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Flashcards · J247 · Topic 7

Practical skills flashcards

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Topic 7: Practical skills

Accuracy vs precision
Accuracy: closeness to the true value. Precision: closeness of repeat readings to each other.
Repeatability vs reproducibility
Repeatable: the same person, method and equipment give the same results. Reproducible: a different person, method or equipment give the same results.
Random vs systematic error
Random error: unpredictable scatter, reduced by repeats and a mean. Systematic error: the same shift in every reading; repeats do not remove it.
Hypothesis vs prediction
A hypothesis is a testable explanation built from scientific theory. A prediction is what you expect to observe if the hypothesis is right.
Repeats read 5.1, 5.4, 5.2 and 5.3. Mean and uncertainty?
Mean = 21.0 ÷ 4 = 5.25. Range = 0.3, half-range 0.15, so the result is 5.25 ± 0.15.
Independent, dependent and control variables
Independent: the one you change. Dependent: the one you measure. Control: kept the same so the test is fair.
Continuous sampling with amylase: the end point is at 50 s. Rate?
Test drops of the mixture with iodine at set intervals; the end point is the first sample that no longer turns blue-black. Rate = 1 ÷ time = 1 ÷ 50 = 0.02 s⁻¹.
A pulse of 21 beats is counted in 15 s. Heart rate?
21 × 4 = 84 beats per minute. Recovery time is the time to return to the resting rate after exercise.
Bacterial lawn practical: safe technique, and the area of a clear zone 14 mm across
Sterilise dishes and agar; work near a Bunsen flame; flame the loop; tape the lid with short strips; incubate at no more than about 25 °C. Area = πr² = π × 7² = 154 mm².
Making 100 cm³ of 0.3 M sucrose solution from a 1.0 M stock
30 cm³ of stock + 70 cm³ of water (0.3 × 100 ÷ 1.0 = 30 cm³ of stock).
A mean of 2.4 plants per 0.25 m² quadrat. How many in 50 m²?
2.4 ÷ 0.25 = 9.6 per m². 9.6 × 50 = 480 plants (an estimate).
Rules for drawing a results graph
Independent variable on the x-axis; axes labelled with quantity and unit; even scales using more than half the grid; points plotted exactly; a line of best fit, not dot to dot.

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