CBSE Class 10 Science — Complete Board Exam Guide

Chapter Overview & Weightage

CBSE Class 10 Science is an 80-mark theory paper with a well-defined unit structure. The paper follows a predictable pattern — once you know the weightage, you can allocate your revision time ruthlessly.

Official Unit-Wise Weightage (2024-25 Syllabus)

Unit	Topics	Marks
Unit I	Chemical Substances (Ch. 1–4)	25
Unit II	World of Living (Ch. 5–8)	25
Unit III	Natural Phenomena (Ch. 9–10)	12
Unit IV	Effects of Current (Ch. 11–12)	13
Unit V	Natural Resources (Ch. 13)	05
Total		80

Chemistry and Biology each carry 25 marks — together that’s 62.5% of your paper. Never neglect either.

The question paper has MCQs (1 mark), short answer (2–3 marks), case-based questions (4 marks), and long answers (5 marks). Since 2022, case-based questions (application-based reading passages) have become a permanent fixture — expect 2–3 such sets every year.

Key Concepts You Must Know

Unit I: Chemical Substances (25 Marks)

Chapter 1 — Chemical Reactions and Equations

Balancing equations: you will always get one 2-mark balancing question
Types of chemical reactions: combination, decomposition, displacement, double displacement, oxidation-reduction
Corrosion and rancidity — 1-mark definitions appear frequently
Identifying oxidising and reducing agents from an equation

Chapter 2 — Acids, Bases and Salts

pH scale and indicators (litmus, phenolphthalein, methyl orange)
Properties of acids and bases — reaction with metals, metal carbonates, metal oxides
Preparation and properties of NaOH (chlor-alkali process), baking soda, washing soda, bleaching powder
Strength vs. concentration distinction — examiners love to trip students here

Chapter 3 — Metals and Non-metals

Activity series and its applications (displacement reactions, extraction)
Ionic bond formation with electron dot structures
Occurrence and extraction of metals: enrichment → reduction → refining
Corrosion: conditions, prevention methods

Chapter 4 — Carbon and its Compounds

Covalent bonding in carbon: why carbon forms long chains (catenation, tetravalency)
Homologous series — IUPAC naming up to 5 carbons
Properties of ethanol and ethanoic acid
Soaps vs. detergents and micelle formation — appears in 5-mark questions

Unit II: World of Living (25 Marks)

Chapter 5 — Life Processes

Nutrition: autotrophic (photosynthesis equation) and heterotrophic
Human digestive system: enzymes, their sources, substrates, products
Respiration: aerobic vs. anaerobic, energy released (ATP)
Double circulation in humans — why it exists
Excretion: nephron structure and ultrafiltration

Chapter 6 — Control and Coordination

Nervous system: neuron structure, types of neurons, reflex arc
Brain: cerebrum, cerebellum, medulla — their functions
Endocrine glands: hormones, deficiency diseases (must memorise the table)
Plant hormones: auxin, gibberellin, cytokinin, abscisic acid, ethylene — and what each does

Chapter 7 — How Do Organisms Reproduce?

Asexual reproduction: binary fission (Amoeba, Leishmania), budding, spore formation, fragmentation, vegetative propagation
Sexual reproduction in flowering plants: pollination, fertilisation, fruit/seed formation
Human reproductive system: male and female — diagram + labelling is guaranteed
Contraceptive methods and their categories

Chapter 8 — Heredity

Mendel’s laws: dominance, segregation, independent assortment
Monohybrid and dihybrid crosses — always practice the Punnett square
Sex determination in humans: XX/XY mechanism
Evolution: acquired vs. inherited traits, speciation

Unit III: Natural Phenomena (12 Marks)

Chapter 9 — Light: Reflection and Refraction

Mirror formula, magnification formula — practise 5+ numericals each
Refraction laws, refractive index, and Snell’s law
Lens formula (thin lens), power of lens (P = 1/f in metres)
Real vs. virtual image rules — draw ray diagrams for every case

Chapter 10 — Human Eye and Colourful World

Defects of vision: myopia, hypermetropia, presbyopia — causes, correction, ray diagrams
Refraction through a glass prism: dispersion
Atmospheric refraction: twinkling of stars, colour of sun at sunrise/sunset
Scattering: Rayleigh scattering and why sky is blue

Unit IV: Effects of Current (13 Marks)

Chapter 11 — Electricity

Ohm’s law, resistance, resistivity
Series and parallel combinations — deriving equivalent resistance
Heating effect: Joule’s law, electric power (P = VI = I²R = V²/R)
Commercial unit of energy: 1 kWh = 3.6 × 10⁶ J

Chapter 12 — Magnetic Effects of Electric Current

Magnetic field of a straight conductor, solenoid
Fleming’s Left-Hand Rule (motor) and Right-Hand Rule (generator)
AC vs. DC: frequency, advantages
Domestic wiring: live, neutral, earth; fuse and MCB

Unit V: Natural Resources (5 Marks)

Chapter 13 — Our Environment

Food chains, food webs, trophic levels
10% energy flow law
Biodegradable vs. non-biodegradable waste
Ozone layer depletion: CFCs, UV radiation effects

Important Formulas

\frac{1}{v} + \frac{1}{u} = \frac{1}{f}

Magnification: $m = -\frac{v}{u} = \frac{h'}{h}$

Use sign convention: distances measured from pole; direction of incident light is positive.

\frac{1}{v} - \frac{1}{u} = \frac{1}{f}

P = \frac{1}{f(\text{in metres})} \quad \text{(unit: Dioptre, D)}

For a combination of lenses in contact: $P = P_1 + P_2 + P_3$

V = IR \qquad R = \rho\frac{l}{A}

Series: $R_s = R_1 + R_2 + R_3$

Parallel: $\dfrac{1}{R_p} = \dfrac{1}{R_1} + \dfrac{1}{R_2} + \dfrac{1}{R_3}$

P = VI = I^2R = \frac{V^2}{R}

E = Pt \quad \text{(joules)} \qquad 1 \text{ kWh} = 3.6 \times 10^6 \text{ J}

n = \frac{\sin i}{\sin r} = \frac{c}{v} = \frac{\lambda_1}{\lambda_2}

Also: $n_{21} = \frac{n_2}{n_1}$ — the refractive index of medium 2 with respect to medium 1.

Solved Previous Year Questions

PYQ 1 — Chemistry (CBSE Board 2023, 3 marks)

Q: An element A burns with a golden-yellow flame, reacts with water vigorously to form a strongly alkaline solution and a gas which burns with a ‘pop’ sound. Identify element A and write balanced equations for its reactions with (i) oxygen and (ii) cold water.

Solution:

Element A is Sodium (Na). The golden-yellow flame, vigorous reaction with water, and formation of a strongly alkaline solution (NaOH) are all characteristic properties of sodium.

(i) Reaction with oxygen:

4\text{Na} + \text{O}_2 \rightarrow 2\text{Na}_2\text{O}

(ii) Reaction with cold water:

2\text{Na} + 2\text{H}_2\text{O} \rightarrow 2\text{NaOH} + \text{H}_2\uparrow

The gas produced is hydrogen (H₂), which burns with a ‘pop’ sound — this is the standard test for hydrogen.

When you see “golden-yellow flame + reacts with water”, think sodium immediately. Similarly, “brick-red flame” → calcium, “lilac flame” → potassium. These identifiers appear as 1-mark fillers regularly.

PYQ 2 — Physics Numerical (CBSE Board 2024, 3 marks)

Q: An electric iron consumes energy at a rate of 840 W when heating is at maximum and 360 W when heating is at minimum. The voltage is 220 V. Calculate the current and resistance in each case.

Solution:

We use $P = VI$ → $I = P/V$ and $R = V/I$ (or $R = V^2/P$ ).

At maximum heating (P = 840 W):

I_1 = \frac{P_1}{V} = \frac{840}{220} \approx 3.82 \text{ A}

$R_1 = \frac{V}{I_1} = \frac{220}{3.82} \approx 57.6 \text{ }\Omega$

At minimum heating (P = 360 W):

I_2 = \frac{P_2}{V} = \frac{360}{220} \approx 1.64 \text{ A}

$R_2 = \frac{V}{I_2} = \frac{220}{1.64} \approx 134.1 \text{ }\Omega$

Notice: higher resistance = lower current = less power = less heating. The heating coil has variable resistance to control temperature.

PYQ 3 — Biology (CBSE Board 2023, 5 marks)

Q: Draw a neat labelled diagram of the human excretory system. State the composition of urine and explain the role of the kidney in maintaining blood composition.

Solution:

The human excretory system consists of two kidneys, two ureters, one urinary bladder, and the urethra.

Composition of urine: Water (95%), urea (~2%), uric acid, creatinine, salts (NaCl, KCl), and trace amounts of bile pigments.

Role of kidney in maintaining blood composition:

The kidney performs three key processes:

Ultrafiltration — Blood enters the glomerulus under high pressure. Water, glucose, amino acids, urea, salts filter into the Bowman’s capsule as glomerular filtrate.
Selective reabsorption — As filtrate passes through the tubule, useful substances (glucose, amino acids, water, needed ions) are reabsorbed back into blood capillaries.
Tubular secretion — Additional waste products (H⁺ ions, potassium) are actively secreted into the filtrate from blood.

The kidney maintains blood osmolarity, pH, and removes nitrogen waste — making it the body’s primary homeostatic organ.

A 5-mark Biology question almost always has a diagram (2 marks) + explanation (3 marks). Practice drawing the nephron, human eye, neuron, flower parts, and binary fission — these are the six most-tested diagrams.

Difficulty Distribution

Chapter	Easy	Medium	Hard	Typical Marks
Chemical Reactions & Equations	60%	30%	10%	6–8
Acids, Bases & Salts	40%	40%	20%	7–9
Metals & Non-metals	40%	40%	20%	7–9
Carbon & Compounds	30%	50%	20%	7–8
Life Processes	35%	45%	20%	8–10
Control & Coordination	45%	40%	15%	5–7
Reproduction	50%	35%	15%	5–7
Heredity	40%	45%	15%	4–6
Light	25%	45%	30%	7–9
Human Eye	50%	35%	15%	4–5
Electricity	25%	40%	35%	8–10
Magnetic Effects	50%	35%	15%	5–6
Our Environment	70%	25%	5%	3–5

Electricity and Light together form 25 marks. They are the most calculation-heavy chapters. Students who practise 20+ numericals from these two chapters consistently score 20+ in the physics section.

Expert Strategy

Weeks 1–2: Foundation Pass

Work through all 13 chapters once — don’t aim for mastery, aim for familiarity. Read your NCERT textbook properly; CBSE sets 70% of questions directly or near-directly from NCERT text, examples, and exercises. The NCERT in-text questions and end-chapter exercises are non-negotiable.

Weeks 3–4: Formula and Diagram Bank

Build a single sheet with all formulas for Physics and a diagram notebook for Biology. The mirror formula, lens formula, Ohm’s law, Joule’s law, and power formulas must come to you without thinking. For Biology, practise drawing diagrams every day — aim for 5 minutes per diagram, clean labels, arrows pointing precisely.

Week 5: PYQ Sprint

Solve the last 5 years of CBSE board papers under timed conditions (3 hours). Analyse your mistakes — most students find they consistently drop marks on the same type of question (usually “give reasons” questions or equation balancing). Fix those specifically.

CBSE rewards structured answers. For “give reasons” type questions, always write: Statement → Reason → Example in that order. Examiners mark against a marking scheme, so if your reason is correct but buried inside a paragraph, you may lose marks.

Week 6: High-Yield Revision

Focus on these high-yield areas for the final week:

All NCERT diagrams (human eye, nephron, neuron, reproductive systems, reflex arc)
Balancing chemical equations (minimum 10 equations per day)
Electricity numericals — at least 3 per day
One-liners for differences: Mitosis vs. Meiosis, Aerobic vs. Anaerobic, Series vs. Parallel

Common Traps

Mirror vs. Lens Sign Convention: Both use the same sign convention (New Cartesian), but the formulas are different. Mirror: $1/v + 1/u = 1/f$ . Lens: $1/v - 1/u = 1/f$ . Students routinely apply the mirror formula to a lens problem. Always write which formula you’re using at the top of your solution.

“Dilute acid is added to…” questions: When dilute H₂SO₄ or HCl reacts with a metal carbonate, the products are salt + water + CO₂. Students write H₂ instead of CO₂. With metal carbonates, it is never hydrogen gas.

Activity series confusion in displacement reactions: Zinc displaces copper from copper sulphate solution — not the other way around. When asked if a reaction will occur, always check: is the displacing metal higher in the activity series? If yes, reaction occurs. Students often get this backwards under exam pressure.

Resistance in parallel: The equivalent resistance in parallel is always less than the smallest individual resistance. If you calculate a parallel equivalent that is bigger than any component, your calculation is wrong. Use this as a quick sanity check.

Mendel’s ratios: Monohybrid cross gives 3:1 phenotypic ratio, 1:2:1 genotypic ratio. Dihybrid gives 9:3:3:1. The exam often asks for the genotypic ratio of a monohybrid cross, and students write 3:1. They are different things.

“Least deviation” in prism: The minimum deviation position is when the ray passes symmetrically through the prism. Questions that ask “at what condition does minimum deviation occur?” expect this specific answer — many students write a formula instead of the condition.

On the day of the exam, attempt Biology questions first if you find them easier — it settles your nerves. Leave numericals for last only if you are confident; otherwise, do them while your mind is fresh. Show all working for numericals: even a wrong final answer with correct method fetches partial marks.

The science paper at Class 10 is genuinely scoring. Students who are methodical about NCERT + PYQs + diagram practice routinely score 75+. We are aiming for that.