Do You Add Sulfuric Acid to Water or Vice Versa?

Man in gloves using an eyedropper to add liquid to a glass jar.

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When you mix concentrated sulfuric acid and water, you pour the acid into a larger volume of water. Mixing the chemicals the other way can present a lab safety hazard.

Whether you add acid to the water or water to the acid is one of those things that's important to remember, but you may need to figure out. Sulfuric acid (H2SO4) reacts very vigorously with water in a highly exothermic reaction. If you add water to concentrated sulfuric acid, it can boil and spit and you may get a nasty acid burn. If you're wondering about the temperature change, mixing 100 ml of concentrated sulfuric acid and 100 ml of water initially at 19 degrees C reaches a temperature over 131 degrees C within a minute. The spitting or splashing of acid that results from mixing them in the incorrect order is from the intense heat produced by delayed boiling.

Sulfuric Acid and Water Safety

If you spill some sulfuric acid on your skin, you want to wash it off with copious amounts of running, cold water as soon as possible. Water is less dense than sulfuric acid, so if you pour water on the acid, the reaction occurs on top of the liquid. If you add the acid to the water, it sinks. Any wild and crazy reactions have to get through the water or beaker to get to you. How do you remember this? Here are some mnemonics:

  • AA: Add Acid
  • Acid to Water, like A&W Root Beer
  • Drop acid, not water
  • If you think your life's too placid, add the water to the acid
  • First the water, then the acid, otherwise it won't be placid

Personally, I don't find any of those mnemonics easy to remember. I get it right because I figure if I get it wrong, I'd rather have a whole container of water splash on me than a whole container of sulfuric acid, so I take my chances with the small volume of acid and the large volume of water.

Sulfuric Acid and Water Reaction

When you mix sulfuric acid and water, sulfuric acid donates a hydrogen ion, producing the hydronium ion. Sulfuric acid becomes its conjugate base, HSO4-. The equation for the reaction is:

H2SO4 + H2O → H3O+ + HSO4-