What Acids and Bases Have in Common & Acid Dilution for CBSE Class 10

When we study hydrochloric acid ($\text{HCl}$), sulphuric acid ($\text{H}_2\text{SO}_4$), and nitric acid ($\text{HNO}_3$), they exhibit remarkably similar chemical behaviors despite possessing different elemental compositions. Similarly, sodium hydroxide ($\text{NaOH}$), potassium hydroxide ($\text{KOH}$), and calcium hydroxide ($\text{Ca(OH)}_2$) share soapy textures and turn red litmus blue. What fundamental microscopic property is common to all acids, and what is common to all bases?

In CBSE Class 10 Science, answering this question reveals the essential role of water as an ionizing solvent, explains electrical conductivity in electrolyte solutions, and establishes the strict laboratory safety protocol for diluting concentrated acids.

What You Will Learn

• The common characteristic of all acids: generation of $\text{H}^+(aq)$ and hydronium ions ($\text{H}_3\text{O}^+$)
• Why dry $\text{HCl}$ gas does not change the colour of dry litmus paper (NCERT Activity 2.9)
• The common characteristic of all bases: generation of hydroxide ions ($\text{OH}^-(aq)$)
• Alkalis: water-soluble bases
• Electrical conductivity in solutions: why acids conduct electricity while glucose and alcohol do not
• The exothermic nature of acid dilution and the universal safety rule: Acid to Water, NEVER Water to Acid

1. What Do All Acids Have in Common?

All acids contain hydrogen in their chemical formulas. However, not all hydrogen-containing compounds are acids (for example, glucose $\text{C}_6\text{H}_{12}\text{O}_6$ and alcohol $\text{C}_2\text{H}_5\text{OH}$ contain hydrogen but are not acidic).

The Defining Characteristic of Acids

All acids produce hydrogen ions, $\text{H}^+(aq)$, when dissolved in water.

$\text{HCl}(g) \xrightarrow{\quad \text{Water} \quad} \text{H}^+(aq) + \text{Cl}^-(aq)$ $\text{HNO}_3(l) \xrightarrow{\quad \text{Water} \quad} \text{H}^+(aq) + \text{NO}_3^-(aq)$ $\text{H}_2\text{SO}_4(l) \xrightarrow{\quad \text{Water} \quad} 2\text{H}^+(aq) + \text{SO}_4^{2-}(aq)$

The Formation of Hydronium Ions ($\text{H}_3\text{O}^+$)

A hydrogen ion is simply a bare proton (an atom of hydrogen stripped of its single electron). Because of its extremely small size and intense positive charge density, a bare $\text{H}^+$ ion cannot exist independently in aqueous solution. It immediately bonds with a polar water molecule to form a hydronium ion ($\text{H}_3\text{O}^+$):

$\text{H}^+ + \text{H}_2\text{O} \longrightarrow \text{H}_3\text{O}^+ \text{ [Hydronium Ion]}$ $\text{HCl} + \text{H}_2\text{O} \longrightarrow \text{H}_3\text{O}^+ + \text{Cl}^-$

Important: <u>Acidic properties are displayed ONLY in the presence of water. Without water, an acid cannot dissociate to produce hydrogen or hydronium ions.</u>

2. NCERT Activity 2.9: The Dry HCl Gas Experiment

Take about $1\text{ g}$ of solid $\text{NaCl}$ in a clean, dry test tube and add concentrated sulphuric acid. A colourless gas ($\text{HCl}$) is evolved: $2\text{NaCl}(s) + \text{H}_2\text{SO}_4(l) \longrightarrow \text{Na}_2\text{SO}_4(s) + 2\text{HCl}(g) \uparrow$

Testing the Gas with Litmus Paper:

1. With Dry Blue Litmus Paper: The dry blue litmus paper shows no change in colour.
2. With Moist (Wet) Blue Litmus Paper: The moist blue litmus paper immediately turns red.

Scientific Reason:

• In the absence of moisture, dry $\text{HCl}$ gas exists purely as covalent molecules and cannot dissociate into ions.
• When moist litmus paper is introduced, $\text{HCl}$ gas dissolves in the water present on the paper and dissociates into $\text{H}^+(aq)$ and $\text{Cl}^-(aq)$ ions.
• It is these free $\text{H}^+(aq)$ ions that trigger the chemical colour change in the litmus dye.

Exam Tip: In board exams, CBSE frequently asks: "Why does dry HCl gas not change the colour of dry litmus paper?" Always emphasize: "Hydrogen ions can only be produced in the presence of water; in the absence of moisture, HCl does not dissociate into $H^+$ ions."

3. What Do All Bases Have in Common?

The Defining Characteristic of Bases

All bases produce hydroxide ions, $\text{OH}^-(aq)$, when dissolved in water.

$\text{NaOH}(s) \xrightarrow{\quad \text{Water} \quad} \text{Na}^+(aq) + \text{OH}^-(aq)$ $\text{KOH}(s) \xrightarrow{\quad \text{Water} \quad} \text{K}^+(aq) + \text{OH}^-(aq)$ $\text{Mg(OH)}_2(s) \xrightarrow{\quad \text{Water} \quad} \text{Mg}^{2+}(aq) + 2\text{OH}^-(aq)$

What are Alkalis?

Most bases are insoluble in water. Bases that dissolve in water are specifically called alkalis.

• Examples of alkalis: $\text{NaOH}$, $\text{KOH}$, $\text{Ca(OH)}_2$, $\text{NH}_4\text{OH}$.
• Non-alkali base: Copper(II) oxide $\text{CuO}$ and ferric hydroxide $\text{Fe(OH)}_3$ are bases, but they do not dissolve in water.

Remember: <u>All alkalis are bases, but not all bases are alkalis.</u>

4. Electrical Conductivity in Solutions (NCERT Activity 2.8)

Set up an electrical circuit with a battery, a bulb, a switch, and two graphite electrodes immersed in a beaker:

          [ Switch ] ---- [ Bulb ] ---- [ 6V Battery ]
             |                                |
             |                                |
        [Electrode 1]                    [Electrode 2]
             \                                /
              +---[ Solution in Beaker ]----+

Experimental Findings:

1. Hydrochloric Acid ($\text{HCl}$) or Sulphuric Acid ($\text{H}_2\text{SO}_4$) Solution: The bulb glows brightly.
   • Explanation: Acids dissociate into mobile ions ($\text{H}^+$ and anions $\text{Cl}^-, \text{SO}_4^{2-}$). These free ions carry electric charge through the solution.
2. Sodium Hydroxide ($\text{NaOH}$) Solution: The bulb glows brightly.
   • Explanation: Mobile $\text{Na}^+$ and $\text{OH}^-$ ions carry the current.
3. Glucose ($\text{C}_6\text{H}_{12}\text{O}_6$) or Alcohol ($\text{C}_2\text{H}_5\text{OH}$) Solution: The bulb does NOT glow.
   • Explanation: Although glucose and alcohol contain hydrogen atoms, they form covalent bonds and do not dissociate into ions in water. In the absence of free ions, no electric current flows.

5. Dilution of Acids: The Crucial Safety Rule

Dilution is the process of mixing an acid or base with water, resulting in a decrease in the concentration of ions ($\text{H}_3\text{O}^+$ or $\text{OH}^-$) per unit volume.

The Thermal Nature of Dilution

The process of dissolving a concentrated mineral acid (especially sulphuric acid $\text{H}_2\text{SO}_4$ or nitric acid $\text{HNO}_3$) in water is extremely exothermic (releases a massive amount of heat energy).

The Inviolable Laboratory Safety Rule:

The acid must ALWAYS be added slowly to water with constant stirring.
NEVER add water to concentrated acid!

$\text{CORRECT: Concentrated Acid } \xrightarrow{\text{Slowly with stirring}} \text{ Water}$ $\text{DANGEROUS: Water } \xrightarrow{\text{STRICTLY FORBIDDEN}} \text{ Concentrated Acid}$

Why is Adding Water to Acid Dangerous?

If water is poured into concentrated acid:

1. A small amount of water meets an overwhelming quantity of concentrated acid.
2. The intense heat generated instantly converts the added water into steam.
3. This explosive boiling causes the corrosive acid to splash out violently, causing severe chemical burns on the face, hands, and clothes.
4. The rapid, uneven localized heating may also cause the glass container to crack or shatter.

6. Summary and Revision Cheat Sheet

Common Mistake: Thinking that distilled water conducts electricity. Pure distilled water does not contain free ions and does not conduct electricity. Rainwater conducts electricity because dissolved atmospheric gases ($\text{CO}_2, \text{SO}_2$) form weak acid ions!