Measurement, Units, and Density
SI units and prefixes
All scientific measurement uses the International System (SI). Learn the base units: metre (length), kilogram (mass), second (time), ampere (current), kelvin (temperature), mole (amount of substance), and candela (luminous intensity). Derived units matter too: newton (force), joule (energy), watt (power), pascal (pressure), and the litre for volume. Prefixes convert cleanly: kilo- (×10³), centi- (×10⁻²), milli- (×10⁻³), micro- (×10⁻⁶). Always write the unit with the number; a bare figure is meaningless.
Reading instruments
A ruler is read to the nearest millimetre, with an estimate of half a division. A measuring cylinder is read at eye level at the bottom of the meniscus. A thermometer and a top-pan balance follow the same rules: read at eye level, record to the smallest graduation plus one estimated digit. Significant figures reflect the precision of the instrument, so a balance reading 12.50 g is more precise than one reading 13 g.
Density
Density is mass per unit volume: ρ = m/V, commonly in g/cm³ or kg/m³. Because 1 cm³ = 1 mL and 1 m³ = 1000 L, conversions are frequent traps. Water has a density of about 1.0 g/cm³, so objects denser than water sink and less dense objects float. To find the volume of an irregular solid, use displacement in a measuring cylinder.
Worked example
A metal block has mass 54 g and volume 20 cm³. Density = 54 ÷ 20 = 2.7 g/cm³ - consistent with aluminium. To convert to kg/m³: 2.7 g/cm³ × 1000 = 2700 kg/m³. If the block were placed in water it would sink, since 2.7 > 1.0 g/cm³.