Water to Acid vs Acid to Water

[JimK]

Ben is absolutely right. 18" of water is not deep enough to safely add the acid directly to the pool. Diluting it first in a better controlled environment makes sense. Sheepishly, I didn't post that here - telling myself that we were already pretty far off topic.
I will still stand by A->W (unless it's already quite dilute). FWIW I've seen a drop of water dance on a volume of HCL and I've seen it explode splashing HCL all around.

The demo I saw of a drop of water dropped into a beaker of HCL resulted in a very violent explosion.

[PoolDoc]

The demo I saw of a drop of water dropped into a beaker of HCL resulted in a very violent explosion.

Was that lab acid, or 31% commercial acid?

I'm going to have to try that.

[PoolDoc]

Hm-mh. Still going to try it, but this video shows exactly what I would have expected (somewhat obnoxious sound). with commercial muriatic acid and water:
https://www.youtube.com/watch?v=hMnwhifRPuw

Mind you, the situation would be very different with sulfuric acid, where the reaction would be strongly exothermic. I haven't worked with other acids, outside of a lab.

[BigDave]

Perhaps the demo was with H2SO4 - not what I recall but it's been more than 35 years. It did make an impression on me. To this day I trust A->W and will likely continue to recommend it until there's a good reason to do otherwise.
I'll have to try it this summer when it's time to adjust my pool's pH.

[JimK]

Was that lab acid, or 31% commercial acid?

I'm going to have to try that.

Don't know. I wouldn't recommend trying it. Look it up on YouTube instead.

[CarlD]

The demo I saw of a drop of water dropped into a beaker of HCL resulted in a very violent explosion.

Hence why you ALWAYS pour the acid into the water, not the other way around! (Ka-BOOOM!!!!)

[chem geek]

The heat (enthalpy) of dilution for some acids is given in this link. 31.45% HCl is around 10 so the heat of dilution is roughly 10 kJ/mol. 38.5% Sulfuric acid, by comparison, has a heat of dilution of around 28 kJ/mol so substantially higher. This link calculates the temperature rise for a specific example of adding acid to water and explains why adding water to acid is not a good idea. It basically has the water heat up so much that it can boil and splatter, causing acid to be propelled with it risking getting splashed with concentrated acid.

[Anonymous [GDPR] European in the UK]

I like adding the acid slowly to the pool all the way round and letting it fume, I grab my air guitar and sing "Smoke on the water"