In a chemical reaction, substances change into new ones: milk turns into curd, iron rusts, coal burns. Many everyday reactions involve acids and bases — the sourness of lemon, the sting of an ant, the relief from an antacid. Exams ask about reaction types, which acid is in which food, litmus and pH, and the names of common salts.
On this page you will learn:
- Types of chemical reaction; exothermic and endothermic
- Oxidation and reduction; rusting and rancidity
- Acids in food and in stings
- Indicators and the pH scale; neutralisation
- Common salts: baking soda, washing soda, bleaching powder, plaster of Paris
0–14
Range of the pH scale
Below 7 acidic, above 7 basic
7
pH of pure water
Neutral
NaHCO3
Baking soda
Asked in SI 2016
Types of chemical reaction
Rule in one line: a balanced equation has the same number of each atom on both sides.
QUICK FACT
Signs that a reaction has taken place: a change of colour, a gas given off, a change of temperature, or an insoluble solid (precipitate) forming. Equations are balanced because mass is conserved: 2H2 + O2 → 2H2O.
| Type | What happens | Example |
|---|---|---|
| Combination | Two or more substances form one | CaO + H2O → Ca(OH)2 |
| Decomposition | One substance breaks into simpler ones | CaCO3 → CaO + CO2 (on heating) |
| Displacement | A more reactive metal pushes out a less reactive one | Fe + CuSO4 → FeSO4 + Cu (blue turns green) |
| Double displacement | Two compounds swap partners, often giving a precipitate | Na2SO4 + BaCl2 → BaSO4 (white) + 2NaCl |
QUICK CHECK
PYQ · RSMSSB LDC 2024 — CaO + H₂O → Ca(OH)₂ + Heat. The given chemical equation is: (a) a double displacement reaction (b) a combination reaction (c) a decomposition reaction (d) exothermic reaction. Choose the most appropriate answer.
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Answer: B — b and d only
Two substances join into one product (combination), and heat is given out (exothermic).
QUICK CHECK
PYQ · PPSC ICS 2016 — If two substances react and the temperature of the mixture increases, the reaction is
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Answer: D — Exothermic
A reaction that gives out heat warms the mixture: it is exothermic. PPSC OHE 2016 asked the same question.
Oxidation and reduction
Rule in one line: oxidation is gain of oxygen or loss of electrons; reduction is the opposite.
Slow oxidation
Rusting
Iron + oxygen + water gives reddish-brown rust.
Slow oxidation
Rancidity
Fats and oils in food go stale; chips packets are filled with nitrogen to stop it.
Fast oxidation
Burning
Coal, wood and LPG join oxygen quickly, giving heat and light.
Oxidation in daily life
QUICK CHECK
PYQ · PPSC OHE 2016 — Oxidation is a chemical reaction involving the:
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Answer: B — Loss of Electrons
In terms of electrons, oxidation is loss and reduction is gain. Protons stay in the nucleus during reactions.
QUICK CHECK
PYQ · RSMSSB LDC 2024 — Burning of coal to form carbon dioxide is a/an _____ reaction.
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Answer: C — Oxidation
C + O2 → CO2: carbon gains oxygen, so it is oxidised. It is also a combination and an exothermic reaction.
Acids and bases around us
Rule in one line: acids taste sour and turn blue litmus red; bases taste bitter, feel soapy and turn red litmus blue.
| Source | Acid |
|---|---|
| Lemon, orange | Citric acid |
| Curd, sour milk | Lactic acid |
| Vinegar | Acetic acid |
| Tamarind, grapes | Tartaric acid |
| Tomato, spinach | Oxalic acid |
| Ant sting, nettle sting | Formic (methanoic) acid |
| Amla, citrus fruits (vitamin C) | Ascorbic acid |
| Apple | Malic acid |
| Our stomach | Hydrochloric acid |
| Base | Common name or use |
|---|---|
| Sodium hydroxide, NaOH | Caustic soda; soap making |
| Calcium hydroxide, Ca(OH)2 | Slaked lime, lime water; whitewash |
| Magnesium hydroxide, Mg(OH)2 | Milk of magnesia; antacid |
| Ammonium hydroxide, NH4OH | Window cleaners |
QUICK CHECK
PYQ · PPSC OHE 2016 — The acid present in lemons and oranges is:
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Answer: C — Citric acid
Citrus fruits get their sour taste from citric acid. Acetic acid is in vinegar; oxalic acid in tomato and spinach.
QUICK CHECK
PYQ · PPSC OHE 2016 — Milk contains …………… acid.
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Answer: C — Lactic
Bacteria turn the milk sugar (lactose) into lactic acid. That is what makes milk sour and sets curd.
QUICK CHECK
PYQ · PSSSB Excise Inspector 2023 — Choose the incorrect acid-source pair.
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Answer: C — Oxalic acid – Ant sting
An ant sting has methanoic (formic) acid, not oxalic acid. The other three pairs are correct.
QUICK CHECK
PYQ · PPSC OHE 2016 — …………… acid is the chemical name of Vitamin C.
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Answer: B — Ascorbic
Vitamin C is ascorbic acid; its lack causes scurvy. Tartaric acid is in tamarind and grapes.
Indicators and the pH scale
Rule in one line: pH below 7 is acidic, 7 is neutral, above 7 is basic.
| Indicator | In acid | In base |
|---|---|---|
| Litmus (from lichens) | Blue turns red | Red turns blue |
| Phenolphthalein | Colourless | Pink |
| Methyl orange | Red | Yellow |
| Turmeric | Stays yellow | Turns reddish-brown |
| China rose (gurhal) | Dark pink | Green |
Reading pH with a test strip
pH strip after an acidic solution (about pH 2): warm red-orange colours
Rupal Dugar, Wikimedia Commons · CC BY-SA 4.0
pH strip after a basic solution (about pH 10): a pad turns blue
Rupal Dugar, Wikimedia Commons · CC BY-SA 4.0
Approximate pH of common substances
| Problem | Cause | Neutralised with |
|---|---|---|
| Indigestion | Too much stomach acid | Antacid such as milk of magnesia |
| Ant sting | Formic acid in the skin | Baking soda or calamine |
| Acidic soil | Too much acid for crops | Quicklime or slaked lime |
| Tooth decay | Acid from food left in the mouth | Basic toothpaste |
QUICK CHECK
PYQ · PPSC AA 2022 — The pH value of acidic medium is:
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Answer: D — Less than 7
On the 0–14 scale, 7 is neutral, below 7 is acidic and above 7 is basic.
Salts and chemicals we meet every day
Rule in one line: learn each salt with its chemical name, formula and main use.
| Common name | Chemical name and formula | Main uses |
|---|---|---|
| Common salt | Sodium chloride, NaCl | Food; raw material for caustic soda and baking soda |
| Baking soda | Sodium bicarbonate (sodium hydrogen carbonate), NaHCO3 | Baking powder, antacid, soda-acid fire extinguisher |
| Washing soda | Sodium carbonate, Na2CO3·10H2O | Cleaning; removing hardness of water; glass making |
| Bleaching powder | Calcium oxychloride, CaOCl2 | Disinfecting drinking water; bleaching cloth |
| Plaster of Paris | Calcium sulphate hemihydrate, CaSO4·½H2O | Plaster for broken bones, statues, toys |
| Gypsum | Calcium sulphate dihydrate, CaSO4·2H2O | Heated to make plaster of Paris; added to cement |
| Blue vitriol | Copper sulphate, CuSO4·5H2O | Turns white on heating as it loses its water |
KEY TERM
Soda water and carbonic acid
Soda water is water with carbon dioxide dissolved in it under pressure. A little of the gas forms the weak carbonic acid, which gives the drink its slight sour bite. When the bottle is opened, the extra gas escapes as bubbles.
QUICK CHECK
PYQ · Punjab Police SI 2016 — Chemical name of baking soda is
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Answer: D — sodium bicarbonate
Baking soda is sodium bicarbonate, NaHCO3. On heating it gives carbon dioxide, which makes cakes rise. Sodium chloride is common salt.
QUICK CHECK
PYQ · Punjab Police Constable 2021 — Soda water contains which of the following gases?
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Answer: A — Carbon Dioxide
Soda water is carbonated: carbon dioxide is dissolved in it under pressure.
QUICK CHECK
PYQ · Punjab Police SI 2016 — The ‘stones’ formed in the human kidney consist mostly of
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Answer: D — calcium oxalate
Most kidney stones are calcium oxalate, a salt. 'Calcium' alone is only part of it, so it is not the best answer.
Traps and practice
Last revision — where the options try to fool you.
COMMON TRAPS
- Oxidation is loss of electrons, not gain. Gain of electrons is reduction.
- Temperature goes up = exothermic; goes down = endothermic.
- Ant sting is formic (methanoic) acid, not oxalic.
- Lemon: citric; milk and curd: lactic; vitamin C: ascorbic; vinegar: acetic.
- Acids have pH less than 7; a lower pH means a stronger acid.
- Baking soda is NaHCO3 (sodium bicarbonate); washing soda is Na2CO3·10H2O.
- Kidney stones: calcium oxalate, not just 'calcium'.
CHECK YOURSELF
An ant bites you. What should you apply, and why?
Show answer
Apply baking soda (or calamine). The sting puts formic acid into the skin; the mild base neutralises it and eases the pain.
FLASHCARDS
Acids and salts flashcards
- Acid in lemon
- Citric acid
- Acid in milk and curd
- Lactic acid
- Acid in ant sting
- Formic (methanoic) acid
- Vitamin C
- Ascorbic acid
- Baking soda
- Sodium bicarbonate, NaHCO3
- Gas in soda water
- Carbon dioxide
- Kidney stones
- Calcium oxalate
- Oxidation (electrons)
- Loss of electrons
QUICK BYTES
- Combination, decomposition, displacement, double displacement
- Lime + water: combination and exothermic
- OIL RIG: oxidation is loss, reduction is gain
- Acid turns blue litmus red; base turns red litmus blue
- pH 7 neutral; blood 7.4; gastric juice 1.2
- Acid + base → salt + water
- Plaster of Paris CaSO4·½H2O; gypsum CaSO4·2H2O