Chemical Reactions, Acids, Bases and Salts
Chemical Reactions, Acids, Bases and Salts
A chemical reaction changes substances into new ones. NTPC asks the type of a given reaction, oxidation and reduction, the pH of common substances, acids found in natural sources and the chemical formulas of salts used at home.
1. Types of Chemical Reactions
| Type | Pattern | Example |
|---|---|---|
| Combination | A + B gives AB | Quicklime and water give slaked lime: CaO + H₂O gives Ca(OH)₂ (exothermic) |
| Decomposition | AB gives A + B; needs heat, light or electricity, so usually endothermic | Heating limestone: CaCO₃ gives CaO + CO₂ |
| Displacement | A more reactive element replaces a less reactive one | Iron nail in copper sulphate turns brown; solution turns from blue to green |
| Double displacement | Ions exchange; often a precipitate forms | AgNO₃ + NaCl gives AgCl (white precipitate) |
| Oxidation | Gain of oxygen or loss of electrons | Rusting of iron |
| Reduction | Loss of oxygen or gain of electrons | CuO + H₂ gives Cu + H₂O |
| Redox | Oxidation and reduction together | Most combustion reactions |
| Rancidity | Oxidation of fats in food | Prevented by antioxidants and nitrogen-filled chip packets |
2. Acids and Bases
| Item | Fact |
|---|---|
| pH scale | 0 to 14; below 7 acidic, 7 neutral, above 7 basic |
| Indicators | Litmus (acid turns blue litmus red), phenolphthalein (pink in base), methyl orange |
| Stomach acid | Hydrochloric acid, pH about 1.5 to 3.5 |
| Tooth decay | Starts when mouth pH falls below 5.5 |
| Bee sting | Methanoic (formic) acid; treated with baking soda |
| Antacids | Milk of magnesia (magnesium hydroxide) |
| Natural source | Acid |
|---|---|
| Lemon, orange | Citric acid |
| Tamarind, grapes | Tartaric acid |
| Curd, sour milk | Lactic acid |
| Vinegar | Acetic acid |
| Tomato | Oxalic acid |
| Ant and nettle sting | Formic (methanoic) acid |
| Apple | Malic acid |
3. Important Salts
| Common name | Chemical name and formula | Use |
|---|---|---|
| Common salt | Sodium chloride, NaCl | Food, raw material for other chemicals |
| Baking soda | Sodium hydrogen carbonate, NaHCO₃ | Baking, antacid |
| Washing soda | Sodium carbonate decahydrate, Na₂CO₃·10H₂O | Removing hardness of water |
| Bleaching powder | Calcium oxychloride, CaOCl₂ | Disinfecting water |
| Plaster of Paris | Calcium sulphate hemihydrate, CaSO₄·½H₂O | Plaster for fractures, statues |
| Gypsum | CaSO₄·2H₂O | Making cement and Plaster of Paris |
✗ Decomposition reactions usually release heat | ✓ Decomposition usually needs energy, so it is endothermic; combination is usually exothermic
✗ Curd contains citric acid | ✓ Curd contains lactic acid; citric acid is in lemons and oranges
हिंदी नोट: अपघटन अभिक्रिया में ऊर्जा लगती है, इसलिए यह प्रायः ऊष्माशोषी होती है। pH 7 से कम अम्लीय और 7 से अधिक क्षारीय होता है। इमली में टार्टरिक अम्ल, दही में लैक्टिक अम्ल और सिरके में ऐसीटिक अम्ल होता है। बेकिंग सोडा NaHCO₃ और धोने का सोडा Na₂CO₃·10H₂O है।
Exam Pointer: Verified NTPC pattern: the reaction type that is always endothermic (April 2016 CBT-1). pH, natural-acid and salt-formula questions are tagged syllabus-based.
Pariksha Pattern: Every Way NTPC Asks This Topic
Pattern 1: Types of reactions and energy change
[PYQ: NTPC CBT-1 29-Apr-2016 Shift-2]
EXAM LEVEL
Q. Burning of natural gas releases heat. Is it exothermic or endothermic?
Exothermic, since heat is released.
Answer: Exothermic
EXAMATLAS LEVEL
Q. Identify the type of each reaction: heating calcium carbonate; an iron nail in copper sulphate solution; adding water to quicklime.
Heating CaCO₃ into CaO and CO₂ is decomposition. Iron pushing copper out of its salt is displacement. Quicklime and water forming slaked lime is combination, which is strongly exothermic.
Answer: Decomposition; displacement; combination
Pattern 2: Oxidation, reduction and everyday effects
[Pattern: syllabus-based, PYQ-style]
EXAM LEVEL
Q. In CuO + H₂ giving Cu + H₂O, which substance is oxidised?
Hydrogen, since it gains oxygen to form water. Copper oxide is reduced.
Answer: Hydrogen
EXAMATLAS LEVEL
Q. What is rancidity, and why are chip packets filled with nitrogen? Name the process when iron rusts.
Rancidity is the oxidation of fats and oils that spoils taste and smell. Nitrogen keeps oxygen out, so oxidation is prevented. Rusting is the oxidation of iron in moist air.
Answer: Oxidation of fats; to keep oxygen out; oxidation
Pattern 3: pH and natural acids
[Pattern: syllabus-based, PYQ-style]
EXAM LEVEL
Q. Which acid is present in tamarind?
Tartaric acid.
Answer: Tartaric acid
EXAMATLAS LEVEL
Q. Arrange in increasing pH: pure water, stomach juice, milk of magnesia, tomato juice. Which acid is injected by a bee sting and how is it treated?
Stomach juice (about 2) is lowest, then tomato juice (about 4), then pure water (7), then milk of magnesia (about 10). A bee sting injects methanoic acid and is treated with a mild base such as baking soda.
Answer: Stomach juice, tomato juice, water, milk of magnesia; methanoic acid, baking soda
Pattern 4: Common salts and their formulas
[Pattern: syllabus-based, PYQ-style]
EXAM LEVEL
Q. What is the chemical formula of baking soda?
NaHCO₃, sodium hydrogen carbonate.
Answer: NaHCO₃
EXAMATLAS LEVEL
Q. Match the substance with its use: bleaching powder, washing soda, Plaster of Paris. Which compound is heated to make Plaster of Paris?
Bleaching powder disinfects water, washing soda removes the hardness of water and Plaster of Paris sets fractured bones. Plaster of Paris is made by heating gypsum, CaSO₄·2H₂O.
Answer: Disinfecting water, softening water, setting fractures; gypsum
60-Second Revision
- Combination is usually exothermic; decomposition is usually endothermic.
- Oxidation: gain of oxygen or loss of electrons; rusting and rancidity.
- pH below 7 acid; citric lemon, tartaric tamarind, lactic curd, acetic vinegar, formic ant.
- NaHCO₃ baking soda; Na₂CO₃·10H₂O washing soda; CaOCl₂ bleaching powder; CaSO₄·½H₂O Plaster of Paris.