a time like our ancient Greek civilizations. Their equivalents of Heron and Archemedes produced synthetic materials.

Move forward a few thousand years, with civilizations falling and rising, and environments changing in an area. You might have some very interesting wildlife, with especially spectacular sterile hybrids. Their Archemedes analogue would have produced weapons...