Hydronium Ions, Dry HCl and Acid Dilution | Class 10 Science

Quick answer: Acids show acidic properties only in water, because they need water to form hydronium ions ($\mathrm{H_3O^+}$): $\mathrm{H^+ + H_2O \rightarrow H_3O^+}$. That is why dry HCl gas does not change the colour of dry blue litmus paper, but turns moist blue litmus red. When diluting, always add acid to water slowly, with stirring, never water to acid, because the process releases a lot of heat.

If dry hydrogen chloride gas is passed over dry blue litmus paper, nothing happens. But if the litmus paper is moistened with water, it quickly turns red. Why does HCl gas behave like an acid only when water is present?

The reason is that an acid produces hydrogen ions only in water, and these hydrogen ions combine with water molecules to form hydronium ions ($\mathrm{H_3O^+}$).

In CBSE Class 10 Science, Chapter 2 (Acids, Bases and Salts), the dry and wet HCl experiment, hydronium ions, strong and weak acids and why acid is added to water are frequently asked board exam topics.

What You Will Learn

• Why hydrogen ions exist in water as hydronium ions ($\mathrm{H_3O^+}$)
• The dry vs moist HCl gas litmus experiment
• The difference between strong and weak acids
• What happens to the concentration of hydronium ions on dilution
• Why acid must be added to water, and not water to acid

1. Hydronium Ions: Why $\mathrm{H^+}$ Does Not Exist Alone in Water

A hydrogen ion, $\mathrm{H^+}$, is a hydrogen atom that has lost its only electron, so it is just a proton. It cannot exist on its own in water. Instead, it combines with a water molecule to form a hydronium ion:

$\mathrm{H^+ + H_2O \longrightarrow H_3O^+}$

Key point: Acids show acidic behaviour only when they are dissolved in water, because only then do they produce hydrogen ions, which exist as hydronium ions ($\mathrm{H_3O^+}$).

2. Dry vs Moist HCl Gas Experiment

Setup

Hydrogen chloride gas is prepared by heating solid sodium chloride with concentrated sulphuric acid:

$2\mathrm{NaCl} + \mathrm{H_2SO_4} \xrightarrow{\text{heat}} \mathrm{Na_2SO_4} + 2\mathrm{HCl}\uparrow$

The gas is passed through a guard tube containing calcium chloride, which absorbs moisture and keeps the gas dry.

Explanation

With dry litmus paper: without water, HCl molecules do not separate into ions. No hydrogen ions are formed, so dry HCl gas shows no acidic behaviour.

With moist litmus paper: HCl dissolves in the water on the paper and forms ions:

$\mathrm{HCl + H_2O \longrightarrow H_3O^+ + Cl^-}$

The hydronium ions turn the moist blue litmus paper red.

3. Strong and Weak Acids

The strength of an acid depends on how much of it breaks up into ions in water, not on how concentrated it is.

Extra fact: Chemists describe this using the degree of ionisation, the fraction of acid molecules that break up into ions. It is close to $1$ for strong acids and much smaller for weak acids. This term goes beyond the Class 10 textbook.

4. What Happens During Dilution?

Dilution means adding water to an acid or base. It decreases the concentration of ions ($\mathrm{H_3O^+}$ or $\mathrm{OH^-}$) per unit volume.

• The solution's volume increases, so the hydronium ions are spread out more.
• The concentration of $\mathrm{H_3O^+}$ per unit volume decreases.
• For an acid, the pH rises towards $7$ (the solution becomes less acidic), but it never becomes neutral just by dilution.

5. Why Is Acid Added to Water, and Not Water to Acid?

Common board question: Why should concentrated acid be added to water and not water to acid?

Answer:

1. Mixing a concentrated acid (especially sulphuric acid) with water is highly exothermic: it releases a lot of heat.
2. If water is added to concentrated acid, the small amount of water heats up very quickly and can boil. This can make the acid splash out and cause burns. The heat can also crack the glass container.
3. If acid is added slowly to water with constant stirring, the large amount of water absorbs the heat safely, so the mixture does not boil or splash.

Safe method: Always add concentrated acid slowly to water, with constant stirring.

6. Summary Table

7. Exam Tips and Common Mistakes

Exam Tip: In the HCl experiment, mention the calcium chloride guard tube. It explains why the gas is dry.

Common Mistake: Saying that dilution increases the number of hydronium ions. Dilution decreases their concentration per unit volume.

Common Mistake: Confusing a strong acid with a concentrated acid. Strength depends on how completely the acid ionises; concentration depends on how much acid is present in a given volume of water.

Frequently Asked Questions

Why does dry HCl gas not change the colour of dry litmus paper?

Without water, HCl does not form hydrogen ions, so it shows no acidic behaviour.

What is a hydronium ion?

It is the ion $\mathrm{H_3O^+}$, formed when a hydrogen ion combines with a water molecule.

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

The process releases a lot of heat. Adding water to acid can make the mixture boil and splash, causing burns, while adding acid slowly to water lets the water absorb the heat safely.

What happens to the concentration of hydronium ions when an acid is diluted?

It decreases, because the same ions are spread through a larger volume of solution.

What is the difference between a strong and a weak acid?

A strong acid ionises almost completely in water, giving many hydronium ions. A weak acid ionises only partially, giving fewer hydronium ions.