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Science & TechnologyNCERT Class 10 · Science (Class 10)

Acids, Bases and Salts

How acids and bases are detected by indicators and behave chemically — releasing H+ and OH- ions in water whose strength is captured on the pH scale.

⏱ 6 min readGS-III6 sections5 memory tricks
Why this matters for UPSC

Everyday chemistry is a recurring Prelims favourite: pH interpretation, the gas evolved in a reaction and its test, indicator colour changes, and acid-base behaviour in daily life (antacids, conductivity of acid solutions). For GS-III it underpins environmental science — non-metallic oxides being acidic is the chemical basis of acid rain — and basic scientific literacy for public safety in handling concentrated chemicals.

Understand the chapter

Detecting Acids and Bases: Indicators

Acids are sour and turn blue litmus red; bases are bitter and turn red litmus blue. Indicators signal acidic or basic nature through a change in colour (or odour). Beyond litmus, the chapter introduces natural, synthetic and olfactory indicators, each useful without tasting the substance.

  • Natural: litmus (from lichen, a Thallophyta plant; purple when neutral), turmeric, red cabbage, Hydrangea/Petunia/Geranium petals
  • Synthetic: methyl orange and phenolphthalein
  • Olfactory indicators: substances whose odour changes in acid/base media — onion and clove work; vanilla loses smell in base
  • Phenolphthalein is colourless in acid, pink in base; methyl orange is red in acid, yellow in base

Reactions with Metals, Carbonates and Bicarbonates

A reactive metal displaces hydrogen from an acid as H2 gas, forming a salt. Metal carbonates and hydrogencarbonates react with acids to release CO2, which turns lime water milky. These reactions give characteristic gas tests used to identify the products.

  • Acid + Metal → Salt + Hydrogen gas (H2 burns with a 'pop'/squeaky sound)
  • Metal carbonate / hydrogencarbonate + Acid → Salt + CO2 + Water
  • CO2 + lime water [Ca(OH)2] → white CaCO3 (milkiness); excess CO2 gives soluble Ca(HCO3)2, milkiness vanishes
  • Some metals react with bases too: 2NaOH + Zn → Na2ZnO2 (sodium zincate) + H2

Neutralisation and Reaction of Oxides

An acid cancels a base and vice-versa, producing a salt and water — a neutralisation reaction. Metallic oxides react with acids exactly like bases do, so they are basic oxides; non-metallic oxides react with bases, so they are acidic oxides.

  • Base + Acid → Salt + Water (e.g., NaOH + HCl → NaCl + H2O)
  • Metal oxide + Acid → Salt + Water (CuO + dilute HCl gives blue-green copper chloride) → metal oxides are basic
  • Non-metallic oxide + Base → Salt + Water (CO2 + Ca(OH)2) → non-metal oxides are acidic
  • This acidic-oxide idea is the chemistry behind acid rain

What All Acids and All Bases Have in Common

All acids conduct electricity in aqueous solution and release the H+ ion; glucose and alcohol contain hydrogen but do not ionise, so they neither conduct nor act as acids. All bases release the OH- ion. Ionisation needs water: dry HCl gas does not turn dry litmus red because no H+ forms without water.

  • Acids give H+(aq); bases give OH-(aq) in water
  • H+ never exists alone — it combines with water as the hydronium ion, H3O+ (HCl + H2O → H3O+ + Cl-)
  • Conduction of current proves ions carry charge through acid solutions
  • Net neutralisation: H+(aq) + OH-(aq) → H2O(l)

Strength of Acids/Bases and the pH Scale

A universal indicator (a mixture of indicators) shows different colours at different H+ concentrations, allowing strength to be judged. The pH scale quantifies hydrogen-ion concentration from 0 (very acidic) to 14 (very alkaline), with 7 as neutral. The relationship is inverse — higher hydronium concentration means lower pH.

  • pH < 7 acidic, = 7 neutral, > 7 basic/alkaline
  • 'p' in pH stands for 'potenz' (German for power)
  • Higher H3O+ concentration → lower pH value
  • Alkali = a base that dissolves in water (soapy, bitter, corrosive)

Dilution and Safe Handling

Dissolving an acid or base in water is highly exothermic. Concentrated acid must always be added slowly to water with stirring — never water to acid — to prevent splashing and burns. Dilution lowers the concentration of H3O+/OH- ions per unit volume.

  • Process is exothermic — beaker base feels hot
  • Add acid to water, not water to acid (avoids violent splash, glass cracking)
  • Hazard warning signs appear on concentrated H2SO4 and NaOH containers

Key terms

Indicator
A substance that signals whether a solution is acidic or basic by a change in colour or odour.
Olfactory indicator
A substance (e.g., onion, clove) whose smell changes in acidic or basic media.
Neutralisation
Reaction of an acid with a base giving salt and water: Base + Acid → Salt + Water.
Alkali
A base that is soluble in water (soapy, bitter, corrosive).
pH scale
A 0–14 scale measuring hydrogen-ion concentration; 7 is neutral, below 7 acidic, above 7 basic.
Hydronium ion (H3O+)
The form in which H+ actually exists in water, after combining with a water molecule.
Universal indicator
A mixture of several indicators showing distinct colours across the range of H+ concentrations.
Basic vs acidic oxide
Metallic oxides react with acids (basic); non-metallic oxides react with bases (acidic).
Dilution
Adding water to an acid/base, decreasing the concentration of H3O+/OH- ions per unit volume.

Must-know facts exam-ready

  • Acids turn blue litmus red; bases turn red litmus blue; litmus is purple when neutral and is extracted from lichen (division Thallophyta).
  • Phenolphthalein: colourless in acid, pink in base. Methyl orange: red in acid, yellow in base.
  • Acid + Metal → Salt + Hydrogen gas; H2 burns with a 'pop' (squeaky-pop) sound.
  • Metal carbonate/hydrogencarbonate + Acid → Salt + CO2 + Water; CO2 turns lime water milky (white CaCO3).
  • Excess CO2 converts CaCO3 to soluble Ca(HCO3)2, so the milkiness disappears.
  • Neutralisation: Base + Acid → Salt + Water; net ionic form H+(aq) + OH-(aq) → H2O.
  • Metallic oxides are basic; non-metallic oxides are acidic.
  • Acids release H+ (as H3O+) and bases release OH- in water; ionisation requires water, so dry HCl gas does not redden dry litmus.
  • Glucose and alcohol contain hydrogen but do not ionise — they are not acidic and do not conduct electricity.
  • 2NaOH + Zn → Na2ZnO2 (sodium zincate) + H2 — but not all metals react with bases.
  • pH scale runs 0–14; neutral = 7; 'p' stands for 'potenz' (German for power); higher H3O+ means lower pH.
  • Dissolving acid/base in water is highly exothermic — always add acid to water, never water to acid.

Memory tricks remember it for good

POP-OUT
POP = metal + acid gives Hydrogen (burns with a pop); OUT = carbonate + acid gives CO2 (puts a flame out, lime water milky)
💡 Recall which gas evolves and its test in acid reactions.
Lower pH = Loads of H+
Three L's link low pH to lots of hydrogen/hydronium ions (more acidic)
💡 Fixes the inverse pH–H+ relationship that traps aspirants.
AAA — Add Acid to Aqua
Always Add Acid to water (Aqua), never the reverse
💡 Remembers the safe, exothermic-dilution rule.
Phenolphthalein = Pink for a Proper base
P-words tie phenolphthalein's pink colour to a base; it is colourless in acid
💡 Recall indicator colours; pair with methyl orange (Red acid, Yellow base).
Metals are Basic, Non-metals are Acidic (oxides)
Metal oxide = basic oxide; non-metal oxide = acidic oxide
💡 Remember oxide character and the acid-rain link.

Traps to avoid

  • All acids contain hydrogen, but not all hydrogen-containing compounds are acids — glucose and alcohol have H yet are not acidic and do not conduct.
  • All alkalis are bases, but not all bases are alkalis — only water-soluble bases are alkalis.
  • Lower pH means MORE acidic (more H+), not less; pH and hydronium concentration are inversely related.
  • Dry HCl gas is not acidic in behaviour — H+ forms only in presence of water, so it won't redden dry litmus.
  • Reversing the dilution rule: water must never be added to concentrated acid; acid is added to water.
  • Swapping gas tests — H2 burns with a pop, whereas CO2 extinguishes a flame and turns lime water milky.

Exam focus

🧠 Prelims angles

  • pH interpretation: acidic/neutral/basic ranges and the inverse pH–H+ relationship.
  • Identifying the gas evolved (H2 vs CO2) and its confirmatory test (pop sound vs lime water turning milky).
  • Matching indicators to colour changes — litmus, phenolphthalein, methyl orange; natural vs synthetic vs olfactory.
  • Source/nature of litmus (lichen, Thallophyta; purple when neutral).
  • Why acid solutions conduct electricity (ions) while glucose/alcohol do not.
  • Character of oxides — metallic (basic) vs non-metallic (acidic) — and neutralisation products in daily life (antacids).

✍️ Mains angles GS-III

  • Acid rain and industrial emissions as an environmental hazard (GS-III).Anchor it in the chapter fact that non-metallic oxides (SO2, NOx) are acidic; discuss sources, ecological/heritage damage and mitigation.
  • Scientific literacy and public safety in handling hazardous chemicals.Use the exothermic-dilution rule and hazard warning labels to argue for safety regulation, awareness and disaster-response preparedness.
Practice Science & Technology questions from this syllabus →

Last-minute revision tick as you recall

  • Acid → blue litmus red; Base → red litmus blue; litmus from lichen, purple when neutral.
  • Acid + Metal → Salt + H2 (pop); Carbonate + Acid → Salt + CO2 + Water (lime water milky).
  • Neutralisation: Acid + Base → Salt + Water.
  • Metal oxides basic; non-metal oxides acidic (acid-rain link).
  • Acids give H+/H3O+, bases give OH-; ionisation needs water (dry HCl ≠ acidic).
  • Alkali = water-soluble base.
  • pH 0–14, neutral 7; lower pH = more H+ = more acidic; p = 'potenz' (power).
  • Dilution is exothermic — always add acid to water.
  • Phenolphthalein: colourless in acid, pink in base; methyl orange: red in acid, yellow in base.

Distilled from NCERT Class 10 · Science (Class 10) for UPSC. Always cross-check facts with the original NCERT.