Re: Battery acid

Students and people who work with chemicals are told that it is a very dangerous practice to add water to sulfuric acid, and that they should not do so. The approved practice is to work out how much water will be needed, put that amount of water in a LARGE container, and add the acid to the water.

But everyone routinely adds distilled water to the cells of car batteries to make the electrolyte up to the proper level, and the acid in car batteries is sulfuric acid!

Why this difference in practice?

Well, the safety problem with water/sulfuric acid systems is the large amount of heat that is generated when water and sulfuric acid are mixed together. Sulfuric acid supplied by chemical companies is normally 98% pure. When you start adding water to this, a lot of heat is generated - enough for temperatures higher than 100 deg C to be attained. The water sizzles and boils, and dangerously corrosive hot acid fumes are formed.

The acid in car batteries, although it is sulfuric acid, and although it is still in a fairly strong and corrosive form, has been broken down already with quite a considerable amount of water. Battery electrolyte is only 30 to 50% sulfuric acid, with the balance being water. But most of the heat produced in mixing water and sulfuric acid is evolved in the first stage of the dilution, between about 98% and 80% sulfuric acid content. Heat is still given out when water is added to battery electrolyte. The electyrolyte, and even the wall of the battery can be felt to be getting a bit warm. But at this lower concentration of sulfuric acid, there is not enough heat generated to cause significant hot acid fuming, or other dangerous effects.