Science & Technology
Chemistry in Everyday Life
Acids, Bases and Salts
Acids, bases and salts for CGPSC: indicators, pH in daily life, neutralisation, and common salts like baking soda, washing soda, bleaching powder and POP.
Prerequisites
- Class 10 idea of elements, compounds and chemical equations: an acid, a base or a salt is a compound, and their reactions are written as balanced equations.
- Physics in Everyday Life — useful for the electrical-conductivity point (acids and bases in water conduct because they form ions).
- Read alongside Life Science and Human Health — digestion, tooth decay and blood pH reappear there.
The Hook: Imli, Chuna and a Bee Sting
Connect
In a Chhattisgarh village, the same household runs a small chemistry lab without knowing it. Tamarind (imli, इमली) is used to scrub brass lotas until they shine. A pinch of chuna (lime, चूना) goes into the paan. When a child is stung by a bee, an elder rubs on baking soda. When a field turns “khatti” (sour), the agriculture officer advises lime.
Why does sour tamarind clean brass? Why does a basic powder soothe a bee sting but not a wasp sting? Why does lime fix a sour field? All three are questions about acids, bases and the salts they form.
Why it matters: Prelims P-I regularly asks one-line facts from this topic: the acid in a fruit or a sting, the chemical name of a common salt, the colour change of an indicator, the pH of a familiar substance. Mains P-IV GS-II can ask short notes on pH, antacids or bleaching powder, and links them to health, agriculture and pollution.
Acids and Bases: The Core Ideas
Definitions you need
| Property | Acid (अम्ल) | Base (क्षार / भस्म) |
|---|---|---|
| Taste | Sour | Bitter |
| Feel | — | Soapy (soluble bases) |
| In water gives | H⁺ ions (as H₃O⁺, hydronium) | OH⁻ ions (hydroxide) |
| Blue litmus | Turns red | Stays blue |
| Red litmus | Stays red | Turns blue |
| With active metals (e.g. zinc) | Releases hydrogen gas | Some (e.g. NaOH with zinc) also release hydrogen |
| With carbonates / hydrogencarbonates | Releases carbon dioxide (turns lime water milky) | No reaction |
- A base that dissolves in water is called an alkali (e.g. NaOH, KOH, Ca(OH)₂). All alkalis are bases, but not all bases are alkalis (copper oxide is a base but does not dissolve).
- Acids and bases conduct electricity in water because they produce ions. Dry HCl gas or glucose and alcohol solutions do not turn litmus red because they produce no H⁺ ions.
- Strong vs weak: strong acids (HCl, H₂SO₄, HNO₃) ionise almost completely; weak acids (acetic, citric, carbonic) ionise only partly. “Strong” is about ionisation, not about concentration. “Concentrated” means a lot of acid in little water.
Safety rule examiners love
Diluting a concentrated acid is highly exothermic. Always add acid to water slowly, with stirring, never water to acid. Adding water to concentrated sulphuric acid can make the mixture boil and splash out.
Acids in nature and in the kitchen
| Natural source | Acid |
|---|---|
| Vinegar (सिरका) | Acetic acid (ethanoic acid) |
| Lemon, orange (citrus fruits) | Citric acid |
| Tamarind (इमली) | Tartaric acid |
| Tomato | Oxalic acid |
| Sour milk, curd (दही) | Lactic acid |
| Ant sting, nettle sting | Methanoic acid (formic acid) |
| Apple | Malic acid |
| Grapes | Tartaric acid |
| Stomach (gastric juice) | Hydrochloric acid |
| Soft drinks (aerated) | Carbonic acid; many colas also contain phosphoric acid |
| Muscles after hard exercise | Lactic acid builds up and causes cramps |
Mineral acids (from minerals): hydrochloric (HCl), sulphuric (H₂SO₄, the “king of chemicals” because of its wide industrial use), nitric (HNO₃). Organic acids come from plants and animals (the table above).
Common bases in daily life: lime water (calcium hydroxide), milk of magnesia (magnesium hydroxide, an antacid), washing soda solution, soap solution, ammonium hydroxide (window cleaners).
Indicators (सूचक)
| Indicator | In acid | In base |
|---|---|---|
| Litmus (natural; from lichens) | Red | Blue |
| Phenolphthalein (synthetic) | Colourless | Pink |
| Methyl orange (synthetic) | Red | Yellow |
| Turmeric (natural) | Yellow (no change) | Reddish-brown |
| China rose (gudhal) petals | Dark pink / magenta | Green |
| Red cabbage juice | Red | Green |
Olfactory indicators change smell, not colour. Onion and clove oil lose their odour in a basic solution; vanilla essence also loses its smell in a base. They help visually impaired students test solutions.
Why a turmeric stain turns red on soap
A curry (haldi) stain on a white kurta turns reddish-brown when you rub soap on it, because soap solution is basic. It turns yellow again after rinsing with water. This is a favourite “everyday observation” MCQ.
The pH Scale
pH (from German potenz, “power”) measures the hydrogen ion concentration of a solution. It runs from 0 to 14 for most everyday solutions.
$$\text{pH} = -\log_{10}[\text{H}^+]$$- pH 7 = neutral (pure water at 25 °C).
- Below 7 = acidic. The lower the pH, the stronger the acid.
- Above 7 = basic. The higher the pH, the stronger the base.
- The scale is logarithmic: a change of 1 pH unit is a 10-fold change in H⁺ concentration. pH 3 is 10 times more acidic than pH 4 and 100 times more than pH 5.
Universal indicator (a mixture of indicators) shows a different colour across the whole pH range: red at strongly acidic, green near neutral, violet at strongly basic.
pH values worth remembering (approximate)
| Substance | Approx. pH | Nature |
|---|---|---|
| Gastric juice | about 1.5–3.5 | Strongly acidic |
| Lemon juice | about 2–2.5 | Acidic |
| Vinegar | about 3 | Acidic |
| Normal rain water | about 5.6 | Slightly acidic (CO₂ dissolves to form carbonic acid) |
| Milk | about 6.5 | Slightly acidic; turns more acidic as it sours into curd |
| Pure water | 7 | Neutral |
| Human blood | 7.35–7.45 | Slightly basic |
| Baking soda solution | about 8–9 | Weak base |
| Milk of magnesia | about 10 | Base |
| Sodium hydroxide solution | about 13–14 | Strong base |
Values are approximate and vary with concentration; Prelims asks the order and the nature, not the decimal.
pH in daily life: six applications
- Digestion and antacids. The stomach makes HCl, which helps digestion and kills germs. Excess acid causes indigestion and acidity. Antacids are mild bases, such as milk of magnesia (Mg(OH)₂), aluminium hydroxide or sodium hydrogencarbonate, that neutralise the excess. More in Soaps, Medicines and Food Chemicals.
- Tooth decay. Tooth enamel is calcium hydroxyapatite, the hardest substance in the body. It starts to corrode when the pH in the mouth falls below 5.5, which happens when mouth bacteria break down sugar and food particles into acids. Toothpastes are basic and neutralise this acid.
- Soil pH and crops. Most crops grow best in a roughly neutral soil (about pH 6.5 to 7.5). Acidic soil is treated with lime (quicklime, slaked lime or chalk). Alkaline (usar) soil is treated with gypsum or organic matter. See Agricultural Science.
- Stings. A bee sting injects methanoic (formic) acid; rubbing a mild base like baking soda gives relief. The nettle plant’s stinging hairs also inject methanoic acid; the traditional remedy is rubbing the leaf of the dock plant. (A wasp sting is often described as alkaline and treated with a mild acid like vinegar, but this is a popular remedy, not a reliable exam fact.)
- Acid rain. Rain with pH below 5.6 is called acid rain. It forms when sulphur dioxide (from coal-burning power plants and smelters) and nitrogen oxides (from vehicles) dissolve in rain to form sulphuric and nitric acids. It damages soils, lakes and limestone and marble monuments (the “marble cancer” of the Taj Mahal). See Environmental Pollution and Waste Management.
- Living bodies work in a narrow pH range. Blood is held near 7.4 by buffers; a fall in blood pH is called acidosis.
Chhattisgarh angle
Chhattisgarh has large coal-based thermal power capacity in districts such as Korba and Raigarh. Sulphur dioxide from coal combustion is a precursor of acid rain, which is why flue-gas desulphurisation (FGD) units for coal plants feature in pollution-control debates. Use this as a Chhattisgarh link in Mains answers on acid rain. See Minerals, Energy and Industries.
Neutralisation and Salts (लवण)
Neutralisation (उदासीनीकरण): an acid and a base react to form a salt and water. The reaction is exothermic.
$$\text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{Water}$$$$\text{HCl} + \text{NaOH} \rightarrow \text{NaCl} + \text{H}_2\text{O}$$A salt can be acidic, basic or neutral, depending on the strength of its parent acid and base:
| Parent acid + base | Salt solution | Example |
|---|---|---|
| Strong acid + strong base | Neutral (pH 7) | NaCl (common salt), K₂SO₄ |
| Strong acid + weak base | Acidic (pH below 7) | NH₄Cl (ammonium chloride) |
| Weak acid + strong base | Basic (pH above 7) | Na₂CO₃ (washing soda), NaHCO₃ (baking soda), CH₃COONa |
Common salt (sodium chloride, NaCl)
- Obtained from sea water by evaporation and from rock salt deposits.
- Raw material for many chemicals: sodium hydroxide, chlorine, hydrogen, baking soda, washing soda, bleaching powder and hydrochloric acid.
- Iodised salt: potassium iodide or potassium iodate added to prevent goitre (iodine deficiency).
Chlor-alkali process: electrolysis of brine (concentrated NaCl solution) gives three products, hence the name (chlor = chlorine, alkali = sodium hydroxide):
| Product | Formed at | Uses |
|---|---|---|
| Chlorine gas | Anode | Water treatment, PVC, bleaching powder, disinfectants, CFC production |
| Hydrogen gas | Cathode | Fuel, hydrogenation of oils (vanaspati ghee), ammonia for fertilisers |
| Sodium hydroxide | Near the cathode | Soaps and detergents, paper making, artificial fibres, degreasing metals |
Five salts every aspirant must know
| Common name | Chemical name | Formula | How it is made | Key uses |
|---|---|---|---|---|
| Bleaching powder | Calcium oxychloride (also described as calcium hypochlorite mixture) | CaOCl₂ | Chlorine passed over dry slaked lime | Bleaching cotton, linen and wood pulp; disinfecting drinking water; oxidising agent in industry |
| Baking soda (मीठा सोडा) | Sodium hydrogencarbonate (sodium bicarbonate) | NaHCO₃ | From NaCl, water, CO₂ and ammonia (Solvay process) | Baking (gives CO₂ that makes dough rise); antacid; soda-acid fire extinguishers |
| Washing soda | Sodium carbonate decahydrate | Na₂CO₃·10H₂O | Heating baking soda gives sodium carbonate; recrystallising it gives washing soda | Glass, soap and paper industries; making borax; household cleaning; removing permanent hardness of water |
| Plaster of Paris (POP) | Calcium sulphate hemihydrate | CaSO₄·½H₂O | Heating gypsum at 373 K (100 °C) | Plaster for fractured bones; toys and decorative ceilings; making surfaces smooth |
| Gypsum | Calcium sulphate dihydrate | CaSO₄·2H₂O | Natural mineral; also forms when POP is mixed with water | Making POP; added to cement to slow its setting; reclaiming alkaline soil |
Key reactions:
$$2\text{NaHCO}_3 \xrightarrow{\text{heat}} \text{Na}_2\text{CO}_3 + \text{H}_2\text{O} + \text{CO}_2$$$$\text{CaSO}_4\cdot 2\text{H}_2\text{O} \xrightarrow{373\,\text{K}} \text{CaSO}_4\cdot \tfrac{1}{2}\text{H}_2\text{O} + 1\tfrac{1}{2}\,\text{H}_2\text{O}$$$$\text{CaSO}_4\cdot \tfrac{1}{2}\text{H}_2\text{O} + 1\tfrac{1}{2}\,\text{H}_2\text{O} \rightarrow \text{CaSO}_4\cdot 2\text{H}_2\text{O} \;(\text{sets hard})$$- Baking powder = baking soda + a mild edible acid such as tartaric acid. The acid neutralises the sodium carbonate that would otherwise form and give the cake a bitter taste.
- POP must be stored in a moisture-proof container, because it absorbs water and sets into hard gypsum.
Water of crystallisation
Some salts hold a fixed number of water molecules in their crystals:
| Salt | Formula | Colour change on heating |
|---|---|---|
| Copper sulphate (blue vitriol, नीला थोथा) | CuSO₄·5H₂O | Blue → white (anhydrous); blue colour returns with a few drops of water |
| Washing soda | Na₂CO₃·10H₂O | Loses water |
| Gypsum | CaSO₄·2H₂O | Becomes POP at 373 K |
Analysis for Mains: Why This Topic Matters Beyond the Lab
- Public health: bleaching powder and chlorine are the cheapest disinfectants for village wells and municipal water; iodised salt has been central to India’s goitre control; antacids are among the most consumed over-the-counter medicines.
- Agriculture: soil pH decides nutrient availability; liming acidic soils and adding gypsum to alkaline soils are standard soil-health card recommendations.
- Environment: acid rain from SO₂ and NOₓ links chemistry to coal power, mining and heritage conservation.
- Industry: the chlor-alkali and soda-ash industries supply paper, glass, soap, textiles and PVC. Mercury-cell chlor-alkali plants were a pollution concern; Indian industry has largely shifted to the cleaner membrane-cell technology.
Memory Tricks
Litmus: 'BRA' and 'RBB'
Blue → Red in Acid. Red → Blue in Base.
Phenolphthalein and methyl orange
“Phenol Pinks in base” (colourless in acid, pink in base).
“Methyl Orange Reddens in Acid” (red in acid, yellow in base).
Natural acids: 'Vinegar Leaves Tamarind To Curdle Ants'
Vinegar – acetic · Lemon – citric · Tamarind – tartaric · Tomato – oxalic · Curd – lactic · Ants – methanoic (formic).
Gypsum has TWO waters, POP has HALF
Gypsum 2 H₂O → heat at 100 °C → POP ½ H₂O. “Plaster is half-baked gypsum.”
Don’t Confuse
| This | vs | That |
|---|---|---|
| Baking soda NaHCO₃ (sodium hydrogencarbonate) | vs | Washing soda Na₂CO₃·10H₂O (sodium carbonate) |
| Washing soda Na₂CO₃ | vs | Caustic soda NaOH (sodium hydroxide) — strong alkali |
| Baking soda (one chemical) | vs | Baking powder (baking soda + tartaric acid) |
| Gypsum CaSO₄·2H₂O | vs | Plaster of Paris CaSO₄·½H₂O |
| Strong acid (fully ionised) | vs | Concentrated acid (little water) |
| Bleaching powder CaOCl₂ | vs | Bleach in shops (often sodium hypochlorite solution) |
| Normal rain about pH 5.6 | vs | Acid rain pH below 5.6 |
Common Mistakes
MCQ traps
- “Pure rain water has pH 7.” No. Normal rain is slightly acidic (about 5.6) because atmospheric CO₂ dissolves in it. Only rain below 5.6 is called acid rain.
- “A lower pH means a weaker acid.” The opposite. Lower pH means more H⁺ and a stronger acidic solution.
- “pH 4 is twice as acidic as pH 5.” It is ten times. The scale is logarithmic.
- “All bases are alkalis.” Only water-soluble bases are alkalis.
- “Ant sting contains acetic acid.” It is methanoic (formic) acid. Acetic acid is in vinegar.
- “Tomato contains citric acid.” The standard textbook answer is oxalic acid.
- “Plaster of Paris is made by heating gypsum strongly.” It is made at 373 K. Heating gypsum above about 473 K gives dead burnt plaster (anhydrous CaSO₄), which does not set.
- “Antacids are acids.” Antacids are mild bases.
Practice
All questions below are original practice questions written in CGPSC style. They are not previous-year questions.
Level 1: Foundation
- The acid present in tamarind is ______, and the acid present in sour curd is ______.
- Tooth decay starts when the pH in the mouth falls below ______.
- Plaster of Paris is prepared by heating gypsum at ______ K.
Answers
- Tartaric acid; lactic acid.
- 5.5.
- 373 K (100 °C).
Level 2: Prelims MCQs
Q1. Consider the following statements:
- A solution with pH 3 has ten times the hydrogen ion concentration of a solution with pH 4.
- Rain water with a pH of 6.0 is classified as acid rain.
- Human blood is slightly basic.
Which of the statements given above is/are correct?
- (a) 1 only
- (b) 1 and 3 only
- (c) 2 and 3 only
- (d) 1, 2 and 3
Answer and explanation
(b) 1 and 3 only. The pH scale is logarithmic, so one unit is a 10-fold change (statement 1 correct). Acid rain is rain with pH below 5.6; pH 6.0 is within the normal range of slightly acidic rain (statement 2 wrong). Blood is held at about 7.35–7.45, slightly basic (statement 3 correct).
Q2. Match List I (Common name) with List II (Chemical formula):
| List I | List II |
|---|---|
| A. Baking soda | 1. CaOCl₂ |
| B. Washing soda | 2. CaSO₄·½H₂O |
| C. Bleaching powder | 3. NaHCO₃ |
| D. Plaster of Paris | 4. Na₂CO₃·10H₂O |
- (a) A-3, B-4, C-1, D-2
- (b) A-4, B-3, C-1, D-2
- (c) A-3, B-4, C-2, D-1
- (d) A-4, B-3, C-2, D-1
Answer and explanation
(a) A-3, B-4, C-1, D-2. Baking soda is sodium hydrogencarbonate; washing soda is sodium carbonate decahydrate; bleaching powder is calcium oxychloride; POP is calcium sulphate hemihydrate. The trap in options (b) and (d) is swapping the two “sodas”.
Q3. A white cotton shirt with a turmeric (haldi) stain turns reddish-brown when washed with soap. This happens because:
- (a) soap solution is acidic
- (b) soap solution is basic and turmeric acts as an indicator
- (c) turmeric reacts with water to form an acid
- (d) soap bleaches turmeric
Answer and explanation
(b). Turmeric is a natural indicator: yellow in acidic and neutral solutions, reddish-brown in basic solutions. Soap solution is basic. The colour returns to yellow when the soap is rinsed away.
Q4. In the chlor-alkali process, which of the following is obtained at the anode?
- (a) Hydrogen
- (b) Sodium hydroxide
- (c) Chlorine
- (d) Sodium metal
Answer and explanation
(c) Chlorine. Electrolysis of brine gives chlorine at the anode, hydrogen at the cathode and sodium hydroxide solution near the cathode. Sodium metal is not formed from aqueous brine; it is obtained by electrolysis of molten NaCl.
Q5. Which of the following pairs is not correctly matched?
- (a) Ant sting – Methanoic acid
- (b) Vinegar – Acetic acid
- (c) Tomato – Citric acid
- (d) Gastric juice – Hydrochloric acid
Answer and explanation
(c). Tomato is matched with oxalic acid in the standard (NCERT) list. Citric acid is the acid of lemon and orange. The other pairs are correct.
Level 3: Mains Answer Writing
Q1. (60 words) What is the pH scale? Give two examples of the importance of pH in everyday life.
Model answer outline
- Definition: pH is the negative logarithm of hydrogen ion concentration; scale 0–14; 7 neutral, below 7 acidic, above 7 basic; one unit = 10-fold change.
- Examples (any two): tooth decay below pH 5.5 and basic toothpaste; antacids (milk of magnesia) neutralising excess stomach HCl; liming acidic soils for crops; acid rain below pH 5.6.
- Close: life processes function only in a narrow pH range (blood about 7.4).
Q2. (150–200 words) Describe the preparation and uses of bleaching powder and Plaster of Paris. Why is bleaching powder important for rural public health?
Model answer outline
- Intro: Both are calcium salts made from cheap raw materials and used widely in homes, hospitals and industry.
- Bleaching powder (CaOCl₂): made by passing chlorine (from the chlor-alkali process) over dry slaked lime. Uses: bleaching textiles and paper pulp, oxidising agent, disinfecting drinking water.
- POP (CaSO₄·½H₂O): made by heating gypsum at 373 K; sets into gypsum on adding water. Uses: orthopaedic casts, statues, false ceilings; must be stored dry.
- Rural public health: chlorinating open wells and tanks with bleaching powder is cheap, simple and effective against water-borne diseases such as cholera, typhoid and diarrhoea, especially during the monsoon; it is routinely used by health departments after floods.
- Chhattisgarh angle: monsoon outbreaks of diarrhoea in rural and forest areas make well-chlorination a recurring public health measure.
- Conclusion: simple salts from common salt and limestone support sanitation, healthcare and construction, which is why the chlor-alkali industry is called a backbone of the chemical sector.
Quick Revision Table
| Fact | Detail |
|---|---|
| Acid in water gives | H⁺ (H₃O⁺) ions |
| Base in water gives | OH⁻ ions; soluble base = alkali |
| Litmus source | Lichens |
| Phenolphthalein | Colourless in acid, pink in base |
| Methyl orange | Red in acid, yellow in base |
| Olfactory indicators | Onion, vanilla, clove oil |
| pH 7 / below / above | Neutral / acidic / basic; 1 unit = 10× |
| Tooth decay | Below pH 5.5 |
| Acid rain | pH below 5.6 |
| Blood pH | 7.35–7.45 |
| Antacid | Milk of magnesia Mg(OH)₂ |
| Chlor-alkali | Cl₂ at anode, H₂ at cathode, NaOH |
| Bleaching powder | CaOCl₂; Cl₂ + dry slaked lime |
| Baking soda | NaHCO₃; antacid, baking, fire extinguisher |
| Washing soda | Na₂CO₃·10H₂O; hard water, glass |
| POP | CaSO₄·½H₂O; gypsum at 373 K |
| Blue vitriol | CuSO₄·5H₂O; turns white on heating |
Key Takeaways
Key Takeaways
- Acids give H⁺, bases give OH⁻; together they neutralise to form salt + water.
- pH is logarithmic: 7 neutral, below 7 acidic, above 7 basic; each unit is a 10-fold change.
- Everyday pH facts: tooth decay below 5.5, acid rain below 5.6, blood about 7.4, antacids are bases.
- Learn the natural acids (vinegar, lemon, tamarind, tomato, curd, ant sting) and the indicator colours.
- Five salts from common salt and limestone, namely bleaching powder, baking soda, washing soda, POP and gypsum, run daily life from baking to bone setting.
What’s Next
- Metals and Non-metals — oxides of metals are basic, oxides of non-metals are acidic
- Soaps, Medicines and Food Chemicals — antacids, soaps and hard water
- Chemistry in Everyday Life — Quick Revision
- Environmental Pollution and Waste Management — acid rain in depth
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