You definitely can use it as an expression of the Acid Principle and a 'lower-level substitute' for 'our water', but it is not an absolute necessity. Other means can be used.
Unless you work completely 'purist' and move through the density scale from there, there is a need to 'magnetize' your initial matter/body by 'opening' it so it can attract AND keep 'our water'. There are various ways to 'magnetize/open/spiritualize' matter. Some (but not all) of these ways may involve acids, such as in the long process with Marcasite (a sulfide), where sulfuric acid forms naturally.
Besides, you have already mentioned that you haven't performed the work yet, so IF this is still the case and all your info comes from reading and from a set of fixed ideas you have formed in your mind, you are in no position to preach what 'alchemy proper' is or isn't, or what 'auxiliary' means are at our disposal to kick-start the process.
Interesting to note what a combination of mild shit-storm as well as positive feedback my admitted memory-related mistake has caused here and in PM as well
What particular knowledge of chemistry would that be?
If he/she doesn't get distracted, as 5 year olds often do ... Also, if you have a preference for a path using high temperatures, would you allow a 5 y/o child near it?
Addendum: As 'proof of concept', in the past I have used more 'common' expressions of the 'acid principle' to 'open' various matters/bodies and 'elevate' them, such as changing 'states' (from 'closed' to 'open', from 'fixed' to 'volatile', from polar-soluble to non-polar soluble, etc...).
To further exemplify, various 'waters' such as dew, rainwater, even a body's own 'waters' are examples of (perhaps relatively week but nevertheless efficient) expressions of the 'Acid Principle'.
Another interesting example of using the 'Acid Principle' is found HERE.
What is often referred to as 'Our Water' (Secret Fire) can be (somewhat and quite loosely) compared with a 'dew on steroids'.
It is archetipally derived from 'Spiritus Mundi Acidus Corporeus' (Golden Chain). Aso in the Golden Chain, 'Our' Subject/Matter is similarly described as 'Spiritus Mundi Fixus Alcalicus Corporeus'.
IF we commence our work with common (corporeal) matter(s), we inevitably work with CHEMICALS ('Code'). Using 'Code against Code' is similar to the Hermetic Way of using 'Law against Law'. A variation of 'hacking', if you will...
It matters little whether I have performed experiments or not, the moment you add a substance foreign to it it is ruined. I am only talking about Alchemy Proper, not side stuff.
Instead of calling it shit storm, you must thank God that you have a bunch of well meaning friends. I am assuming they PM'ed you regarding acid part.
Regarding chemistry, the knowledge of certain salts and their behavior.
Regarding 5yr old, talking about the easiness.