Why Acid Is Added to Water, Not Water to Acid | Class 10

Quick answer: Mixing a concentrated acid with water is highly exothermic (it gives out a lot of heat). If water is added to concentrated acid, the heat produced locally can make the mixture boil and splash out, causing burns, and the glass container may crack from excessive local heating. So the acid must always be added slowly to water, with constant stirring, so the large amount of water absorbs the heat.

What happens when acid mixes with water

The process of dissolving an acid in water is highly exothermic. For example, concentrated sulphuric acid reacts with water to form hydronium ions, releasing a lot of heat:

$\mathrm{H_2SO_4 + H_2O \longrightarrow H_3O^+ + HSO_4^-} \quad \text{(heat given out)}$

The same is true when a strong base such as sodium hydroxide dissolves in water.

Dilution means adding water to an acid to decrease the concentration of $\mathrm{H_3O^+}$ ions per unit volume.

Why you must not add water to concentrated acid

 water → into concentrated acid
          ↓
 heat produced in a small spot
          ↓
 water boils, mixture splashes
 glass may crack

1. The small amount of water added mixes with a large amount of concentrated acid at one spot.
2. A large amount of heat is produced locally, in a very small volume.
3. This can make the liquid boil suddenly and splash out, which can cause serious burns to the skin and eyes.
4. The glass container may also crack or break because of excessive local heating.

The safe method: add acid to water

 acid (slowly) → into water
          ↓  stir constantly
 heat spread through lots of water
          ↓
 gentle temperature rise

1. Add the acid slowly, a little at a time, to a larger volume of water.
2. Stir constantly with a glass rod so the heat spreads through the whole liquid.
3. Water has a high specific heat capacity, so a large amount of water can absorb the heat with only a moderate rise in temperature.
4. Wear safety goggles and gloves, and work in a suitable container such as a borosilicate beaker.

Memory line: "Add acid to water, as you ought to."

Model answer (NCERT question)

Question: Why should acid be added to water and not water to the acid?

Answer: Dissolving an acid in water is a highly exothermic process. If water is added to a concentrated acid, the heat generated may cause the mixture to splash out and cause burns, and the glass container may break due to excessive local heating. So the acid must always be added slowly to water with constant stirring.

Summary table

Exam tips

• Start your answer with "the dissolution of an acid in water is highly exothermic".
• Include both dangers: splashing that causes burns and breaking of the glass container.
• End with the correct method: add acid slowly to water with constant stirring.
• The same rule applies when dissolving solid sodium hydroxide, which is also highly exothermic.

Common mistakes

• Saying the order does not matter because the final solution is the same. The danger is during mixing.
• Forgetting to mention constant stirring.
• Calling the process endothermic. It gives out heat.
• Writing that acid dilution makes the acid stronger. Dilution decreases the concentration of $\mathrm{H_3O^+}$ ions.

Frequently Asked Questions

Why should acid be added to water and not water to acid?

Mixing acid and water is highly exothermic. Adding water to concentrated acid causes intense local heating, so the mixture may splash out and cause burns and the glass may crack. Adding acid slowly to water with stirring spreads the heat safely.

Is the dilution of an acid exothermic or endothermic?

It is highly exothermic, which means it gives out a large amount of heat.

What happens if water is added to concentrated sulphuric acid?

The heat produced in a small area can make the mixture boil and splash out, causing serious burns, and can crack the container.

Why is stirring important while diluting an acid?

Stirring spreads the heat evenly through the whole liquid and prevents dangerous hot spots.

What does dilution of an acid mean?

Dilution means adding water to an acid so that the concentration of hydronium ions per unit volume decreases.