Overview
This lecture introduces the International System of Units (SI), its base and derived units, common prefixes, and the importance of standardized measurement in science.
SI Base Units
- The SI system ensures consistency in scientific measurements worldwide.
- There are seven base SI units: meter (m) for length, kilogram (kg) for mass, second (s) for time, mole (mol) for amount, kelvin (K) for temperature, ampere (amp) for electrical current, candela (cd) for luminous intensity.
- Chemistry primarily uses meter, kilogram, second, mole, and kelvin.
- Celsius (°C) is also used for temperature, with K = °C + 273.
SI Prefixes
- Prefixes are added to units to represent multiples or fractions by powers of ten (e.g., kilo- = 1,000; centi- = 1/100).
- Combining a prefix abbreviation with a unit abbreviation forms units like kg (kilogram) or mL (milliliter).
- Common prefixes: giga- (G, 10⁹), mega- (M, 10⁶), kilo- (k, 10³), centi- (c, 10⁻²), milli- (m, 10⁻³), micro- (μ, 10⁻⁶), nano- (n, 10⁻⁹).
Derived SI Units
- Derived units are made by combining base units (e.g., area: m², volume: m³).
- Volume in chemistry is often measured in liters (L), where 1 L = 1,000 cm³ = 1,000 mL.
- Energy’s SI unit is the joule (J), equal to kg·m²/s²; 1 calorie (cal) = 4.184 J.
- Density is mass divided by volume; common units are g/cm³ or g/mL for solids and liquids, g/L for gases.
Conversions and Examples
- 1 km = 1,000 m; 1 m = 100 cm.
- 1 kg = 1,000 g; 1 Mg = 1,000,000 g.
- 1 mL = 1 cm³.
- To find density: divide mass by volume (density = mass/volume).
Key Terms & Definitions
- SI — International System of Units, standard for scientific measurement.
- Base unit — Fundamental SI unit (e.g., meter, kilogram).
- Derived unit — Combination of base units (e.g., m³ for volume).
- Prefix — Modifies unit size by power of ten (e.g., kilo-, milli-).
- Density — Mass per unit volume (g/cm³, g/mL).
- Joule (J) — SI unit of energy, equals kg·m²/s².
- Calorie (cal) — Non-SI energy unit, 1 cal = 4.184 J.
Action Items / Next Steps
- Practice converting between different SI units and prefixes.
- Complete the skill-building exercises and example problems on unit abbreviations and density.
- Review the differences between base and derived units and memorize common SI prefixes.