Safety and source boundary
Instruction uses fictional readings, numerical models, and paper-based SDS literacy. No mixing, heating, or handling chemicals is authorized. Use product-specific SDS information and approved supervision for any real laboratory activity.
Explicit instruction
Density describes mass per unit volume: density = mass/volume. Always keep the units attached and report appropriate significant figures based on the precision of the given measured quantities. To compare an experimental value with a trusted reference, percent error = |measured − reference|/|reference| ×100 when the reference is nonzero. Do not confuse percent error against a reference with percent difference between two measurements.
Vocabulary: density; significant figures; dimensional analysis; percent error.
Worked reasoning example
Worked synthetic case: Mass = 24 g and volume = 8.0 mL. Density = (24 g)/(8.0 mL) = 3.0 g/mL, stated to two significant figures. With a hypothetical reference of 2.8 g/mL, percent error = |3.0−2.8|/2.8×100 ≈7.1%. The reference is invented for instruction, not a verified value for any substance. For unit conversion, 25 mL ×(1 L/1000 mL)=0.025 L.
Guided practice
Why do grams cancel neither milliliters nor liters in the density ratio? Explain why density must include units and how the reference value determines the percent-error denominator.
Independent student task
- Compute density for a fictional 18.0 g sample occupying 6.0 mL; include significant figures.
- Convert 125 mL into liters using a conversion factor with explicit canceling units.
- If the hypothetical accepted density is 2.5 g/mL, compute percent error for a measurement of 2.7 g/mL and state the formula.
Misconception correction
Dropping units or reporting excessive decimal places can make a correct arithmetic result scientifically misleading.
Formative concept check
What is 18.0 g divided by 6.0 mL using two significant figures?
Reveal worked response
3.0 g/mL. 18.0/6.0 = 3, and 6.0 has two significant figures; 3.0 records two significant figures.
Self-check only; never an official exam or mastery attempt.
Teacher notes
All data are synthetic; no actual material identification is possible from a density calculation alone. Explicitly distinguish measured precision from actual uncertainty.
Learning record return
Record explanations, calculations, safety decisions, and corrections in the existing Week 01 classroom field. Existing local records and mastery authority remain unchanged.
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