Scientific Measurement and SI Units
Measurement is the foundation of science. A qualitative observation ("the water is hot") tells you little; a quantitative measurement ("the water is 85.3°C") enables calculation, comparison, prediction, and reproducibility. The ability to measure precisely and express results accurately is a core skill in chemistry and every other natural science.
The SI System
The modern international standard for measurement is the International System of Units, abbreviated SI (from the French Système International d'Unités). The SI system, adopted globally in 1960 and periodically updated, provides a coherent framework of seven base units from which all other units are derived.
| Base Quantity | SI Unit | Symbol |
|---|---|---|
| Length | meter | m |
| Mass | kilogram | kg |
| Time | second | s |
| Electric current | ampere | A |
| Temperature | kelvin | K |
| Amount of substance | mole | mol |
| Luminous intensity | candela | cd |
In chemistry, the most frequently used SI units are the meter, kilogram, kelvin, and mole. Non-SI units such as the liter (L) and the degree Celsius (°C) are also widely used with SI approval.
SI Prefixes
SI prefixes multiply a base unit by a power of ten, making it convenient to express very large or very small quantities:
| Prefix | Symbol | Factor |
|---|---|---|
| giga | G | 10⁹ |
| mega | M | 10⁶ |
| kilo | k | 10³ |
| deci | d | 10⁻¹ |
| centi | c | 10⁻² |
| milli | m | 10⁻³ |
| micro | μ | 10⁻⁶ |
| nano | n | 10⁻⁹ |
| pico | p | 10⁻¹² |
Examples: - 1 kilometer (km) = 1,000 m = 10³ m - 1 milliliter (mL) = 0.001 L = 10⁻³ L - 1 nanometer (nm) = 10⁻⁹ m (used for atomic dimensions; a hydrogen atom is ~0.1 nm in diameter) - 1 microgram (μg) = 10⁻⁶ g (used in pharmacology for very potent drugs)
Temperature Scales
Three temperature scales are used in science:
Celsius (°C): Based on water's freezing point (0°C) and boiling point (100°C) at standard pressure. Used in everyday life and most scientific contexts.
Kelvin (K): The SI unit. Zero kelvin (0 K = −273.15°C) is absolute zero — the theoretical temperature at which all thermal motion stops. There are no negative Kelvin values. Kelvin is essential for gas law calculations and thermodynamics.
Conversion: K = °C + 273.15
Fahrenheit (°F): Used primarily in the United States. Not used in scientific work.
Conversion: °F = (°C × 9/5) + 32
Key reference points: - Absolute zero: 0 K = −273.15°C - Water freezes: 273.15 K = 0°C = 32°F - Room temperature: ~298 K = 25°C = 77°F - Water boils: 373.15 K = 100°C = 212°F
Precision and Accuracy
These two terms are often confused but mean distinct things in science:
Accuracy describes how close a measurement is to the true (accepted) value. Precision describes how closely repeated measurements agree with each other (reproducibility).
A dart board analogy: - Accurate and precise: darts clustered tightly around the bullseye. - Precise but not accurate: darts clustered tightly together, but far from the bullseye (systematic error). - Accurate but not precise: darts scattered around the bullseye on average, but widely spread. - Neither accurate nor precise: darts scattered randomly.
In chemistry, we want measurements to be both accurate and precise. Systematic errors (instrument calibration problems, procedural errors) affect accuracy. Random errors (small, unpredictable variations in reading) affect precision.
Significant Figures
Significant figures (sig figs) are the meaningful digits in a measurement that convey information about the precision of the measurement.
Rules for counting significant figures: 1. All non-zero digits are significant: 345 has 3 sig figs. 2. Zeros between non-zero digits are significant: 3,045 has 4 sig figs. 3. Leading zeros are NOT significant: 0.0045 has 2 sig figs. 4. Trailing zeros after a decimal point ARE significant: 3.450 has 4 sig figs. 5. Trailing zeros without a decimal point are ambiguous: 3,400 might have 2, 3, or 4 sig figs; use scientific notation (3.400 × 10³) to be clear.
Rules for calculations: - Multiplication/Division: The result should have the same number of sig figs as the measurement with the fewest sig figs. - 12.34 × 1.2 = 14.808 → rounds to 15 (2 sig figs) - Addition/Subtraction: The result should have the same number of decimal places as the measurement with the fewest decimal places. - 12.34 + 1.2 = 13.54 → rounds to 13.5 (one decimal place)
Scientific Notation
Scientific notation expresses numbers as a coefficient (between 1 and 10) multiplied by a power of ten:
- Avogadro's number: 602,214,076,000,000,000,000,000 = 6.022 × 10²³
- Mass of an electron: 0.000000000000000000000000000000911 kg = 9.11 × 10⁻³¹ kg
- Diameter of a carbon atom: 0.00000000015 m = 1.5 × 10⁻¹⁰ m
Scientific notation makes it easy to handle very large and very small numbers, keeps significant figures explicit, and simplifies multiplication and division.
Units in Calculations: Dimensional Analysis
Dimensional analysis (the factor-label method) is a technique for converting between units by multiplying by conversion factors that equal 1:
Example: Convert 5.00 kilometers to centimeters. 5.00 km × (1,000 m / 1 km) × (100 cm / 1 m) = 500,000 cm = 5.00 × 10⁵ cm
Example: Convert a density of 0.789 g/mL to kg/m³. 0.789 g/mL × (1 kg / 1,000 g) × (1,000 mL / 1 L) × (1,000 L / 1 m³) = 789 kg/m³
Units cancel like fractions; if the units work out correctly, the calculation is almost certainly set up correctly. This technique is invaluable for stoichiometry and any multi-step unit conversion.
Common Chemistry Measurements
| Quantity | Common Unit | SI Unit | Notes |
|---|---|---|---|
| Volume | mL, L | m³ | 1 L = 1 dm³ = 1,000 mL |
| Mass | g, kg | kg | 1 kg = 1,000 g |
| Density | g/mL, g/cm³ | kg/m³ | d = m/V |
| Pressure | atm, mmHg, bar, Pa | Pa | 1 atm = 101,325 Pa = 760 mmHg |
| Energy | kJ, cal, kcal | J | 1 cal = 4.184 J |
| Concentration | mol/L (M) | mol/m³ | 1 M = 1 mol/L |
| Amount | mol | mol | 6.022 × 10²³ particles/mol |