Chemistry — Changes, Atomic Structure, Acids, Bases, Salts & Everyday Products
Chemistry in the GAT is mostly recall of facts that touch daily life: which gas is which, what an acid does to litmus, what soap is made from, why a cut apple turns brown. There are no organic mechanisms, but the official syllabus does list chemical equations, so you are expected to read an equation, name the type of reaction it shows, and know when one is balanced — all of which this chapter covers. Beyond that the examiner tests relentlessly the pairing of a common name with a chemical name — baking soda, washing soda, quicklime, bleaching powder — so those pairs are built into tables you can revise in two minutes before the paper.
Atomic Structure
| Particle | Charge | Where | Discovered by |
|---|---|---|---|
| Electron | Negative | Shells around the nucleus | J. J. Thomson |
| Proton | Positive | Nucleus | Goldstein / Rutherford |
| Neutron | Neutral | Nucleus | James Chadwick |
- Atomic number (Z) = number of protons. It defines the element and never changes for it.
- Mass number (A) = protons + neutrons. Electrons are too light to count.
- Isotopes — same Z, different A (¹H, ²H, ³H; C-12 and C-14). Same chemistry, different mass.
- Isobars — same A, different Z (Ar-40 and Ca-40). Different elements entirely.
- Rutherford's gold-foil experiment showed the atom is mostly empty space with a tiny dense nucleus.
⚠️ Isotopes vs isobars — one letter, opposite meanings
Isotopes share the proton count; isobars share the mass number A. Fixing the letter to the shared quantity stops the confusion. And remember that a neutral atom has electrons equal to protons — if the numbers differ, you are looking at an ion, not an atom.Acids, Bases, Salts & the pH Scale
| Test | Acid | Base |
|---|---|---|
| Taste | Sour (lemon, vinegar) | Bitter, feels soapy |
| Blue litmus | Turns red | No change |
| Red litmus | No change | Turns blue |
| pH | Less than 7 | More than 7 |
| Ion released in water | H⁺ | OH⁻ |
The pH scale runs 0 to 14 with 7 exactly neutral (pure water). Acid + base → salt + water, which is neutralisation, and it is behind several everyday remedies: an antacid (a mild base) for acidity in the stomach, baking soda paste on a bee sting, and lime added to acidic soil.
🎯 Where the natural acids come from
Citrus fruit → citric acid. Vinegar → acetic acid. Curd → lactic acid. Tamarind → tartaric acid. Ant and bee sting → formic acid. Tomato → oxalic acid. Apple → malic acid. Stomach → hydrochloric acid. This list is asked as a match-the-pairs question year after year, and it is eight facts for a near-certain mark.| Common name | Chemical name & formula |
|---|---|
| Baking soda | Sodium bicarbonate, NaHCO₃ |
| Washing soda | Sodium carbonate, Na₂CO₃·10H₂O |
| Common salt | Sodium chloride, NaCl |
| Quicklime | Calcium oxide, CaO |
| Slaked lime | Calcium hydroxide, Ca(OH)₂ |
| Bleaching powder | Calcium oxychloride, CaOCl₂ |
| Plaster of Paris | Calcium sulphate hemihydrate |
| Blue vitriol | Copper sulphate, CuSO₄·5H₂O |
| Dry ice | Solid carbon dioxide |
Chemical Equations & the Five Types of Reaction
An equation is a sentence: reactants on the left, products on the right, an arrow for "becomes". Because matter is never created or destroyed, the count of every kind of atom must be identical on both sides — that is all "balancing" means. You are not asked to balance a hard equation against the clock; you are asked to spot whether one is balanced and to name the reaction type, and both are quick once you know the five families.
| Type | Pattern | Standard example |
|---|---|---|
| Combination | A + B → AB (two things become one) | CaO + H₂O → Ca(OH)₂ — quicklime slaked, releasing much heat |
| Decomposition | AB → A + B (one becomes two, needing heat, light or electricity) | CaCO₃ → CaO + CO₂ on heating; electrolysis of water gives H₂ and O₂ |
| Displacement | A + BC → AC + B (a more reactive metal pushes out a less reactive one) | Fe + CuSO₄ → FeSO₄ + Cu — the blue solution turns green and copper deposits |
| Double displacement | AB + CD → AD + CB (partners swap; usually gives a precipitate) | Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl — white barium sulphate precipitate |
| Redox (oxidation–reduction) | One substance gains oxygen (oxidised), the other loses it (reduced) | CuO + H₂ → Cu + H₂O — CuO is reduced, H₂ is oxidised |
🧠 OIL RIG, and how to name a redox reaction in five seconds
Oxidation Is Loss, Reduction Is Gain — of electrons. At GAT level you can work in oxygen instead: whatever gained oxygen was oxidised, whatever lost it was reduced. Two consequences worth remembering because they are asked as separate questions: the substance that is reduced is the oxidising agent, and the one oxidised is the reducing agent — they are always named the opposite way round from what happened to them, which is exactly why the question is asked. Rusting and respiration are both slow oxidations; corrosion and rancidity (fats going stale) are the two everyday examples the examiner favours.⚠️ You may change the coefficient, never the formula
To balance H₂ + O₂ → H₂O you write 2H₂ + O₂ → 2H₂O. What you must never do is turn H₂O into H₂O₂ to make the oxygen count work — changing a subscript changes the substance itself, and hydrogen peroxide is not water. So balancing only ever means putting numbers in front of formulae. A quick check for the multiple-choice version of this question: count one element at a time, leave hydrogen and oxygen for last, and if an odd number of oxygen atoms is blocking you, double everything.Two more labels complete the set, and they describe heat rather than atoms. An exothermic reaction releases heat — respiration, burning, and quicklime meeting water. An endothermic one absorbs it, which is why photosynthesis needs sunlight and why dissolving ammonium nitrate makes the beaker cold. Practically every combination reaction is exothermic and practically every decomposition is endothermic, which is a useful default when a question gives you nothing else to go on.
Everyday Chemical Products
- Soap is the sodium or potassium salt of a long-chain fatty acid, made by boiling oil or fat with alkali — a process called saponification. Soap fails in hard water because calcium and magnesium ions form an insoluble scum; detergents work there because their calcium salts stay soluble.
- Glass is made by fusing sand (silica) with soda ash and limestone. It is not a true solid but a supercooled liquid — an amorphous substance with no fixed melting point.
- Ink is a pigment or dye in a solvent; permanent inks use tannic acid with iron salts, which is why old documents fade to brown.
- Cement is limestone plus clay, heated in a kiln; gypsum is added to slow the setting so it can be worked.
| Gas | Fact the examiner wants |
|---|---|
| Nitrogen (78%) | Most abundant gas in air; inert, used in food packing |
| Oxygen (21%) | Supports combustion but does not itself burn |
| Carbon dioxide | Turns lime water milky — the standard test; used in fire extinguishers |
| Carbon monoxide | Deadly because it binds haemoglobin ~200× better than oxygen |
| Helium | Lighter than air and non-flammable — used in balloons and airships |
| LPG / CNG | LPG is butane + propane; CNG is mainly methane |
🧠 Why cooking gas is given a smell
LPG is odourless, so a strong-smelling sulphur compound — ethyl mercaptan — is added deliberately so that leaks are noticed. That single fact is a recurring question, and it belongs to a family the examiner likes: things added on purpose for safety. Others are gypsum in cement to slow setting, iodine in salt to prevent goitre, and chlorine in water to kill bacteria.Key takeaways
- Chemical change signs: colour change, gas, precipitate, or heat/light exchanged.
- A burning candle is BOTH — wax melts physically, then burns chemically.
- Atomic number = protons; mass number = protons + neutrons.
- IsotOPes share Protons; isobArs share mass number A.
- Acid turns blue litmus red and has pH below 7; base does the reverse.
- Baking soda NaHCO₃, washing soda Na₂CO₃·10H₂O — different compounds entirely.
- Five reaction types: combination, decomposition, displacement, double displacement, redox.
- Balance by changing coefficients in front of formulae, never the subscripts inside them.
- The substance reduced is the oxidising agent; the one oxidised is the reducing agent.
- Soap fails in hard water; detergents do not. Glass is a supercooled liquid.
- CO₂ turns lime water milky; CO is lethal because it out-binds oxygen on haemoglobin.
Practice questions (10)
Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.
The burning of a candle is:
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Answer: C — Both a physical and a chemical change
The wax first melts and vaporises, which is physical and reversible, and the vapour then burns to give carbon dioxide and water, which is chemical. Both happen at once, which is why this is a favourite question.The neutron was discovered by:
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Answer: C — James Chadwick
Chadwick discovered the neutron in 1932. Thomson found the electron, Rutherford established the nucleus, and Bohr gave the shell model of the atom.Atoms of the same element having the same atomic number but different mass numbers are called:
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Answer: B — Isotopes
Isotopes share the proton count (hence the same element) but differ in neutrons, like C-12 and C-14. Isobars share the mass number while being different elements.The acid present in curd is:
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Answer: B — Lactic acid
Bacteria convert the lactose in milk into lactic acid, which is what sours it. Citric acid is in citrus fruit, tartaric in tamarind and acetic in vinegar.The chemical name of baking soda is:
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Answer: B — Sodium bicarbonate
Baking soda is sodium bicarbonate, NaHCO₃. Sodium carbonate (Na₂CO₃·10H₂O) is washing soda — a different compound, and the two are deliberately offered together.Soap does not clean effectively in hard water because:
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Answer: B — Calcium and magnesium ions form an insoluble scum with soap
Hard water is rich in calcium and magnesium ions, whose soap salts are insoluble and form scum instead of lather. Detergents are used instead because their calcium and magnesium salts remain soluble.Carbon monoxide is highly poisonous because it:
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Answer: B — Combines with haemoglobin far more readily than oxygen
CO binds haemoglobin roughly 200 times more strongly than oxygen does, forming carboxyhaemoglobin, so the blood can no longer carry oxygen. The victim suffocates even in a room with plenty of air.A strong-smelling substance is deliberately added to LPG in order to:
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Answer: B — Make leaks detectable
LPG is odourless, so ethyl mercaptan is added purely as a safety warning. It belongs with gypsum in cement, iodine in salt and chlorine in water — substances added deliberately for a practical reason.The reaction Fe + CuSO₄ → FeSO₄ + Cu is an example of:
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Answer: C — Displacement
Iron is more reactive than copper, so it pushes copper out of its salt — a single displacement, pattern A + BC → AC + B. The visible signs are the blue solution turning green and copper depositing on the iron. In a double displacement both partners swap and a precipitate usually forms, as in Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl.In the reaction CuO + H₂ → Cu + H₂O, which statement is correct?
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Answer: B — CuO is reduced and acts as the oxidising agent
CuO loses oxygen, so it is reduced; H₂ gains oxygen, so it is oxidised. The naming is deliberately counter-intuitive and is exactly why the question is set: the substance that is reduced is the oxidising agent, and the one oxidised is the reducing agent — each is named for what it does to the other, not for what happens to it.