Subarea I - Nature of Science and Measurement
Subarea I tests how well you can design, execute, and interpret chemistry investigations: SI measurement, significant figures, uncertainty, data analysis, and safety. The exam favors reasoning over memorized steps, so expect to decide which quantity limits precision and whether an error is random or systematic.
Measurement and units
Use SI base units: kilogram, meter, second, kelvin, mole, ampere, candela. Convert with dimensional analysis so units cancel. Temperature conversion: K = °C + 273.15. Density is a derived unit (g/mL or g/cm³). Percent error = |experimental − accepted| / accepted × 100%.
Significant figures and uncertainty
For multiplication/division, the result has the same number of sig figs as the least precise factor. For addition/subtraction, keep the fewest decimal places. Exact counts and definitions (12 in 1 dozen, 100 cm in 1 m) do not limit sig figs. Accuracy is closeness to true value; precision is reproducibility. Random error scatters results; systematic error shifts all results in one direction, such as an uncalibrated balance.
Experimental design
Identify independent, dependent, and controlled variables. A control group isolates the treatment. Replication reduces random error; calibration reduces systematic error. Use a graph with labeled axes, units, and a best-fit line; slope often carries the physical meaning.
Worked example
A sample has mass 12.34 g and volume 6.2 mL. Density = 12.34 g / 6.2 mL = 1.9903... g/mL. The volume has two significant figures, so report 2.0 g/mL. Reasoning: division limits sig figs to the least precise measurement, and rounding 1.99 to two sig figs gives 2.0.
Traps
Do not round intermediate steps too early. Do not confuse precision with accuracy. Do not use Celsius in gas-law or thermodynamics formulas that require kelvin. Do not report more decimal places than the instrument supports.