Indian Constitution — Structure & Amendments

Why This Chapter Matters

Chemistry contributes roughly 3 to 5 marks in SSC MTS and CHSL General Awareness papers, and much of it comes straight from things you have watched happen in your own kitchen — milk turning sour, an iron gate turning orange with rust, a tablet fizzing in water. This makes chemistry one of the easiest scoring areas in the whole exam once you connect the textbook term to the real event you already know.

The single biggest mistake aspirants make is trying to memorise the periodic table and chemical formulas like a phone directory, without understanding why elements behave the way they do. SSC MTS and CHSL rarely test deep chemistry. They test basic structure: what matter is made of, how the periodic table is organised, what acids and bases do, and why everyday reactions like rusting and digestion happen. This chapter builds that foundation in plain language, tying every concept to something you have seen with your own eyes, so the facts stick without needing rote repetition.

1. States of Matter

Everything around you, solid, liquid, or gas, is called matter — anything that has mass and occupies space. Matter exists mainly in three familiar states, and a fourth state that exams sometimes mention.

Solids have a fixed shape and a fixed volume. Their particles are packed tightly together and can only vibrate in place, not move around freely. This is why a block of ice keeps its shape whether you put it in a round bowl or a square tray.

Liquids have a fixed volume but no fixed shape — they take the shape of whatever container holds them. Their particles are close together but can slide past one another, which is why water poured into a glass, a bottle, or a bucket always adjusts its shape to match.

Gases have neither a fixed shape nor a fixed volume. Their particles are far apart and move freely in every direction, spreading out to fill whatever space is available, which is why the smell of cooking spices from a kitchen can fill an entire house within minutes.

Memory hook: Think of a crowded local train at three stages. A fully packed train at rush hour, where nobody can move, is like a solid. A moderately full train where people shuffle and shift positions but still stay bunched together is like a liquid. An empty train where a few passengers spread out and roam freely across the compartment is like a gas.

Matter changes between these states when heat is added or removed. Ice (solid) melts into water (liquid) when heated, and this is called melting. Water turns into steam (gas) when heated further, and this is called vaporisation. Steam cools back into water through condensation, and water freezes back into ice through freezing. One special case that skips the liquid stage entirely is called sublimation — a solid turns directly into gas without becoming liquid first, as seen with camphor (kapoor) tablets shrinking away in a cupboard, or dry ice (solid carbon dioxide) smoking away directly into gas.

Exam trap: Students often assume all solids melt before turning to gas. Sublimation is a favourite exception SSC likes to test, and naphthalene balls and camphor are the two most commonly asked examples.

There is a fourth state called plasma, an extremely hot, ionised gas found in lightning, neon signs, and stars including our own Sun. You do not need deep detail here, just recognition that plasma exists as the fourth state of matter beyond solid, liquid, and gas.

2. Atoms and Elements Basics

An atom is the smallest unit of matter that keeps the identity of an element. Atoms are built from three tiny particles: protons (positively charged, found in the nucleus), neutrons (no charge, also in the nucleus), and electrons (negatively charged, orbiting around the nucleus in shells). The nucleus, made of protons and neutrons packed tightly together, sits at the centre and carries almost all the atom's mass, while electrons circle around it at a relative distance, similar to how planets orbit far from the compact, heavy Sun at the centre of the solar system.

Exam trap: A neutral atom always has an equal number of protons and electrons, which balances the positive and negative charges to zero overall. If an atom gains or loses electrons, it becomes a charged particle called an ion — losing electrons creates a positive ion (cation), gaining electrons creates a negative ion (anion). This distinction between a neutral atom and an ion comes up frequently.

The number of protons in an atom is called its atomic number, and this number alone defines which element the atom is. Every atom with exactly 1 proton is hydrogen; every atom with exactly 6 protons is carbon; every atom with exactly 8 protons is oxygen. Change the proton count and you get a completely different element.

An element is a pure substance made of only one type of atom that cannot be broken down into simpler substances by ordinary chemical means. Gold, oxygen, carbon, and iron are all elements. When two or more elements combine chemically in a fixed ratio, they form a compound — water (hydrogen and oxygen combined) and common salt or sodium chloride (sodium and chlorine combined) are compounds. This is different from a mixture, where substances are simply mixed together physically without a fixed chemical ratio and can be separated back out by physical means, like sand mixed with water, or salt dissolved in water, which can be separated again by evaporation.

Exam trap: A compound has entirely new properties different from its original elements — table salt is nothing like the poisonous gas chlorine or the reactive metal sodium that combine to form it. A mixture, on the other hand, keeps the individual properties of its ingredients, and this exact distinction between compound and mixture is tested repeatedly.

Molecules are the smallest particle of an element or compound that can exist independently while still showing all the chemical properties of that substance. Two oxygen atoms bonded together form an oxygen molecule (O₂), the form of oxygen you actually breathe.

3. Periodic Table Basics

The periodic table is an organised chart of all known elements, arranged so that elements with similar chemical behaviour line up in the same column. Russian chemist Dmitri Mendeleev is credited with creating the first widely accepted version of the periodic table in 1869, arranging elements by increasing atomic weight and noticing that their properties repeated in a pattern, which he called periodicity. The modern periodic table, used today, arranges elements by increasing atomic number instead, a refinement that fixed a few inconsistencies in Mendeleev's original ordering.

The table has two main directions that matter for exams. Horizontal rows are called periods, and there are 7 of them. Vertical columns are called groups, and there are 18 of them. Elements in the same group share similar chemical properties because they have the same number of electrons in their outermost shell, which is the shell most responsible for how an element reacts.

Memory hook: Think of the periodic table like a classroom seating chart organised by roll number and subject group. Students sitting in the same row (period) are simply numbered in sequence, but students grouped in the same column (like all the students who chose the same optional subject) tend to behave similarly in exams because they share the same "subject" — just like elements in the same group share similar chemical behaviour because of matching outer electrons.

A few groups have special names worth remembering. Group 1 elements (excluding hydrogen) are called alkali metals — lithium, sodium, potassium — and they are famously soft and highly reactive, especially with water. Group 17 elements are called halogens — fluorine, chlorine, bromine, iodine — and they are reactive nonmetals often used in disinfectants and salts. Group 18 elements are called noble gases — helium, neon, argon — and they are almost completely unreactive because their outermost electron shell is already full, which is exactly why neon is used safely in glowing signboards without reacting with anything around it.

Elements are broadly divided into three categories. Metals are shiny, good conductors of heat and electricity, malleable (can be hammered into sheets) and ductile (can be drawn into wires) — iron, copper, gold, aluminium. Nonmetals are generally poor conductors, brittle if solid, and lack shine — oxygen, carbon, sulphur. Metalloids show properties of both metals and nonmetals — silicon and boron are classic examples, and silicon's in-between behaviour is exactly why it is used as a semiconductor in computer chips.

Exam trap: Mercury is the only metal that is liquid at normal room temperature, and this single fact is one of the most repeated questions in SSC-level science sections. Similarly, bromine is the only nonmetal that is liquid at room temperature — pairing these two facts together makes them far easier to recall correctly instead of mixing them up.

A handful of key elements come up again and again in exams:

Element	Symbol	Atomic Number	Key Fact
Hydrogen	H	1	Lightest element, most abundant in the universe
Oxygen	O	8	Essential for respiration and combustion
Carbon	C	6	Basis of all organic life and organic chemistry
Sodium	Na	11	Soft, reactive alkali metal, part of table salt
Iron	Fe	26	Most widely used metal, tends to rust
Gold	Au	79	Highly unreactive, used in jewellery and currency reserves
Mercury	Hg	80	Only liquid metal at room temperature

4. Acids and Bases in Daily Life

An acid is a substance that tastes sour, turns blue litmus paper red, and releases hydrogen ions (H⁺) when dissolved in water. You taste acids constantly without thinking of them as chemistry — lemon and citrus fruits contain citric acid, vinegar contains acetic acid, curd and sour milk contain lactic acid, and even your own stomach produces hydrochloric acid to help digest food.

A base is a substance that tastes bitter, feels soapy or slippery to touch, turns red litmus paper blue, and releases hydroxide ions (OH⁻) when dissolved in water. Common bases include soap, baking soda solution, and lime water. A base that dissolves in water is specifically called an alkali.

Exam trap: Every alkali is a base, but not every base is an alkali — a base only counts as an alkali if it actually dissolves in water. This distinction between "base" and "alkali" is a favourite trap because most students use the two words interchangeably in daily speech, but the exam treats them as technically different.

The pH scale measures how acidic or basic (alkaline) a substance is, running from 0 to 14. A pH of exactly 7 is neutral (pure water sits here). Values below 7 are acidic, and the lower the number, the stronger the acid. Values above 7 are basic, and the higher the number, the stronger the base.

Memory hook: Picture the pH scale as a thermometer for sourness and bitterness. The coldest end (0) is the strongest acid, the middle (7) is perfectly neutral like plain water, and the hottest end (14) is the strongest base. The further you move from the neutral middle in either direction, the more extreme the substance behaves.

Litmus paper is the simplest indicator used to test whether something is acidic or basic. Blue litmus turns red in acid, and red litmus turns blue in base. This one fact, phrased in exactly this way, has appeared in various forms across SSC-level papers repeatedly.

Here are pH values of common substances you should recognise instantly:

Substance	Approximate pH	Nature
Stomach acid (gastric juice)	1.5–2	Strongly acidic
Lemon juice	2–3	Acidic
Vinegar	3	Acidic
Milk	6.5	Mildly acidic
Pure water	7	Neutral
Blood (human)	7.4	Mildly basic
Baking soda solution	9	Basic
Soap solution	9–10	Basic

Exam trap: Human blood is actually mildly basic (pH around 7.4), not neutral and not acidic — students often assume it must be exactly 7 like pure water, but that is incorrect and frequently tested.

When an acid and a base are mixed together in the right proportion, they cancel each other's effects in a reaction called neutralisation, producing a salt and water. This is exactly why doctors recommend antacid tablets, which contain a mild base, to relieve the burning discomfort of excess stomach acid — the base in the antacid neutralises the excess hydrochloric acid in your stomach. Farmers also use this principle, adding lime (a base) to acidic soil to bring its pH closer to neutral before planting crops.

5. Common Chemical Reactions We See Daily

Rusting is a chemical reaction you have watched happen on iron gates, bicycle chains, and old tools left out in the rain. Iron reacts slowly with oxygen and moisture in the air to form iron oxide, the reddish-brown flaky substance called rust. This reaction needs both oxygen and water to happen — iron kept perfectly dry, or iron kept fully underwater with no dissolved oxygen, rusts far more slowly. This is exactly why rusting speeds up dramatically during the monsoon and why coastal areas, with salty humid air, see faster rusting than dry inland regions.

Exam trap: Rust is not simply "iron turning old." It is a genuine new chemical compound, iron oxide (Fe₂O₃, roughly), formed by a real chemical reaction with oxygen and water, not a physical change like scratching or bending. This "chemical change versus physical change" framing is a favourite way SSC phrases rusting questions.

Preventing rust is also commonly tested. Painting iron surfaces, applying grease or oil, and galvanisation (coating iron with a thin layer of zinc) are the standard prevention methods, because all three block oxygen and moisture from reaching the iron surface directly.

Cooking involves several chemical changes happening at once. When you heat raw batter to make a dosa or roti, the starches and proteins undergo permanent chemical changes that cannot be reversed — you cannot turn a cooked roti back into raw dough, which is the classic marker of a chemical change as opposed to a physical one. Boiling an egg is another everyday chemical change students often mistake for merely a physical change caused by heat; the proteins inside actually change their structure permanently, called denaturation, and no amount of cooling brings the egg back to its raw liquid state.

Exam trap: Melting butter or ice is a physical change (reversible, no new substance formed), but frying an egg or baking bread is a chemical change (irreversible, new substance formed). SSC often gives a list of daily kitchen activities and asks you to identify which is physical and which is chemical — the reversibility test is your fastest way to decide.

Fermentation is another kitchen reaction worth knowing. Curd forms from milk through fermentation, where bacteria (mainly Lactobacillus) convert milk sugar (lactose) into lactic acid, which thickens the milk and gives curd its sour taste. Idli and dosa batter rising overnight, and bread dough rising with yeast, are further everyday examples of fermentation, where microorganisms produce carbon dioxide gas that gets trapped in the batter and makes it light and fluffy.

Digestion inside your own body is a chain of chemical reactions too. Your stomach produces hydrochloric acid along with enzymes like pepsin, which break down proteins from your food into smaller, absorbable units. Saliva in your mouth contains an enzyme called amylase, which starts breaking down starches into simpler sugars right from the first bite, which is exactly why chewing rice or bread for a long time slowly starts to taste faintly sweet. Digestion is, at its core, a long series of chemical reactions that break down large, complex food molecules into small, simple molecules your body can absorb and use for energy.

Photosynthesis and respiration, covered more fully in the biology chapter, are also chemical reactions worth a quick mention here — plants combine carbon dioxide and water using sunlight energy to produce glucose and oxygen, while respiration in your own cells does roughly the reverse, breaking down glucose with oxygen to release energy, water, and carbon dioxide.

Combustion is the chemical reaction of burning, where a substance combines rapidly with oxygen to release heat and light. Burning of LPG gas in your kitchen stove, wood in a fire, or petrol in a vehicle engine are everyday combustion reactions. Complete combustion, with enough oxygen supply, produces carbon dioxide and water and burns with a clean blue flame, while incomplete combustion, with insufficient oxygen, produces soot (carbon particles) and carbon monoxide and burns with a yellow, sooty flame — which is exactly why a well-adjusted gas stove burner gives a clean blue flame, while a poorly adjusted one gives a dirty yellow flame with black soot forming on your vessels.

Exam trap: Carbon monoxide, produced by incomplete combustion, is dangerous because it is colourless, odourless, and binds to your blood's haemoglobin far more strongly than oxygen does, which is why running a petrol generator or coal fire in a closed room without ventilation can be fatal even though nothing looks or smells wrong.

Quick Revision — One-Line Facts

• Matter exists mainly in three states: solid, liquid, and gas, plus a fourth state called plasma.
• Solids have fixed shape and volume; liquids have fixed volume but no fixed shape; gases have neither.
• Sublimation is a solid turning directly into a gas, as seen with camphor and naphthalene balls.
• An atom contains protons and neutrons in the nucleus, and electrons orbiting around it.
• Atomic number equals the number of protons and defines which element an atom is.
• A neutral atom has equal protons and electrons; gaining or losing electrons creates an ion.
• An element has only one type of atom; a compound forms when elements combine chemically in fixed ratio.
• A mixture combines substances physically and can be separated back by physical methods.
• Dmitri Mendeleev created the first widely accepted periodic table in 1869.
• The modern periodic table arranges elements by increasing atomic number.
• The periodic table has 7 periods (rows) and 18 groups (columns).
• Group 1 elements (except hydrogen) are alkali metals; Group 17 are halogens; Group 18 are noble gases.
• Mercury is the only metal that is liquid at room temperature; bromine is the only liquid nonmetal.
• Metals are shiny, malleable, and ductile; nonmetals are generally brittle and poor conductors.
• Silicon is a metalloid, used as a semiconductor in computer chips.
• Acids turn blue litmus red and taste sour; bases turn red litmus blue and taste bitter.
• Every alkali is a base, but not every base dissolves in water to become an alkali.
• The pH scale runs from 0 to 14, with 7 as neutral.
• Human blood has a pH of about 7.4, making it mildly basic, not neutral.
• Neutralisation is the reaction between an acid and a base, forming salt and water.
• Rusting is a chemical reaction between iron, oxygen, and moisture, forming iron oxide.
• Galvanisation protects iron from rust by coating it with zinc.
• Melting and freezing are physical changes; cooking, rusting, and burning are chemical changes.
• Fermentation by bacteria converts milk into curd through lactic acid production.
• Saliva contains amylase, which begins starch digestion in the mouth.
• Combustion is burning, a chemical reaction combining a substance with oxygen to release heat and light.
• Complete combustion gives a clean blue flame; incomplete combustion gives soot and carbon monoxide.
• Carbon monoxide is dangerous because it binds haemoglobin more strongly than oxygen does.
• Stomach acid, mainly hydrochloric acid, helps digest proteins with the enzyme pepsin.

Memory Tables

Table 1: States of Matter and Their Changes

Change	From → To	Everyday Example
Melting	Solid → Liquid	Ice melting into water
Vaporisation	Liquid → Gas	Water boiling into steam
Condensation	Gas → Liquid	Steam turning to water droplets on a cold plate
Freezing	Liquid → Solid	Water freezing into ice
Sublimation	Solid → Gas directly	Camphor or naphthalene balls shrinking away

Table 2: Acid vs Base at a Glance

Property	Acid	Base
Taste	Sour	Bitter
Feel	—	Soapy, slippery
Effect on blue litmus	Turns red	No change
Effect on red litmus	No change	Turns blue
pH range	Below 7	Above 7
Daily example	Lemon juice, vinegar, curd	Soap, baking soda, lime water

Table 3: Physical Change vs Chemical Change

Feature	Physical Change	Chemical Change
New substance formed	No	Yes
Reversible	Usually yes	Usually no
Example	Ice melting, butter melting	Iron rusting, egg frying, wood burning

Practice MCQs

Q1. Which state of matter has a fixed volume but no fixed shape? (a) Solid (b) Liquid (c) Gas (d) Plasma

Q2. Camphor balls disappearing directly into the air without turning into liquid is an example of: (a) Evaporation (b) Sublimation (c) Condensation (d) Dissolution

Q3. What determines the atomic number of an element? (a) Number of neutrons (b) Number of electrons only (c) Number of protons (d) Total mass of the atom

Q4. Which of the following is an example of a compound? (a) Sand and water mixed together (b) Salt water (c) Common salt (sodium chloride) (d) Air

Q5. Who is credited with creating the first widely accepted periodic table? (a) Isaac Newton (b) Dmitri Mendeleev (c) Marie Curie (d) John Dalton

Q6. Which metal is liquid at normal room temperature? (a) Iron (b) Gold (c) Mercury (d) Sodium

Q7. Group 18 elements of the periodic table are known as: (a) Alkali metals (b) Halogens (c) Noble gases (d) Metalloids

Q8. Blue litmus paper turns red when dipped into: (a) A base (b) An acid (c) Pure water (d) A neutral salt solution

Q9. What is the approximate pH of pure water? (a) 0 (b) 5 (c) 7 (d) 14

Q10. The approximate pH of human blood is: (a) 5.0, mildly acidic (b) 7.0, exactly neutral (c) 7.4, mildly basic (d) 10.0, strongly basic

Q11. Rusting of iron requires the presence of: (a) Only oxygen (b) Only water (c) Both oxygen and moisture (d) Neither oxygen nor water

Q12. Which method is commonly used to protect iron from rusting by coating it with zinc? (a) Galvanisation (b) Fermentation (c) Neutralisation (d) Sublimation

Q13. Which of the following is a chemical change? (a) Melting of ice (b) Boiling of water into steam (c) Burning of wood (d) Dissolving sugar in water

Q14. The souring of milk into curd happens due to: (a) Evaporation (b) A physical change only (c) Fermentation by bacteria (d) Neutralisation

Q15. Incomplete combustion of fuel produces a dangerous gas known as: (a) Carbon dioxide (b) Oxygen (c) Carbon monoxide (d) Nitrogen

Answer Key

Q	Answer	Reason
1	(b) Liquid	Liquids keep a fixed volume but take the shape of their container, unlike solids or gases.
2	(b) Sublimation	Sublimation is a solid changing directly to gas without passing through the liquid state.
3	(c) Number of protons	Atomic number is defined strictly by the count of protons in the nucleus.
4	(c) Common salt (sodium chloride)	It is formed by sodium and chlorine combining chemically in a fixed ratio, unlike a mixture.
5	(b) Dmitri Mendeleev	Mendeleev arranged elements by atomic weight in 1869, creating the first accepted periodic table.
6	(c) Mercury	Mercury is the only metal that remains liquid at normal room temperature.
7	(c) Noble gases	Group 18 elements have a full outer electron shell, making them almost entirely unreactive.
8	(b) An acid	Acids turn blue litmus red; this is the standard test used to identify acidic substances.
9	(c) 7	A pH of 7 marks the exact neutral point on the 0 to 14 pH scale.
10	(c) 7.4, mildly basic	Human blood sits slightly above neutral pH, making it mildly basic rather than neutral.
11	(c) Both oxygen and moisture	Rusting is a chemical reaction that needs both oxygen and water present together to proceed.
12	(a) Galvanisation	Galvanisation coats iron with a protective layer of zinc to block oxygen and moisture.
13	(c) Burning of wood	Burning creates new substances like ash and gases, making it irreversible and chemical.
14	(c) Fermentation by bacteria	Lactobacillus bacteria convert milk sugar into lactic acid, thickening milk into curd.
15	(c) Carbon monoxide	Insufficient oxygen during combustion produces this colourless, dangerous gas instead of carbon dioxide.