T&T: Acid dilution WARNING

[Candy Chapman and Gary Bell]

"...NEVER add water to muriatic acid!!!.  Or POOOOffffffffffffffff"

Randy said:
  Actually, the reason to add acid to water and not the other way around is
that if you splash when adding acid to water, the drops will be mostly acid.
If you add water to acid the drops splashed out will be acid.

  Regards,
  Randy Pickelmann

Sorry, no disrespect intended here, but Very Wrong Answer, although you are of
course logically correct about the concentrations.  The HUGE reason to NEVER
add water to any acid is that the water molecule is almost instantly broken
apart into ions (H+ and -OH), and that reaction is quite exothermic -- it
releases a LOT of energy in the form of heat.  As you first begin to add
water, the still strong acid solution can instantly boil, often appearing like
an explosion.  Lots of splashing, and you don't have to jiggle it.
Exceptionally Bad News.  In labs, industry and commerce acid dilution is
ALWAYS done acid into water, and is invariably done with safety equipment like
face shields, acid proof gloves, aprons/smocks and shoe covers/boots.  Of
course the amount of heat released depends on the concentration of the acid
and the amount of water added: so if your already diluted muratic acid (in
this case) is very dilute, and/or the water added verrrry slowly and
carefully, perhaps under controlled conditions with a thermometer to measure
the inevitable temperature rise, you could get away from the consequences of
breaking one of the primary safety rules of chemistry.  Or perhaps not...  I
know, everyone would rather pick up a vessel of water to pour, rather than one
of acid, but the consequences can sometimes be dire.  Please cover your eyes,
use rubber gloves, and pick a safe place to do this.  In the case of topping
up batteries on a boat, you have little realistic choice, and honoring the
reality that the sulpheric acid in a wet cell battery is quite concentrated,
please wear some appropriate safety stuff, dribble the water in slowly, and
don't put your face where splatters could come out.

The surprisingly excitable Mister Science.