Chapter 2 · Introduction

Chapter 2 — Acids, Bases and Salts

Sour and bitter, corrosive and soothing: how to detect acids and bases, how they cancel each other, how strong they are on the pH scale, and the salts they leave behind — from baking soda to plaster of Paris.

A squeeze of lemon on your dal, a spoonful of curd with a spicy meal, a pinch of baking soda in the pakora batter, a tablet of antacid after too much food, a sting from an ant that is soothed by a paste of baking soda — acids and bases are part of every Indian kitchen and every human body. Your stomach makes hydrochloric acid to digest food; your teeth decay when the mouth becomes too acidic; farmers test whether their soil is too sour or too bitter for a crop; and acid rain damages forests, lakes and the marble of the Taj Mahal.

This chapter explains what acids and bases are and what they do. You will learn to detect them with indicators, to predict their reactions with metals, carbonates, oxides and each other, and to see the single idea behind all of it — acids give $\mathrm{H^{+}}$ ions in water and alkalis give $\mathrm{OH^{-}}$ ions. Then you will measure how acidic or basic a solution is on the pH scale, and finally meet the salts that neutralisation leaves behind: common salt and the chemicals made from it, baking soda, washing soda, bleaching powder and plaster of Paris.

Indicators→Reactions of acids & bases→H⁺ and OH⁻ ions→Neutralisation→pH scale→Salts→Everyday chemicals
🎯 By the end of this chapter you can…
  • identify acids, bases and neutral substances with litmus, phenolphthalein, methyl orange, turmeric, red cabbage and olfactory indicators;
  • predict and write balanced equations for the reactions of acids with metals, carbonates, hydrogencarbonates, bases and metal oxides, and of bases with non-metal oxides;
  • name and test the gases formed — hydrogen (pop test) and carbon dioxide (lime water test);
  • explain acidic and basic behaviour in terms of $\mathrm{H^{+}}$ ($\mathrm{H_{3}O^{+}}$) and $\mathrm{OH^{-}}$ ions, and explain why acid must be added to water;
  • use the pH scale and universal indicator to compare strong and weak acids and bases, and explain the role of pH in digestion, tooth decay, soil and stings;
  • predict whether a salt solution is acidic, neutral or basic, and describe the preparation and uses of sodium hydroxide, bleaching powder, baking soda, washing soda and plaster of Paris.
Teaser — Neutralisation, Ion by Ion

The beaker holds dilute hydrochloric acid — H⁺ ions and Cl⁻ ions — with a few drops of phenolphthalein. Each drop of sodium hydroxide solution brings in Na⁺ and OH⁻ ions. Watch every OH⁻ find an H⁺ and become a water molecule. When does the colour change?

drops of NaOH added—
H⁺ left—
OH⁻ in excess—
solution is—

Sections in this chapter

2.1

Properties of Acids and Bases

Indicators; reactions with metals, carbonates, oxides and each other; H⁺ and OH⁻ ions in water; alkalis; diluting acids safely.

$\mathrm{H^{+}(aq) + OH^{-}(aq) \rightarrow H_{2}O(l)}$
Open section →2.2

Strength of Acids and Bases: the pH Scale

Universal indicator and the pH scale 0–14; strong and weak acids and bases; pH in digestion, teeth, soil, stings and rain.

$\mathrm{pH} \lt 7$ acidic · $7$ neutral · $\mathrm{pH} \gt 7$ basic
Open section →2.3

Salts and Their Everyday Uses

Families of salts and their pH; common salt and the chlor-alkali process; bleaching powder, baking soda, washing soda, plaster of Paris; water of crystallisation.

$\mathrm{NaHCO_{3}}$ · $\mathrm{Na_{2}CO_{3}\cdot 10H_{2}O}$ · $\mathrm{CaOCl_{2}}$
Open section →
✅ Before you start
You need to write and balance chemical equations with state symbols (Section 1.1), recognise double displacement and displacement reactions (Section 1.2), and know the Class 9 idea of ions and valency — for example that $\mathrm{Na^{+}}$ and $\mathrm{Cl^{-}}$ make $\mathrm{NaCl}$, and $\mathrm{Ca^{2+}}$ and $\mathrm{OH^{-}}$ make $\mathrm{Ca(OH)_{2}}$ (Class 9 Science, Atoms and Molecules). If the formulas of compounds such as $\mathrm{H_{2}SO_{4}}$, $\mathrm{Na_{2}CO_{3}}$ and $\mathrm{CaCO_{3}}$ feel shaky, revisit the formula builder on the welcome page first.
← 1.3 Oxidation and Reduction in Everyday Life2.1 Properties of Acids and Bases →