Last reviewed: 1 November 2026
Updated 1 November 2026. Searching for KCSE 2025 Chemistry Paper 1 answers the day before your own paper? Here are original practice questions written in the style of the 2025 Chemistry Paper 1 (233/1), each with full working, so you can check method as well as the final answer. They are not the KNEC paper and this is not the KNEC marking scheme. The 2026 Chemistry Paper 1 is on Monday 2 November, 2.00 to 4.00 pm, per the KNEC 2026 KCSE Examination Timetable.
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How Chemistry Paper 1 (233/1) Is Structured
Chemistry Paper 1 is a two-hour theory paper of short structured questions, all compulsory, marked out of 80. A typical paper has around 25 to 30 questions worth two to four marks each, drawn from all four years of the syllabus. Paper 2 (233/2) follows on Wednesday 4 November and the practical, 233/3, on Friday 6 November. A non-programmable calculator is allowed, and you may use only KNEC-printed mathematical tables.
The 2025 cohort did better overall than 2024: 27.18% of candidates reached C+ and above, up from 25.53%, according to the Kenya News Agency report on the KCSE 2025 results release. Chemistry was also among the subjects in which male candidates recorded the higher mean score, as reported by Citizen Digital.
KCSE 2025 Chemistry Paper 1 Answers: Practice Questions With Working
1. Element Q has atomic number 17. Write its electron arrangement and state its group and period. (2 marks)
Answer: 2.8.7; Group VII (17), Period 3. Seven outer electrons give the group; three energy levels give the period.
2. Explain why sodium chloride conducts electricity when molten but not when solid. (2 marks)
Answer: In the solid the ions are held in fixed positions in the lattice and cannot move; when molten the ions are mobile and carry charge.
3. Calculate the mass of calcium oxide formed when 25 g of calcium carbonate is completely decomposed. (Ca = 40, C = 12, O = 16) (3 marks)
Answer: CaCO₃ → CaO + CO₂. Molar mass CaCO₃ = 100 g; moles = 25 ÷ 100 = 0.25 mol. Mole ratio 1 : 1, so CaO = 0.25 mol × 56 = 14 g. Marks: equation or ratio, moles, final mass with unit.
4. 20.0 cm³ of 0.1 M sodium hydroxide neutralised 25.0 cm³ of hydrochloric acid. Find the molarity of the acid. (3 marks)
Answer: NaOH + HCl → NaCl + H₂O. Moles NaOH = 0.1 × 20 ÷ 1000 = 0.002 mol = moles HCl. Molarity HCl = 0.002 × 1000 ÷ 25 = 0.08 M.
5. Name the method used to separate a mixture of ethanol and water and state the property it relies on. (2 marks)
Answer: Fractional distillation; the two liquids are miscible but have different boiling points (ethanol 78 °C, water 100 °C).
6. A gas occupies 200 cm³ at 27 °C and 1 atm. Find its volume at 127 °C at the same pressure. (2 marks)
Answer: Charles’ law: V₂ = V₁ × T₂ ÷ T₁ = 200 × 400 ÷ 300 = 266.7 cm³. Temperatures must be in kelvin; using °C loses both marks.
7. Draw the structure and name the first member of the alkene series. (2 marks)
Answer: Ethene, C₂H₄, with a carbon-carbon double bond: H₂C=CH₂. The double bond must be shown.
8. State two observations made when a piece of sodium is dropped into water containing phenolphthalein. (2 marks)
Answer: It floats and darts about, melting into a silvery ball; effervescence; the solution turns pink because sodium hydroxide, an alkali, forms. Any two.
9. Explain why aluminium pans do not corrode easily even though aluminium is a reactive metal. (2 marks)
Answer: Aluminium reacts with oxygen to form a thin, non-porous layer of aluminium oxide that sticks to the surface and prevents further reaction.
10. Give one use of each: (a) sulphur(IV) oxide, (b) helium. (2 marks)
Answer: (a) Bleaching agent or food preservative; (b) filling weather balloons and airships because it is light and unreactive.
Common Ways Candidates Lose Chemistry Paper 1 Marks
- Unbalanced equations or missing state symbols where the question asks for them.
- Rounding too early in mole calculations, so the final figure drifts.
- Writing observations as explanations (“hydrogen is produced” is not an observation; “effervescence” is).
- Using °C instead of K in gas-law questions.
Last-Day Plan Before 233/1
On the morning of the paper, redo five mole calculations and five gas-law problems, then read through the reactivity series, the first five members of each homologous series, and the tests for common gases and ions. Sleep the night before matters more than one more hour of reading. KNEC requires you to be seated 15 minutes before the start, and phones in the exam centre lead to cancelled results.
Likely Topics for Chemistry Paper 1 in 2026
Teachers’ predictions usually put the mole, the periodic table, bonding, acids, bases and salts, and organic chemistry I at the top, because almost every past Paper 1 has tested them. Use any prediction list only as a checklist.
Keep Practising Chemistry
For topic summaries use the KCSE Chemistry Paper 1 revision guide; for speed, the timed Chemistry Paper 1 challenge and the KCSE Chemistry quiz. More questions sit on the Chemistry Paper 1 practice papers page, and Paper 2 practice is on the Chemistry Paper 2 questions and answers page. Everything else is on the KCSE hub and KCSE quizzes.
Frequently Asked Questions
Where can I find the KCSE 2025 Chemistry Paper 1 marking scheme?
KNEC does not publish its marking schemes for sale, and we do not host them. Use original questions with worked answers like the ones on this page.
How many marks is Chemistry Paper 1?
80 marks, all questions compulsory, in 2 hours.
When is KCSE Chemistry Paper 1 in 2026?
Monday 2 November 2026, 2.00 pm to 4.00 pm.
Are Chemistry Paper 1 papers sold online before the exam real?
No. They are scams, and possessing exam material is an irregularity that can lead to cancelled results.
One more timed paper before tomorrow? Start free, then a Daily Pass (KSh 20) opens every KCSE practice paper with worked answers for 24 hours, or take a Monthly Pass (KSh 500) to cover Papers 2 and 3. Paid once by M-Pesa. All passes.