The acid should always be added to water , not water to acid, because diluting a concentrated acid is a highly exothermic (heat‑releasing) process and this order prevents violent splashing and accidents.

Core idea in simple words

  • When you mix acid and water, a lot of heat is produced.
  • If you pour water into concentrated acid, the tiny amount of water gets super‑heated instantly and can boil and splash acid out of the container.
  • If you pour acid slowly into a large amount of water, the heat gets spread out and absorbed by the water, so heating is gentler and safer.

Easy way to remember (often taught in labs):
“Do as you oughta, add acid to water.”

What exactly happens?

  1. Exothermic reaction
    • Mixing strong acids with water releases a lot of heat (it is an exothermic process).
 * Acids like sulfuric acid can heat the mixture well above the boiling point of water when diluted.
  1. If you add water to acid
    • The acid is usually denser than water, so water stays on top initially.
 * All the heat is produced in a thin layer where water touches the acid, so that small amount of water can suddenly boil.

 * This boiling can throw hot, concentrated acid out of the container, and may even crack the glass due to local overheating.
  1. If you add acid to water (the safe way)
    • You start with a large volume of water, which can absorb the heat more evenly because of its high heat capacity.
 * You add the acid slowly with constant stirring, so the solution stays more dilute and the temperature rises more gradually.

 * This greatly reduces the risk of splashing, boiling, and container breakage.

Short, exam‑style answer

While diluting an acid, the acid should be added slowly to water and not water to acid because dilution is highly exothermic; adding water to concentrated acid can cause sudden boiling and splashing of hot, concentrated acid, which may burn the person and even break the container, whereas adding acid to plenty of water allows the heat to be safely absorbed.

Meta description (SEO‑style):
Learn why, while diluting an acid, it is always recommended to add acid to water and not water to acid, with a clear safety explanation of the exothermic reaction involved and the risks of splashing and burns.