Image lesson · 14 cardsIntroduction
This topic is a guaranteed source of 1-2 APPSC questions. The pH scale, indicator colour changes, and important salts (baking soda, washing soda, bleaching powder, Plaster of Paris) are perennial exam favourites.
Section 1: Acids
- Produce H⁺ ions in aqueous solution. Sour taste. Turn blue litmus red.
- Strong acids (completely ionise): HCl, H₂SO₄, HNO₃.
- Weak acids (partially ionise): CH₃COOH (vinegar), H₂CO₃, citric acid.
| Type | Examples |
|---|---|
| Mineral/inorganic | HCl (hydrochloric), H₂SO₄ (sulphuric — "oil of vitriol"), HNO₃ (nitric) |
| Organic | Acetic (vinegar), Citric (lemon), Lactic (curd), Oxalic (tomato), Tartaric (tamarind), Formic (ant sting) |
- Acids + metals → salt + H₂ gas.
- Acids + carbonates → salt + H₂O + CO₂ (turns lime water milky).
- Aqua regia: Conc. HCl : Conc. HNO₃ in 3:1 ratio. Dissolves gold and platinum.
Section 2: Bases
- Produce OH⁻ ions in aqueous solution. Bitter taste. Soapy feel. Turn red litmus blue.
- Strong bases: NaOH (caustic soda), KOH (caustic potash), Ca(OH)₂ (slaked lime).
- Weak bases: NH₄OH, Mg(OH)₂ (milk of magnesia).
- Alkali: A base soluble in water. All alkalis are bases, but not all bases are alkalis.
Section 3: Neutralization
Acid + Base → Salt + Water (exothermic reaction)
Example: HCl + NaOH → NaCl + H₂O
Section 4: pH Scale
- pH = potential of Hydrogen. Measures H⁺ concentration. Range: 0-14.
- pH < 7: acidic | pH = 7: neutral | pH > 7: basic/alkaline
| Substance | pH |
|---|---|
| Gastric juice | ~1.2 |
| Lemon juice | ~2.2 |
| Milk | ~6.5 |
| Pure water | 7.0 |
| Blood | 7.35-7.45 |
- Most crops grow best in pH 6-7. Tooth enamel corrodes below pH 5.5.
Section 5: Indicators
| Indicator | In Acid | In Base |
|---|---|---|
| Litmus (from lichen) | Blue → Red | Red → Blue |
| Phenolphthalein | Colourless | Pink |
| Methyl orange | Red/Orange | Yellow |
| Turmeric (haldi) | Yellow | Red/Brown |
Section 6: Important Salts
| Salt | Formula | Preparation | Uses |
|---|---|---|---|
| Baking soda | NaHCO₃ | Solvay process | Cooking (leavening), antacid, fire extinguisher |
| Washing soda | Na₂CO₃·10H₂O | Solvay process | Glass making, softening hard water, cleaning |
| Bleaching powder | CaOCl₂ | Cl₂ + Ca(OH)₂ | Bleaching textiles/paper, water purification |
| Plaster of Paris | CaSO₄·½H₂O | Heating gypsum to 373 K | Fracture casts, sculptures, construction |
| Caustic soda | NaOH | Electrolysis of brine (chlor-alkali) | Soap making, degreasing |
- Water of crystallisation: CuSO₄·5H₂O (blue vitriol — blue; anhydrous = white). FeSO₄·7H₂O = green vitriol. CaSO₄·2H₂O = gypsum.
Section 7: Hard Water
| Type | Due To | Removal |
|---|---|---|
| Temporary hardness | Bicarbonates of Ca/Mg | Boiling |
| Permanent hardness | Chlorides/sulphates of Ca/Mg | Washing soda or ion-exchange |
Section 8: Likely Exam Questions
The pH of human blood is: Ans: 7.35-7.45
Baking soda is chemically: Ans: Sodium bicarbonate (NaHCO₃)
Aqua regia is a mixture of HCl and HNO₃ in the ratio: Ans: 3:1
Plaster of Paris is obtained by heating: Ans: Gypsum (CaSO₄·2H₂O) to 373 K
Phenolphthalein turns ___ in a basic solution: Ans: Pink
Bleaching powder is prepared by passing chlorine over: Ans: Slaked lime — Ca(OH)₂
Temporary hardness in water is caused by: Ans: Bicarbonates of calcium and magnesium
The acid present in curd is: Ans: Lactic acid
Turmeric stain turns red when it comes in contact with: Ans: Soap (basic substance)
H₂SO₄ is commonly known as: Ans: Oil of vitriol
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5 terms- Aqua regia
- Conc. HCl : Conc. HNO₃ in 3:1 ratio. Dissolves gold and platinum.
- Strong bases
- NaOH (caustic soda), KOH (caustic potash), Ca(OH)₂ (slaked lime).
- Weak bases
- NH₄OH, Mg(OH)₂ (milk of magnesia).
- Alkali
- A base soluble in water. All alkalis are bases, but not all bases are alkalis.
- Water of crystallisation
- CuSO₄·5H₂O (blue vitriol — blue; anhydrous = white). FeSO₄·7H₂O = green vitriol. CaSO₄·2H₂O = gypsum.
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