pH Scale, Acids, Bases and Antacids
~9 min read
- pH: -log[H⁺]. Scale 0-14. Neutral 7. Acidic < 7, basic > 7.
- Indicators: Litmus (red in acid, blue in base). Phenolphthalein (colourless in acid, pink in base). Methyl orange (red in acid, yellow in base).
- Antacids: Mild bases (Mg(OH)₂, NaHCO₃, CaCO₃) neutralise excess stomach acid (HCl).
pH measures the H⁺ concentration. CDS/OTA tests the 0-14 scale, common indicators, and applications like antacids and the role of pH in soil, blood, rain and oral health.
pH Scale
- pH = -log₁₀[H⁺]. Each unit difference = 10× change in H⁺ concentration.
- Scale runs 0 to 14 (in aqueous solutions).
- pH < 7: acidic. pH = 7: neutral (pure water). pH > 7: basic.
- Coined by S.P.L. Sørensen, 1909.
pH of Common Substances
| Substance | Approx pH |
|---|---|
| Gastric acid (stomach) | 1-2 |
| Lemon juice | 2 |
| Vinegar | 3 |
| Tomato juice | 4 |
| Black coffee | 5 |
| Milk | 6.5 |
| Pure water | 7 |
| Human blood | 7.35-7.45 |
| Sea water | 8 |
| Baking soda | 9 |
| Milk of magnesia | 10.5 |
| Household ammonia | 11 |
| NaOH (lye) 1 M | 14 |
Indicators
| Indicator | In acid | In base |
|---|---|---|
| Litmus | Red | Blue |
| Phenolphthalein | Colourless | Pink |
| Methyl orange | Red | Yellow |
| Turmeric | Yellow | Reddish-brown |
Olfactory indicators (onion, vanilla) change odour with pH. Universal indicator shows different colours for the entire pH range 1-14.
Acids, Bases and Antacids
- Acids in daily life: citric (lemon, orange), acetic (vinegar), tartaric (tamarind), lactic (curd), oxalic (spinach), formic (ant sting).
- Bases: NaOH (soap), KOH (alkaline batteries), Ca(OH)₂ (whitewash), NH₄OH (cleaners), Mg(OH)₂ (antacid).
- Antacids: mild bases that neutralise excess stomach HCl. Common — milk of magnesia (Mg(OH)₂), sodium bicarbonate (NaHCO₃), calcium carbonate (CaCO₃).
- Tooth decay begins when oral pH falls below 5.5 — bacteria convert sugar to acid that dissolves enamel.
- Acid rain — pH below 5.6, from SO₂ and NOₓ pollution.
- Blood pH is tightly buffered at 7.35-7.45; deviation can be fatal.
CDS/OTA PYQ Examples
Q: pH of pure water at 25 °C is:
(a) 0 (b) 1 (c) 7 (d) 14
Answer: (c) 7 — neutral.
Q: Which is most acidic?
(a) pH 4 (b) pH 7 (c) pH 9 (d) pH 11
Answer: (a) pH 4 — lower pH means more acidic.
Q: Antacid commonly used to neutralise stomach acid is:
(a) HCl (b) NaCl (c) Mg(OH)₂ (d) H₂SO₄
Answer: (c) Milk of magnesia — Mg(OH)₂.
Q: Phenolphthalein in basic solution is:
(a) Red (b) Colourless (c) Pink (d) Blue
Answer: (c) Pink.
Q: Tooth decay starts when mouth pH falls below:
(a) 7 (b) 6.5 (c) 5.5 (d) 4
Answer: (c) 5.5.
Drill pH Scale, Acids, Bases and Antacids for CDS/OTA
CDS/OTA-pattern items on pH Scale, Acids, Bases and Antacids with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
Why does milk turn sour?
Bacteria convert lactose into lactic acid, lowering pH. Below pH ~4.6, milk proteins (casein) coagulate, forming curd.
Why is acid rain harmful?
It lowers pH of soil, lakes and rivers, harming plants and aquatic life. It also corrodes marble (CaCO₃ + H₂SO₄ → CaSO₄ + CO₂ + H₂O) — damaging buildings like the Taj Mahal.
Why is blood pH so tightly controlled?
Enzymes work efficiently only in a narrow pH range. Deviation beyond 7.0-7.8 can be lethal. Buffers (bicarbonate, phosphate, protein) maintain the balance.