Practical Skills
Practical Skills tests your ability to use apparatus, make observations, record data and interpret experimental results. STPM practical assessment includes microscopy, dissection, chromatography, colorimetry, aseptic technique, and accurate drawing and labelling.
Core rules
- Calibrate the eyepiece graticule with a stage micrometer before measuring specimens; calculate the value of one eyepiece division in µm.
- Magnification = image size ÷ actual size; resolution is the ability to distinguish two close points, not the same as magnification.
- Biological drawings use clear continuous lines, no shading, correct proportions, and labels with straight, non-crossing leader lines.
- Chromatography separates pigments by differential solubility and adsorption; Rf = distance moved by solute ÷ distance moved by solvent.
- Colorimetry uses a calibration curve of absorbance against known concentration; measure within the linear range.
- Aseptic technique prevents contamination: sterile equipment, flame the neck, work near a Bunsen flame.
Worked example
A stage micrometer has 1 mm divided into 100 divisions, so each division is 10 µm. If 50 eyepiece divisions align with 20 stage-micrometer divisions, then 50 eyepiece divisions = 20 × 10 µm = 200 µm, so one eyepiece division = 4 µm. Reasoning: calibrate first, then use the calibrated value to convert specimen measurements into real sizes.
Traps
- Do not confuse magnification with resolution.
- Convert mm to µm by multiplying by 1,000.
- Do not shade drawings or use arrowheads instead of clear label lines.