Chapter 2 · Question 6

Why should an acid always be added to water and not water to the acid while diluting? What happens to the concentration of $\text{H}_3\text{O}^+$ ions when an acid is diluted?

Answer Revealed

Direct Answer:

Mixing an acid or a base with water is highly exothermic — it releases a large amount of heat. If water is added to a concentrated acid, the heat released can cause the water to boil instantly, splashing the concentrated acid out of the container and causing severe burns. The container itself may also break due to localised excessive heating. When acid is added slowly to water with stirring, the large volume of water absorbs and dissipates the heat safely. When an acid is diluted, the concentration of

\text{H}_3\text{O}^+

ions per unit volume decreases because the same number of ions is now spread over a larger volume of solution.

Simple Explanation

Mixing acid with water releases heat — so much that if you pour water into acid, it can boil and splash corrosive acid everywhere. That is how serious chemical burns happen. The safe way: pour acid slowly into water while stirring, like you are feeding it in bit by bit. The water absorbs the heat without boiling over. When you dilute acid, the

\text{H}_3\text{O}^+

concentration goes down — same number of acid particles, but now swimming in more water.

Exam-Ready Structure

Dilution of concentrated acids is a safety-critical procedure justified by the highly exothermic nature of the dissolution process. 1. The process is exothermic: Activity 2.10 demonstrates that when concentrated

\text{H}_2\text{SO}_4

is slowly added to water in a beaker and the beaker is swirled, the base of the beaker becomes warm to touch — confirming that heat is released. Dissolving NaOH pellets in water also produces a temperature rise. 2. The danger of adding water to acid: Concentrated acids, especially

\text{H}_2\text{SO}_4

, have a strong affinity for water. If water is added to the acid, the released heat is so intense and localised that the water can boil almost instantaneously. This violent boiling may splash the concentrated acid out of the container, causing severe chemical burns to the person. The localised heating may also cause the glass container to crack. 3. The correct procedure (add acid to water): (a) Take the required volume of water first. (b) Add the concentrated acid slowly and in small portions to the water while continuously stirring. (c) The bulk of the water absorbs the heat and dissipates it, keeping the temperature rise gradual and safe. (d) The warning sign shown in Figure 2.5 (a hand being corroded by acid splashing from a test tube) appears on containers of concentrated acids and NaOH pellets for this reason. 4. Effect on ion concentration: Dilution means adding more solvent (water) to a given amount of solution. Since the number of moles of acid (and hence the number of

\text{H}_3\text{O}^+

ions) remains the same while the volume increases, the concentration of

\text{H}_3\text{O}^+

ions per unit volume decreases. The acid is said to become 'diluted.'

Key Points

Dissolving acid or base in water is highly exothermic (releases large amount of heat)
Adding water to acid: water boils instantly, acid splashes out, causes burns
Correct method: add acid slowly to water with continuous stirring
Dilution: same number of $\text{H}_3\text{O}^+$ ions spread over larger volume $\rightarrow$ concentration decreases
Warning sign (Figure 2.5) appears on containers of concentrated acids and NaOH

Common Mistakes

Assuming 'dilution = weaker acid,' confusing dilution (decreasing concentration) with weakness (low degree of dissociation)
Memorising the rule without understanding the exothermic reason — the key is heat dissipation, not a chemical trick

Why should an acid always be added to water and not water to the acid while diluting? What happens to the concentration of $\text{H}_3\text{O}^+$ ions when an acid is diluted?