Updated: 19 October 2026
Practical preparation includes reading a table, selecting a relationship and distinguishing an observation from an inference. Work through this original data-only exercise. No live chemical procedure is required, and the material is not an official Paper 3 or confidential instruction.
On this page: original practice, explained answers and 20 short FAQs. Complete the tasks before reading the answers.
Check readings before calculating
Write the measured quantity and unit next to each value. For a burette, a titre is the final reading minus the initial reading. Compare repeated values before choosing the measurements to use. Follow the instructions of the actual paper for recording precision and acceptable results.
Use the balanced equation
A volume alone does not determine concentration. Identify the reacting substances, balance their equation and use the mole ratio. Convert cubic centimetres to cubic decimetres when concentration is expressed in mol/dm³.
Keep observations separate
“A white solid formed” records visible evidence. Naming an ion is an inference that depends on the complete test and its context. Avoid writing a conclusion in place of the requested observation.
Practice questions and explained answers
1. A burette reads 0.20 cm³ initially and 24.80 cm³ finally. Find the titre.
Show suggested answer
Titre = 24.80 − 0.20 = 24.60 cm³.
2. Three titres are 24.60, 24.65 and 26.10 cm³. The task directs you to use the closest pair. Find their mean.
Show suggested answer
The closest pair is 24.60 and 24.65. Mean = 24.625 cm³, or 24.63 cm³ to two decimal places. Do not average the outlier into this specified pair.
3. Calculate moles in 25.0 cm³ of 0.100 mol/dm³ hydrochloric acid.
Show suggested answer
Volume = 25.0/1,000 = 0.0250 dm³. Moles = cV = 0.100 × 0.0250 = 0.00250 mol.
4. HCl + NaOH → NaCl + H₂O. If 0.00250 mol HCl reacts with 20.0 cm³ NaOH, find NaOH concentration.
Show suggested answer
The ratio is 1:1, so NaOH amount = 0.00250 mol. Volume = 0.0200 dm³. Concentration = 0.00250/0.0200 = 0.125 mol/dm³.
5. A solution cools from 35°C to 22°C. State the temperature change with its sign.
Show suggested answer
Final minus initial = 22 − 35 = −13°C, a decrease of 13°C.
6. A salt has solubility 60 g per 100 g water when hot and 25 g per 100 g water when cool. Find the mass crystallised from a saturated solution containing 100 g water, assuming no water loss.
Show suggested answer
Mass crystallised = 60 − 25 = 35 g.
7. Why is a gas test result alone insufficient to identify an unknown mixture completely?
Show suggested answer
One test supports a limited inference. Other substances may be present and further appropriate evidence is required.
8. Correct the answer “the solid disappeared so a new element was formed.”
Show suggested answer
The disappearance is an observation. It may reflect dissolving or reaction; it does not by itself prove formation of a new element.
9. Why should readings be recorded immediately?
Show suggested answer
Immediate recording reduces reliance on memory and helps retain the actual results rather than expected values.
10. What is wrong with altering a reading to fit a familiar value?
Show suggested answer
It replaces evidence with an invented result and undermines the interpretation.
Frequently asked questions
Is a titre a final burette reading?
No. It is the difference between the final and initial readings.
Should I always average every trial?
Follow the task and assess consistency; the supplied question specifies the closest pair.
Why divide cm³ by 1,000?
There are 1,000 cubic centimetres in one cubic decimetre.
Can every acid-base reaction use a 1:1 ratio?
No. Use the balanced equation for the substances involved.
Is a temperature decrease always evidence of one particular reaction?
No. The experimental context and other evidence matter.
Does the solubility answer assume constant water mass?
Yes. Water loss would change the calculation.
What is an inference?
An interpretation supported by observations and a suitable test.
Can I perform these tests at home?
This is a data exercise. Laboratory work needs approved supervision and procedures.
Do these examples reproduce the current practical?
No. They are original revision examples.
Where do I find real past practical papers?
Use the existing Chemistry subject archive and check the year and paper number.
Why preserve significant figures?
They communicate the precision justified by the given measurements and instructions.
What is the difference between amount and concentration?
Amount is measured in moles; concentration relates amount to solution volume.
Does the neutralisation equation need balancing?
Yes. Mole ratios come from the balanced equation.
Can a negative temperature change be reported as a rise?
No. The sign and description must agree.
Why state the no-water-loss assumption?
Evaporation would alter the mass of water available and change the crystallisation calculation.
Are colour changes always conclusive?
No. Interpret them in the context of the appropriate test.
What should a results table include?
Clear quantity labels, units and the actual readings.
How can I identify an arithmetic slip?
Estimate the result and check unit conversions before repeating all the working.
Should I replace an unexpected observation?
No. Record it honestly and review the method as instructed.
Can this page supply confidential instructions?
No. It contains only original public revision tasks.
Related revision resources
- KCSE Chemistry Past Papers by Year and Paper
- Titration Calculations: Worked Examples and Answers
- How to solve stoichiometry problems in KCSE Chemistry
- KCSE Mathematics Paper 1: Mixed Worked Revision Check
- KCSE Biology Practical: Tables, Drawings and Conclusions
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For official assessment notices and examination instructions, consult KNEC. These practice answers are not official scoring guides.
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