Acids, Bases & Salts: pH Scale, Theories & Indicators

Acid-base chemistry provides the theoretical framework for analyzing ionic equilibria, chemical reactivity, and industrial synthesis. Scientific classification developed through three classical models. Svante Arrhenius formulated the ionic theory in 1884, defining acids as substances releasing hydrogen ions (H⁺) and bases as donors of hydroxide ions (OH⁻) in aqueous media. Johannes Brønsted and Thomas Lowry expanded this in 1923 by defining acids as proton donors and bases as proton acceptors, establishing conjugate acid-base pairs. Gilbert N. Lewis generalized the concept, classifying acids as electron-pair acceptors and bases as electron-pair donors. Quantitative acidity measurement advanced in 1909 when S.P.L. Sørensen formulated the logarithmic pH scale (pH = -log₁₀[H⁺]), defining values below 7 as acidic, 7 as neutral, and above 7 as alkaline at 25 degrees Celsius.

Aqueous neutralization occurs when hydronium and hydroxide ions react to produce water and ionic salts: H⁺(aq) + OH⁻(aq) → H₂O(l). Chemical indicators detect equivalence points through reversible structural alterations induced by pH shifts. Litmus, derived from lichens such as Roccella tinctoria, turns red in acids and blue in alkalis; phenolphthalein transitions from colourless to pink above pH 8.2; and methyl orange shifts from red to yellow between pH 3.1 and 4.4. Industrial processing utilizes chlor-alkali brine electrolysis to synthesize sodium hydroxide, chlorine, and hydrogen. Major commercial salts include bleaching powder (calcium hypochlorite, CaOCl₂), baking soda (sodium hydrogen carbonate, NaHCO₃), washing soda (sodium carbonate decahydrate, Na₂CO₃·10H₂O), and Plaster of Paris (calcium sulphate hemihydrate, CaSO₄·0.5H₂O), produced through the thermal dehydration of gypsum at 373 Kelvin.

Acid-base mechanisms maintain physiological homeostasis, exemplified by human arterial blood buffered precisely between pH 7.35 and 7.45 by the carbonic acid-bicarbonate system (H₂CO₃ / HCO₃⁻), preventing fatal acidosis or alkalosis. Industrial applications govern municipal water purification, soil remediation using agricultural lime (calcium hydroxide) to counteract acidity, and antacid formulations neutralizing gastric hydrochloric acid. In the UPSC Civil Services Examination, SSC CGL, and State PSCs, this subject appears consistently in questions covering Arrhenius, Brønsted-Lowry, and Lewis acid-base classifications, indicator colour transitions across acidic and basic ranges, chemical formulas of bleaching powder and Plaster of Paris, the chlor-alkali process, and logarithmic pH calculations of strong versus weak monoprotic acids.

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#1
Acid-base definitions span Arrhenius (H+/OH- ions in aqueous media), Bronsted-Lowry (proton exchange), and Lewis (electron-pair exchange).
#2
S.P.L. Sorensen introduced the logarithmic pH scale in 1909 (pH = -log10[H+]), where neutral aqueous solutions at 25 degrees Celsius exhibit pH 7.
#3
Human arterial blood pH is tightly regulated within a narrow physiological buffer range of 7.35 to 7.45.
#4
Chemical indicators identify solution acidity: litmus turns red in acid and blue in alkali, phenolphthalein turns pink in base, and methyl orange turns red in acid.
#5
Commercial industrial salts include Bleaching Powder (CaOCl2), Baking Soda (NaHCO3), Washing Soda (Na2CO3.10H2O), and Plaster of Paris (CaSO4.0.5H2O).
#6
A Lewis acid acts as an electron-pair acceptor (such as BF3 and AlCl3), whereas a Lewis base acts as an electron-pair donor (such as NH3 and H2O) forming coordinate covalent bonds.
#7
The ionic product of pure water (Kw = [H+][OH-]) equals 1.0 x 10^-14 mol^2/L^2 at 25 degrees Celsius, increasing with rising temperature due to the endothermic dissociation of water.
#8
Strong mineral acids, including Hydrochloric acid (HCl), Nitric acid (HNO3), and Sulphuric acid (H2SO4), dissociate completely in aqueous solution, exhibiting ionization constants far exceeding unity.
#9
Aqua regia is a fuming corrosive mixture composed of concentrated hydrochloric acid and concentrated nitric acid in a 3:1 volumetric ratio, capable of dissolving noble metals like gold and platinum.
#10
Acid rain is defined as precipitation with a pH below 5.6, caused by atmospheric dissolution of sulphur dioxide (SO2) and nitrogen oxides (NOx) forming sulphurous and nitric acids.
#11
The chlor-alkali process electrolyzes aqueous sodium chloride (brine) to produce chlorine gas at the anode, hydrogen gas at the cathode, and sodium hydroxide (NaOH) in solution.
#12
Baking powder is a chemical leavening mixture consisting of sodium hydrogen carbonate (baking soda) and a mild edible tartaric acid to prevent a bitter alkaline taste.
#13
Plaster of Paris (calcium sulphate hemihydrate) is produced by heating gypsum (CaSO4.2H2O) to 373 Kelvin, setting into a hard crystalline solid upon mixing with water.
#14
Buffer solutions resist changes in pH upon addition of small amounts of acid or base, exemplified by the carbonic acid-bicarbonate buffer system (H2CO3/HCO3-) in human plasma.
#15
Natural acid-base indicators include turmeric, which remains yellow in acidic solutions and turns reddish-brown in basic solutions, and red cabbage extract, which turns red in acid and green-yellow in base.

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Analytical perspective & practical exam advice from the Master10 academic board

Educator's Insight

Acids, bases, and salts form the foundation of everyday chemistry, from digestion to industrial manufacturing. While Arrhenius defined acids as hydrogen ion donors in water, the broader Lewis theory defines acids as electron-pair acceptors. The Danish chemist Sorensen introduced the pH scale in 1909 to measure acidity, where numbers below seven represent acidic solutions and values above seven represent alkaline solutions. In the human body, chemical buffers keep blood pH tightly regulated between 7.35 and 7.45.

In SSC CGL and State PSC exams, questions frequently test chemical formulas, indicators, and common salts. A popular MCQ trap confuses baking soda with washing soda; remember that baking soda is sodium bicarbonate, whereas washing soda is sodium carbonate decahydrate. For indicator tests, memorize that phenolphthalein turns bright pink in bases but stays colorless in acids. Also note for prelims revision that aqua regia combines concentrated hydrochloric and nitric acids in a strict three-to-one ratio.

Acids, Bases & Salts: pH Scale, Theories & Indicators

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