Chemical Properties of Acids and Bases for CBSE Class 10 Science

Acids and bases are among the most pervasive chemical compounds encountered in daily life and industrial chemistry. The sour taste of lemons and vinegar is caused by acids (citric acid and acetic acid), while the bitter taste and slippery feel of soap and baking soda arise from bases. However, tasting or touching unknown substances in a chemistry laboratory is dangerous; chemists rely instead on acid-base indicators and characteristic chemical reactions to identify and classify them.

In CBSE Class 10 Science, Chapter 2 (Acids, Bases and Salts) explores the fundamental reactions that define acidic and basic behavior. Understanding these reactions enables students to predict chemical products, explain test-tube observations, and write balanced chemical equations for board examinations.

What You Will Learn

• Types of acid-base indicators: natural, synthetic, and olfactory
• Reactions of acids and bases with active metals (hydrogen gas evolution)
• Reactions of acids with metal carbonates and metal hydrogen carbonates
• The definitive carbon dioxide lime water confirmation test
• Neutralisation reactions: the interaction between acids and bases
• Acid reactions with basic metallic oxides vs. base reactions with acidic non-metallic oxides
• High-yield board exam questions, observations, and common mistakes

1. Acid-Base Indicators

An indicator is a chemical substance that undergoes a distinct change in colour (or odour) in the presence of an acid or a base.

                         Acid-Base Indicators
                                   |
       +---------------------------+---------------------------+
       |                           |                           |
Natural Indicators          Synthetic Indicators        Olfactory Indicators
(Litmus, Turmeric,          (Phenolphthalein,           (Vanilla, Onion,
 Red Cabbage)                Methyl Orange)              Clove Oil)

1. Natural Indicators

• Litmus: Extracted from lichens (a symbiotic association between algae and fungi). Natural colour is purple.
  • In acidic solution: turns red.
  • In basic solution: turns blue.
• Turmeric: Yellow in acidic or neutral solutions; turns reddish-brown in basic solutions (such as soap).
• Red Cabbage Leaf Extract: Red in acidic solutions; turns green or yellow in basic solutions.

2. Synthetic Indicators

• Phenolphthalein: Colourless in acidic and neutral solutions; turns deep pink in basic solutions.
• Methyl Orange: Red / reddish-orange in acidic solutions; turns bright yellow in basic solutions.

3. Olfactory Indicators

Substances whose odour (smell) changes in acidic or basic media are called olfactory indicators. They are especially useful for visually impaired students.

• Onion extract & Vanilla essence: Retain their characteristic smell in acidic medium, but their smell is completely destroyed (lost) in basic medium.
• Clove oil: Odour diminishes or vanishes in basic medium; retained in acidic medium.

2. Reactions of Acids and Bases with Metals

1. Reaction of Acids with Active Metals

When an acid reacts with an active metal, a salt is formed along with the liberation of hydrogen gas:

$\text{Acid} + \text{Metal} \longrightarrow \text{Salt} + \text{Hydrogen Gas} \uparrow$

• NCERT Activity 2.3 (Zinc Granules with Dilute Sulphuric Acid): $\text{Zn}(s) + \text{H}_2\text{SO}_4(aq) \longrightarrow \text{ZnSO}_4(aq) + \text{H}_2(g) \uparrow$
• With hydrochloric acid: $\text{Zn}(s) + 2\text{HCl}(aq) \longrightarrow \text{ZnCl}_2(aq) + \text{H}_2(g) \uparrow$

Important: <u>The presence of hydrogen gas is confirmed by passing the gas through soap solution (forming gas-filled soap bubbles) and bringing a burning candle near a bubble. The gas ignites with a characteristic "pop" sound.</u>

2. Reaction of Bases with Metals

Most metals do not react with bases. However, certain amphoteric metals like zinc and aluminium react with strong bases to liberate hydrogen gas: $2\text{NaOH}(aq) + \text{Zn}(s) \longrightarrow \text{Na}_2\text{ZnO}_2(aq) \text{ [Sodium Zincate]} + \text{H}_2(g) \uparrow$ $2\text{NaOH}(aq) + 2\text{Al}(s) + 2\text{H}_2\text{O}(l) \longrightarrow 2\text{NaAlO}_2(aq) \text{ [Sodium Aluminate]} + 3\text{H}_2(g) \uparrow$

Remember: Such reactions with bases are NOT possible with all metals; only amphoteric metals react with concentrated alkalis.

3. Reactions of Acids with Metal Carbonates and Bicarbonates

When an acid reacts with a metal carbonate or metal hydrogen carbonate (bicarbonate), it forms a salt, water, and carbon dioxide gas:

$\text{Metal Carbonate / Bicarbonate} + \text{Acid} \longrightarrow \text{Salt} + \text{Water} + \text{Carbon Dioxide} \uparrow$

Specific Reactions:

1. Sodium Carbonate with Dilute Hydrochloric Acid: $\text{Na}_2\text{CO}_3(s) + 2\text{HCl}(aq) \longrightarrow 2\text{NaCl}(aq) + \text{H}_2\text{O}(l) + \text{CO}_2(g) \uparrow$
2. Sodium Hydrogen Carbonate with Dilute Hydrochloric Acid: $\text{NaHCO}_3(s) + \text{HCl}(aq) \longrightarrow \text{NaCl}(aq) + \text{H}_2\text{O}(l) + \text{CO}_2(g) \uparrow$

The Lime Water Test for Carbon Dioxide (NCERT Activity 2.5)

• When the liberated $\text{CO}_2$ gas is passed through clear lime water (calcium hydroxide solution), the lime water turns milky due to the precipitation of insoluble white calcium carbonate: $\text{Ca(OH)}_2(aq) \text{ [Lime water]} + \text{CO}_2(g) \longrightarrow \text{CaCO}_3(s) \downarrow \text{ [White ppt / Milky]} + \text{H}_2\text{O}(l)$
• If excess $\text{CO}_2$ is passed continuously, the milkiness disappears because the insoluble $\text{CaCO}_3$ converts into soluble calcium hydrogen carbonate: $\text{CaCO}_3(s) + \text{H}_2\text{O}(l) + \text{CO}_2(g) \longrightarrow \text{Ca(HCO}_3)_2(aq) \text{ [Soluble / Clear Solution]}$

Exam Tip: This two-step lime water test is one of the most frequently asked questions in CBSE Class 10 Science board examinations!

4. Neutralisation Reactions

Formal Definition

The reaction between an acid and a base to form salt and water is known as a neutralisation reaction.

$\text{Acid} + \text{Base} \longrightarrow \text{Salt} + \text{Water} + \text{Heat}$

In ionic terms, neutralisation is the combination of hydrogen ions ($\text{H}^+$) from the acid and hydroxide ions ($\text{OH}^-$) from the base to form unionized water molecules: $\text{H}^+(aq) + \text{OH}^-(aq) \longrightarrow \text{H}_2\text{O}(l)$

• Example: $\text{HCl}(aq) + \text{NaOH}(aq) \longrightarrow \text{NaCl}(aq) + \text{H}_2\text{O}(l)$ $\text{H}_2\text{SO}_4(aq) + 2\text{KOH}(aq) \longrightarrow \text{K}_2\text{SO}_4(aq) + 2\text{H}_2\text{O}(l)$

5. Reactions of Oxides with Acids and Bases

1. Reaction of Metallic Oxides with Acids

Metal oxides react with acids to form salt and water, exactly like bases do. Therefore, <u>metallic oxides are basic in nature</u>: $\text{Metal Oxide} + \text{Acid} \longrightarrow \text{Salt} + \text{Water}$

• Example: When black copper(II) oxide powder is treated with dilute hydrochloric acid, the solution turns blue-green due to the formation of copper(II) chloride: $\text{CuO}(s) \text{ [Black]} + 2\text{HCl}(aq) \longrightarrow \text{CuCl}_2(aq) \text{ [Blue-green]} + \text{H}_2\text{O}(l)$

2. Reaction of Non-Metallic Oxides with Bases

Non-metal oxides react with bases to form salt and water. Therefore, <u>non-metallic oxides are acidic in nature</u>: $\text{Non-Metal Oxide} + \text{Base} \longrightarrow \text{Salt} + \text{Water}$

• Example: Carbon dioxide reacts with calcium hydroxide (a base) to form calcium carbonate salt and water: $\text{CO}_2(g) + \text{Ca(OH)}_2(aq) \longrightarrow \text{CaCO}_3(s) + \text{H}_2\text{O}(l)$

6. Summary and Examination Tips

Common Mistake: Stating that bases react with carbonates to liberate $\text{CO}_2$. Bases do NOT react with metal carbonates; only acids liberate $\text{CO}_2$ from carbonates!