Measurement and Scientific Investigation
Introduction
General Science in the Kenyan Senior School CBC (KICD) integrates biology, chemistry and physics. All three rely on careful observation, measurement and investigation, which are the focus of this first topic.
The scientific method
- Observe a phenomenon.
- Ask a question, e.g. "Does fertiliser increase maize height?"
- Form a hypothesis - a testable prediction.
- Design and carry out an experiment.
- Record data in tables.
- Analyse results using graphs or calculations.
- Conclude whether the hypothesis is supported.
- Report findings.
Variables
- Independent variable - the factor you change.
- Dependent variable - the factor you measure.
- Controlled (constant) variables - factors kept the same for a fair test.
- A control experiment gives a baseline for comparison.
SI units and quantities
| Quantity | Unit | Symbol |
|---|---|---|
| Length | metre | m |
| Mass | kilogram | kg |
| Time | second | s |
| Temperature | kelvin | K |
| Volume | cubic metre | m3 |
Common conversions: 1 km = 1000 m; 1 m = 100 cm; 1 kg = 1000 g; 1 litre = 1000 cm3; 1 hour = 3600 s.
Measuring instruments
- Metre rule, tape measure - length
- Vernier callipers - small lengths to 0.1 mm
- Beam or electronic balance - mass
- Measuring cylinder, burette, pipette - volume
- Stopwatch - time
- Thermometer - temperature
Accuracy, precision and errors
- Accuracy - closeness to the true value.
- Precision - closeness of repeated readings.
- Parallax error occurs when the eye is not level with the scale; read at eye level.
- Zero error occurs when an instrument does not read zero at the start. Repeat readings and take the average to reduce error.
Laboratory safety
Wear a lab coat and goggles, tie hair back, never taste chemicals, handle heat carefully, label containers, know where the fire extinguisher and first aid kit are, and wash hands afterward.
Worked example 1
Convert 2.5 km to metres. Answer: 2.5 x 1000 = 2500 m.
Worked example 2
Three readings of a pendulum's 20 swings are 32.1 s, 32.3 s and 32.2 s. Find the average and the time for one swing. Answer: Average = (32.1 + 32.3 + 32.2) / 3 = 32.2 s. One swing = 32.2 / 20 = 1.61 s.
Try it
- Measure the length of your desk in cm and convert to metres.
- Design a fair test for how salt affects boiling point of water, naming all variables.
- Explain how to avoid parallax error when reading a measuring cylinder.
- Convert 450 g to kg.