Concept

A physical change alters only the form or state of a substance, with no new substance formed, and is usually reversible. A chemical change produces one or more new substances and is generally irreversible.

Identifying which type a process is, is a common question.

Key Points & Formulas

• Physical change: melting of ice, boiling water, dissolving salt (reversible, no new substance).
• Chemical change: burning, rusting, cooking, digestion (new substance, often irreversible).
• Signs of a chemical change: gas evolved, colour change, heat/light, precipitate.
• Mass is conserved in both types of change.

Worked Example

Q. Classify: (i) melting of wax, (ii) burning of wax.

Melting of wax changes only its state (solid→liquid) — physical change.

Burning of wax forms CO₂ and water vapour — a chemical change.

Practice MCQs

Q1. Which of the following is a chemical change?

(A) Rusting of iron (B) Melting of ice (C) Boiling of water (D) Breaking of glass

Answer: A. Rusting forms a new substance (iron oxide); the rest are physical changes.

Q2. A physical change is usually

(A) Reversible (B) Irreversible (C) Explosive (D) Permanent

Answer: A. Physical changes can normally be reversed (e.g., water ⇌ ice).

Concept

An element is made of one kind of atom; a compound is two or more elements chemically combined in a fixed ratio; a mixture is a physical combination in any ratio that keeps the components' properties.

Compounds have fixed properties; mixtures do not.

Key Points & Formulas

• Element: e.g., oxygen, gold, hydrogen.
• Compound: e.g., water (H₂O), common salt (NaCl) — fixed ratio, new properties.
• Mixture: e.g., air, soil, brass — variable ratio, separable physically.
• Mixtures are separated by filtration, distillation, evaporation, etc.

Worked Example

Q. Classify water, air and oxygen as element/compound/mixture.

Oxygen — element (single kind of atom).

Water — compound (H and O combined in a fixed ratio).

Air — mixture (several gases, variable proportions).

Practice MCQs

Q1. Which of the following is a compound?

(A) Common salt (NaCl) (B) Air (C) Brass (D) Oxygen

Answer: A. NaCl is a compound; air and brass are mixtures and oxygen is an element.

Q2. Brass is an example of a

(A) Mixture (alloy) (B) Compound (C) Element (D) Pure metal

Answer: A. Brass is an alloy (mixture) of copper and zinc.

Concept

Air is a mixture of gases (mainly nitrogen and oxygen); water is a compound essential to life. Their composition and properties are basic but frequently tested.

Water shows an anomalous expansion below 4 °C.

Key Points & Formulas

• Air ≈ 78% nitrogen, 21% oxygen, plus CO₂, argon and water vapour.
• Oxygen supports combustion and respiration; nitrogen is inert and dilutes oxygen.
• Water (H₂O) is a universal solvent; its density is maximum at 4 °C.
• Hard water contains calcium and magnesium salts.

Worked Example

Q. Why does aquatic life survive in frozen lakes during winter?

Water is densest at 4 °C, so the 4 °C layer sinks to the bottom.

Ice (less dense) floats on top and insulates the water below.

Water beneath stays liquid, allowing aquatic life to survive.

Practice MCQs

Q1. The most abundant gas in air is

(A) Nitrogen (B) Oxygen (C) Carbon dioxide (D) Argon

Answer: A. Nitrogen makes up about 78% of the atmosphere.

Q2. Water has its maximum density at

(A) 4 °C (B) 0 °C (C) 100 °C (D) 25 °C

Answer: A. Water is densest at 4 °C — its anomalous expansion.

Concept

A chemical equation represents a reaction using symbols and formulae and must be balanced to obey the law of conservation of mass. The laws of chemical combination govern how elements combine.

Balancing ensures equal atoms of each element on both sides.

Key Points & Formulas

• Law of conservation of mass: total mass of reactants = total mass of products.
• Law of constant proportions: a compound always has the same elements in a fixed ratio by mass.
• Balance by adjusting coefficients, never subscripts.
• States: (s) solid, (l) liquid, (g) gas, (aq) aqueous.

Worked Example

Q. Balance: H₂ + O₂ → H₂O.

Oxygen is unbalanced (2 left, 1 right).

Balance: 2H₂ + O₂ → 2H₂O.

Now H: 4=4 and O: 2=2. Balanced.

Practice MCQs

Q1. Balancing equations follows the law of

(A) Conservation of mass (B) Gravitation (C) Constant motion (D) Reflection

Answer: A. Atoms are neither created nor destroyed, so mass is conserved.

Q2. In a balanced equation, what must be equal on both sides?

(A) Number of atoms of each element (B) Number of molecules (C) Volume (D) Temperature

Answer: A. Each element must have equal atoms on both sides.

Concept

Acids taste sour and turn blue litmus red; bases taste bitter, feel soapy and turn red litmus blue. A salt is formed when an acid reacts with a base (neutralisation).

The pH scale (0–14) measures acidity/alkalinity.

Key Points & Formulas

• Acid + Base → Salt + Water (neutralisation).
• pH < 7 acidic, pH = 7 neutral, pH > 7 basic.
• Litmus: acids turn blue→red; bases turn red→blue.
• Common salt is NaCl; baking soda is NaHCO₃.

Worked Example

Q. What is formed when hydrochloric acid reacts with sodium hydroxide?

This is a neutralisation reaction.

HCl + NaOH → NaCl + H₂O.

Products are common salt (NaCl) and water.

Practice MCQs

Q1. The pH of a neutral solution is

(A) 7 (B) 0 (C) 14 (D) 1

Answer: A. pH 7 is neutral on the 0–14 scale.

Q2. Acids turn blue litmus

(A) Red (B) Green (C) Yellow (D) Colourless

Answer: A. Acids turn blue litmus red; bases do the reverse.

Concept

Oxygen and hydrogen are key non-metals with distinctive preparations and tests. Oxidation is the gain of oxygen / loss of electrons; reduction is the reverse. Together they form redox reactions.

Use the memory aid OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).

Key Points & Formulas

• Oxygen supports combustion; relights a glowing splint.
• Hydrogen burns with a 'pop' sound; it is the lightest gas.
• Oxidation: gain of oxygen / loss of hydrogen or electrons.
• Reduction: loss of oxygen / gain of hydrogen or electrons.

Worked Example

Q. In the reaction 2Mg + O₂ → 2MgO, what is oxidised?

Magnesium gains oxygen to form MgO.

Gain of oxygen = oxidation.

So magnesium is oxidised (and oxygen is reduced).

Practice MCQs

Q1. The gas that burns with a 'pop' sound is

(A) Hydrogen (B) Oxygen (C) Nitrogen (D) Carbon dioxide

Answer: A. Hydrogen gives the characteristic 'pop' test.

Q2. Loss of electrons in a reaction is called

(A) Oxidation (B) Reduction (C) Neutralisation (D) Condensation

Answer: A. OIL RIG: Oxidation Is Loss of electrons.

Concept

Carbon is a versatile element forming millions of compounds. Its allotropes (different structural forms of the same element) include diamond, graphite and fullerenes, each with very different properties.

Allotropy is the same element existing in different physical forms.

Key Points & Formulas

• Diamond: hardest natural substance; each carbon bonded to four others.
• Graphite: soft, slippery, conducts electricity; used in pencils and as a lubricant.
• Both diamond and graphite are pure carbon — allotropes.
• Carbon forms covalent compounds (organic chemistry).

Worked Example

Q. Graphite conducts electricity but diamond does not. Why?

In graphite each carbon bonds to only three others, leaving free electrons.

These free electrons carry current.

In diamond all four electrons are bonded, leaving none free — so it does not conduct.

Practice MCQs

Q1. Diamond and graphite are

(A) Allotropes of carbon (B) Compounds of carbon (C) Different elements (D) Mixtures

Answer: A. They are different structural forms (allotropes) of the same element, carbon.

Q2. The hardest known natural substance is

(A) Diamond (B) Graphite (C) Quartz (D) Iron

Answer: A. Diamond, an allotrope of carbon, is the hardest natural material.

Concept

An atom has a central nucleus of protons and neutrons, surrounded by electrons in shells. The atomic number is the number of protons; the mass number is protons plus neutrons.

Electrons fill shells in order, determining chemical behaviour (valency).

Key Points & Formulas

• Proton: +1 charge; neutron: neutral; electron: −1 charge.
• Atomic number (Z) = number of protons; mass number (A) = protons + neutrons.
• Isotopes: same Z, different mass number.
• Valency relates to electrons in the outermost shell.

Worked Example

Q. An atom has atomic number 11 and mass number 23. Find its protons, electrons and neutrons.

Protons = atomic number = 11.

Electrons (neutral atom) = 11.

Neutrons = mass − protons = 23 − 11 = 12.

Practice MCQs

Q1. The particle with no electric charge is the

(A) Neutron (B) Proton (C) Electron (D) Ion

Answer: A. Neutrons are electrically neutral.

Q2. Atoms of the same element with different mass numbers are called

(A) Isotopes (B) Ions (C) Isobars (D) Molecules

Answer: A. Isotopes share atomic number but differ in neutrons (mass number).

Concept

Fertilizers supply nutrients — mainly nitrogen, phosphorus and potassium (NPK) — to improve soil fertility and crop yield. They may be natural (manure) or artificial (chemical).

Nitrogenous fertilizers like urea are widely used.

Key Points & Formulas

• Primary nutrients: Nitrogen (N), Phosphorus (P), Potassium (K).
• Urea is a common nitrogenous fertilizer (high N content).
• Natural fertilizers: compost, farmyard manure; improve soil structure.
• Over-use of chemical fertilizers can harm soil and water.

Worked Example

Q. Which nutrient does urea mainly supply to crops?

Urea, CO(NH₂)₂, is rich in nitrogen.

It mainly supplies nitrogen, essential for leafy growth.

Practice MCQs

Q1. The three primary nutrients supplied by fertilizers are

(A) N, P, K (B) C, H, O (C) Fe, Cu, Zn (D) Na, Cl, Ca

Answer: A. Nitrogen, phosphorus and potassium (NPK) are the primary nutrients.

Q2. Urea is chiefly a source of

(A) Nitrogen (B) Phosphorus (C) Potassium (D) Calcium

Answer: A. Urea is a nitrogenous fertilizer.

Concept

The syllabus covers the everyday chemistry behind common manufactured materials — soap, glass, cement, paper, ink, paints, safety matches and gunpowder — and the raw materials used.

Knowing one key raw material or use for each is usually enough.

Key Points & Formulas

• Soap: made by saponification of fats/oils with an alkali (NaOH).
• Glass: mainly silica (sand), soda and limestone.
• Cement: limestone and clay (calcium silicates).
• Gunpowder: potassium nitrate, charcoal and sulphur.

Worked Example

Q. Name the chemical process by which soap is made and its raw materials.

Soap is made by saponification.

Fats or oils are boiled with an alkali (sodium hydroxide).

This yields soap and glycerol.

Practice MCQs

Q1. The main raw material in the manufacture of glass is

(A) Silica (sand) (B) Limestone only (C) Gypsum (D) Bauxite

Answer: A. Glass is made chiefly from silica (sand) with soda and limestone.

Q2. Gunpowder is a mixture of charcoal, sulphur and

(A) Potassium nitrate (B) Sodium chloride (C) Calcium carbonate (D) Silica

Answer: A. Potassium nitrate (saltpetre) is the oxidiser in gunpowder.