When you mix strong acids and water, it makes a difference whether you add acid to water or water to acid. Always add acid to water and not the other way around. This prevents dangerous splattering caused by a sudden release of heat.
If you add a little water to a lot of concentrated acid, the resulting solution is still concentrated. The hydration reaction uses all the water (acid is the limiting reactant), generating a lot of heat. The solution violently boils, spitting concentrated acid out of its container. If you add a little concentrated acid to a lot of water, water is the limiting reactant and the resulting solution is more dilute. Here, all the acid reacts, but there is extra water to absorb the heat, lessening the chance of boiling.
Key Takeaways
- Always add acid to water, not the other way around.
- Adding water to acid can cause violent boiling and splashing.
- Diluting acids and bases is exothermic—it releases heat.
- Use proper glassware and cool solutions when necessary.
- This rule is critical for strong acids like sulfuric acid and strong bases like sodium hydroxide.
Why Add Acid to Water
There are a few factors that make it better to add acid to water. Diluting acid with water is exothermic, so it’s easier to boil and splash water added to acid than acid added to water because water has a high heat capacity and can absorb a lot of heat. Also, some strong acids are denser than water. If you add water on top of acid, the heat produced occurs at the interface, causing localized boiling and potentially dangerous spattering. On the other hand, if you pour acid on top of or into water, the water rises over the acid before mixing and contains the reaction.
Diluting Sulfuric Acid
Sulfuric acid (H2SO4) is the most dangerous common acid to dilute. Partly, this is because it reacts so violently with skin and clothing. Sulfuric acid quickly dehydrates proteins and carbohydrates in skin and muscle. The acid is much heavier than water, so water added to it reacts with the top layer first. A lot of heat gets generated, even when sulfuric acid and water are mixed properly. Mixing 100 ml of concentrated sulfuric acid and 100 ml of water at 19 °C reaches a temperature over 131 °C (well past the boiling point of water) in under a minute.
It’s the hydration reaction that generates all that heat:
H2SO4 + H2O → H3O+ + HSO4–
The reaction between sulfuric acid and water is highly exothermic. Sulfuric acid acts as a strong dehydrating agent, binding water molecules and forming hydronium (H₃O⁺) and hydrogen sulfate (HSO₄⁻) ions in solution.
Similarly, sulfuric acid readily strips water from organic molecules, dehydrating them.
Dilution Safety Tips
In general, the higher the concentration the acid, the greater the heat increase and the higher the chance of boiling and splashing. Be extremely careful diluting any concentrated strong acid. Always wear proper safety gear and work under a fume hood.
No matter what acid you’re diluting, remember reaction rates double for every 10 degree Celsius temperature increase. So, if you make a serial dilution of an acid, the change of boiling or splashing the acid increases because the stock solution is already hot. There are a couple of ways to avoid this. One is to cool the stock solution before diluting it. You can rest the container in an ice bath before making the dilution. Another method is pouring the hot acid solution over ice made from de-ionized water and then diluting it with room temperature water to reach the final volume.
Finally, choose your glassware wisely. An Erlenmeyer flask or volumetric flask is a better choice than a beaker or graduated cylinder because the flask shape acts as a sort of splash shield (also, graduated cylinders are notoriously unstable and easy to tip over).
Tips to Remember to Add Acid to Water
Here are some easy mnemonic devices to help you remember to add acid to water:
- 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
Diluting Strong Bases
Like strong acids, strong bases react with water in an exothermic reaction. Examples of strong bases include sodium hydroxide (NaOH), potassium hydroxide (KOH), and calcium hydroxide (Ca(OH)₂) Also, like the strong acids, strong bases tend to be corrosive and able to cause chemical and thermal burns on skin. It’s best to add base to water, just like you would acid.
References
- Bishop, Michael L.; Fody, Edward P.; Schoeff, Larry E. (2004). Clinical Chemistry: Principles, Procedures, Correlations. Lippincott Williams & Wilkins. ISBN 0-7817-4611-6.
- Cleapss.org. (2007). Student Safety Sheets – Sulfuric(VI) Acid.
- Pauling, L.C. (1988) General Chemistry. Dover Publications.