UKH

Long story but we now own a hot tub, it really helps the Mrs medical problems. I have made a chlorine dispenser from an old pill bottle that I put chlorine tablets in to give a slow release in to the water. 

I started with 1 pin hole but that wasn't enough so I heated up a pin and made a second hole, as the hot pin melted through the bottle there was a tiny explosion / release of energy and a squirt of water flew out of the original hole. Tonight I found that 2 holes isn't enough so I put a third hole in, thinking it might have been trapped pressure the first time I took the lid off then put it back on again and melted the new hole with a hot pin again. Once again as the pin broke through there was a mini explosion and water was squirted from the existing 2 holes. 

I know chlorine isn't flammable so I'm wondering if any of the boffins on here can shed any light on what is happening. 

Your 'chlorine' tablets are probably chlorine dioxide, which (as a gas) will explosively decompose into chlorine and oxygen when heated

Swimming pool chlorine tablets are marked up as an oxidising agent, I presume the chlorine gas itself is an oxidiser unless there's an intermediate. Your pin and the plastic provide the volatile fuel and the ignition source.

Jk

Bingo, thank you. I suppose the bottle sitting in the 38 degree tub is enough to cause the decomposition then when the hot pin comes in to contact it creates the full reaction. 

Don't f_ with chemistry unless you know what you are doing.

What you want to do is fit a UV steriliser to the filtration loop and go ultra low chlorine.  It’s even nicer than a no-chlorine, peroxide based tub in my humble opinion...

It's got a uv steriliser but the book of words says to maintain a given chlorine level. Until I'm more familiar with looking after it I want to stick to the book. My only deviation is making a slow release tablet dispenser rather that adding it as powder with a spoon.

The pill bottle is fastened to the filter, unoccupied only the bottom holes are in the water then when we are in it the raising level submerge the top holes. I'm trying to balance it so that it maintains its level when not in use and adds a little bit more to cover losses whilst we use it, a 2 hour soak takes the level down from 5pp to not actually registering on the test strip.  Mk1 had too many holes in it so I'm going really slow and steady with mk2 (it's my last spare bottle) adding 1 pin hole every few days until I get the balance right. 

Fully agree, I don't class making a basic dosing system as F_with chemistry though. 

His comment had me thinking about known knowns, known unknowns, and unknown unknowns...beware of unknown unkowns I guess.