Acids, Bases and Salts — RBSE Class 10 (Science)

Squeeze a lemon and your tongue knows instantly — sour. Rub a slip of soap between wet fingers — slippery, slightly bitter if it touches your lips. Your senses are doing crude chemistry: detecting acids and bases. This chapter turns those gut reactions into a measurable, predictable science built around one number — the pH.

1. Indicators — how we see acids and bases

We can't taste chemicals in a lab (never do). Instead we use indicators — substances that change colour (or smell) in acids and bases.

There are also olfactory indicators — substances whose smell changes with acidity. Onion and vanilla lose their smell in a base; clove oil's smell changes too. These are useful for visually impaired students and as a neat demonstration.

2. Chemical properties of acids and bases

Acids + metals → salt + hydrogen

$\text{Zn}+{\text{H}}_2{\text{SO}}_4\to {\text{ZnSO}}_4+{\text{H}}_2\uparrow$

The hydrogen gas burns with a "pop" sound when a lit matchstick is brought near — the standard test.

Bases + metals → salt + hydrogen

Reactive bases also release hydrogen with certain metals:

$2\text{NaOH}+\text{Zn}\to {\text{Na}}_2{\text{ZnO}}_2+{\text{H}}_2\uparrow$

Acids + metal carbonates / hydrogencarbonates → salt + water + CO₂

${\text{Na}}_2{\text{CO}}_3+2\text{HCl}\to 2\text{NaCl}+{\text{H}}_2\text{O}+{\text{CO}}_2\uparrow$

The CO₂ turns limewater milky — the test for carbon dioxide.

Acids + bases → salt + water (neutralisation)

$\text{NaOH}+\text{HCl}\to \text{NaCl}+{\text{H}}_2\text{O}$

Metal oxides + acids; non-metal oxides + bases

Metal oxides are basic (CuO + 2HCl → CuCl₂ + H₂O); non-metal oxides are acidic (CO₂ + 2NaOH → Na₂CO₃ + H₂O). These reactions confirm the basic/acidic nature of the oxides.

3. What is an acid or a base, really?

Acids produce hydrogen ions, H⁺(aq) (which exist as hydronium ions, H₃O⁺) in water. Bases produce hydroxide ions, OH⁻(aq) in water.

Two key consequences:

• Acids need water to show acidic behaviour — dry HCl gas does not turn blue litmus red. The H⁺ ion must be free in solution.
• Diluting an acid or base is highly exothermic. Always add acid to water, slowly, while stirring — never water to acid, or the heat can splash the concentrated acid out.

Strong vs weak: a strong acid (HCl, H₂SO₄, HNO₃) ionises completely; a weak acid (acetic acid in vinegar, carbonic acid) ionises only partly. Same for strong bases (NaOH, KOH) and weak bases (NH₄OH).

4. The pH scale — strength on a ruler

The pH scale runs from 0 to 14 and measures the concentration of H⁺ ions:

• pH < 7 → acidic (the lower, the more acidic)
• pH = 7 → neutral (pure water)
• pH > 7 → basic (the higher, the more basic)

A universal indicator gives a different colour at each pH. The scale is logarithmic: pH 3 is ten times more acidic than pH 4.

Why pH rules everyday life

• In your stomach: dilute HCl (pH ~1.5) digests food. Excess acid → acidity; an antacid (a mild base like milk of magnesia, Mg(OH)₂) neutralises it.
• In your mouth: below pH 5.5, tooth enamel (calcium phosphate) starts to dissolve → cavities. Bacteria make acid from leftover sugar; brushing with mildly basic toothpaste neutralises it.
• In the soil: plants grow best in a narrow pH band; farmers in Rajasthan treat acidic soil with lime and alkaline soil with gypsum.
• Self-defence in nature: a bee sting injects acid (soothe with baking soda); a nettle sting is acidic too. A wasp sting is alkaline.
• Acid rain (pH < 5.6) lowers river pH and harms aquatic life.

5. Salts — a big, useful family

A salt is the product (besides water) of an acid–base neutralisation. The acid and base that "make" a salt give it a family name: all chlorides (NaCl, KCl) come from HCl; all sulphates from H₂SO₄.

pH of a salt depends on its parents:

• strong acid + strong base → neutral salt (NaCl, pH 7)
• strong acid + weak base → acidic salt (NH₄Cl)
• weak acid + strong base → basic salt (Na₂CO₃)

Common salt (NaCl) — the raw material

From common salt we industrially make a whole shelf of chemicals (the chlor-alkali process electrolyses brine):

$2\text{NaCl}(aq)+2{\text{H}}_2\text{O}(l)\stackrel{\text{electricity}}{\to }2\text{NaOH}(aq)+{\text{Cl}}_2(g)+{\text{H}}_2(g)$

NaOH, chlorine and hydrogen — each feeds dozens of products.

The salts you must know cold

Water of crystallisation — some salts hold fixed water molecules in their crystals. Copper sulphate is CuSO₄·5H₂O (blue); heating drives off the water leaving white anhydrous CuSO₄, and adding water turns it blue again. Gypsum is CaSO₄·2H₂O; heating it to 373 K gives plaster of Paris, CaSO₄·½H₂O, which sets back to gypsum when mixed with water.

6. Closing thought

You started with a sour tongue and a slippery bar of soap. You end with:

• a definition (acids give H⁺, bases give OH⁻ in water),
• a measurement (the pH scale), and
• a family tree of salts that explains your antacid, your baking, your blackboard chalk and the plaster on a broken arm.

The single most useful takeaway is the pH scale — it shows up again in biology (blood pH, soil), in agriculture, and in every chemistry exam you will ever sit. Internalise "below 7 acidic, above 7 basic," and the rest of this chapter hangs neatly off it.