📑 Topics on this page (Jump to Topic):
📌 1. Acids, Bases & Indicators
| Acid | Base | |
|---|---|---|
| Taste | Sour | Bitter, soapy |
| Ions in water | H⁺ (H₃O⁺) | OH⁻ |
| Blue litmus | Red | Stays blue |
| Red litmus | Stays red | Blue |
| Phenolphthalein | Colourless | Pink |
| Methyl orange | Red | Yellow |
| Turmeric | Stays yellow | Reddish-brown |
| Examples | HCl, H₂SO₄, HNO₃, CH₃COOH | NaOH, KOH, Ca(OH)₂, NH₄OH |
- Water-soluble bases are alkalis; all alkalis are bases but not all bases are alkalis.
- Acid + metal → salt + hydrogen gas ("pop" with a burning splint). Acid + carbonate → salt + water + CO₂ (turns lime water milky).
- When diluting, add acid to water slowly, never water to acid (intense heat).
- Aqua regia = 3 parts conc. HCl + 1 part conc. HNO₃ — dissolves gold and platinum. H₂SO₄ = "king of chemicals".
📌 2. pH Scale & Natural Acids
- pH (Sørensen, 1909) 0–14: 7 neutral, below 7 acidic, above 7 basic. A fall of 1 pH unit means 10 times more H⁺.
| Substance | Approx. pH |
|---|---|
| Gastric juice (HCl) | 1–2 |
| Lemon juice | 2–2.5 |
| Acid rain | < 5.6 |
| Pure water | 7 |
| Human blood | 7.35–7.45 |
| Sea water | ≈ 8 |
| Milk of magnesia | ≈ 10 |
| NaOH solution | ≈ 14 |
| Source | Acid |
|---|---|
| Lemon, orange | Citric acid |
| Tamarind, grapes | Tartaric acid |
| Tomato | Oxalic acid |
| Vinegar | Acetic acid |
| Sour milk, curd | Lactic acid |
| Ant and nettle sting | Formic (methanoic) acid |
| Apple | Malic acid |
| Stomach | Hydrochloric acid |
| Amla | Ascorbic acid (vitamin C) |
- For acidity, antacids — milk of magnesia Mg(OH)₂, baking soda. Baking soda for ant/bee stings; wasp sting is alkaline (vinegar).
- Below mouth pH 5.5 tooth decay starts (enamel — calcium hydroxyapatite) — basic toothpaste helps. Acidic soil is treated with lime, alkaline soil with gypsum/organic manure.
📌 3. Important Salts
| Common name | Chemical name / formula | Uses |
|---|---|---|
| Common salt | Sodium chloride NaCl | Food; raw material for NaOH, washing soda |
| Baking soda | Sodium bicarbonate NaHCO₃ | Baking powder (+ tartaric acid), antacid, soda-acid fire extinguisher |
| Washing soda | Na₂CO₃·10H₂O | Removing permanent hardness of water, glass, cleaning |
| Bleaching powder | Calcium oxychloride CaOCl₂ | Disinfecting water, bleaching cloth |
| Plaster of Paris | CaSO₄·½H₂O | Plaster for fractures, statues; sets hard into gypsum with water |
| Gypsum | CaSO₄·2H₂O | In cement (slows setting) |
| Blue vitriol | CuSO₄·5H₂O | Fungicide (Bordeaux mixture) |
| Alum | Potash alum K₂SO₄·Al₂(SO₄)₃·24H₂O | Purifying water (coagulation), stopping bleeding |
⚠️ Exam Traps:
- Ant sting = formic acid; vinegar = acetic acid.
- Tomato contains oxalic acid, not citric acid.
- Phenolphthalein is colourless in acid and pink in base.
📌 4. Practice MCQs
📝 Practice MCQs (prepared by SF Prep — not PYQs)
Q1. The pH of pure water is: Practice
Correct answer: (A)
A neutral solution has pH 7.
A neutral solution has pH 7.
Q2. Which acid is found in lemon? Practice
Correct answer: (D)
Citrus fruits contain citric acid.
Citrus fruits contain citric acid.
Q3. Which acid is present in an ant sting? Practice
Correct answer: (C)
Methanoic acid; baking soda gives relief.
Methanoic acid; baking soda gives relief.
Q4. The main acid in tomato is: Practice
Correct answer: (C)
Tomato = oxalic; tamarind = tartaric.
Tomato = oxalic; tamarind = tartaric.
Q5. Phenolphthalein in a basic solution turns: Practice
Correct answer: (B)
Colourless in acid, pink in base.
Colourless in acid, pink in base.
Q6. The chemical formula of bleaching powder is: Practice
Correct answer: (B)
Calcium oxychloride.
Calcium oxychloride.
Q7. Baking soda is: Practice
Correct answer: (C)
NaHCO₃ — releases CO₂ on heating, making cakes rise.
NaHCO₃ — releases CO₂ on heating, making cakes rise.
Q8. Aqua regia is a mixture of: Practice
Correct answer: (C)
It can dissolve gold and platinum.
It can dissolve gold and platinum.
Q9. The pH of human blood is about: Practice
Correct answer: (D)
7.35–7.45, slightly basic.
7.35–7.45, slightly basic.
Q10. Which is used to relieve stomach acidity? Practice
Correct answer: (A)
Mg(OH)₂ is a mild base that neutralises excess HCl.
Mg(OH)₂ is a mild base that neutralises excess HCl.
Q11. The formula of plaster of Paris is: Practice
Correct answer: (D)
With water it sets into gypsum (CaSO₄·2H₂O).
With water it sets into gypsum (CaSO₄·2H₂O).
Q12. Acids react with metals to release: Practice
Correct answer: (B)
A "pop" sound with a burning splint.
A "pop" sound with a burning splint.
Q13. Permanent hardness of water is removed using: Practice
Correct answer: (A)
Na₂CO₃ precipitates calcium/magnesium as carbonates.
Na₂CO₃ precipitates calcium/magnesium as carbonates.
Q14. The correct way to dilute a concentrated acid is to: Practice
Correct answer: (B)
Intense heat can make acid splash out.
Intense heat can make acid splash out.
Q15. A solution of pH 2 is how many times more acidic than one of pH 4? Practice
Correct answer: (A)
Each unit = 10×; two units = 100×.
Each unit = 10×; two units = 100×.
Q16. Turmeric turns which colour in a base? Practice
Correct answer: (C)
A turmeric stain changes colour with soap.
A turmeric stain changes colour with soap.
Q17. Acidic soil is treated by adding: Practice
Correct answer: (D)
Lime is basic and neutralises acidity.
Lime is basic and neutralises acidity.
Q18. How does alum help purify water? Practice
Correct answer: (D)
Alum ions clump suspended particles so they settle.
Alum ions clump suspended particles so they settle.
Q19. Curd contains which acid? Practice
Correct answer: (B)
Lactobacillus makes lactic acid from lactose.
Lactobacillus makes lactic acid from lactose.
Q20. Which is called the "king of chemicals"? Practice
Correct answer: (A)
H₂SO₄ has the widest industrial use.
H₂SO₄ has the widest industrial use.