Units and Symbols
A measurement without units is meaningless! The International System of Units, internationally known by the abbreviation SI (from French Système international d'unités), is the modern form of the metric system and the world's most widely used system of measurement.
Value Being Measured |
Description of Value |
SI Unit |
Measurement Tool |
To reduce error... |
Mass |
The amount of matter in an object. Determined by the number and type of atoms in a substance. |
kilogram (kg) |
Electronic Balance Triple Beam Balance |
Calibrate scale regularly. Place scale on a level surface. Tare "zero" the scale before each use. |
Volume |
The amount of three-dimensional space that a substance or object occupies. |
Cubic meter (m³). While the liter (L) is also commonly used to express volume, especially for liquids, it is not the SI unit; it is a derived unit equivalent to a cubic decimeter (dm³). 1 m³ = 1,000 dm³ = 1,000,000 cm³ 1 dm³ = 1 L = 1,000 mL |
For liquids, use a graduated cylinder, burette, or pipette. For regularly shaped solids (like a cube), calculate volume using a ruler and a geometric formula (V=l×w×h). For irregularly shaped solids, use the water displacement method by submerging the object in a graduated cylinder with a known volume of water and noting the change in volume. The volume of a gas produced in a reaction can be measured with a gas syringe. |
When reading the volume of a liquid, you must read the bottom of the meniscus (the curved surface of the liquid) at eye level to avoid parallax error. |
Time |
The duration of an event. |
Second (s). it is acceptable to use minutes, days or hours when the experiment spans over a significant period of time. |
Stopwatch Clock |
Repeating the measurement multiple times and calculating the average can reduce human error (reaction time) and provide a more reliable result. |
Temperature |
A measure of the kinetic energy of particles. |
Kelvin (K). The Kelvin scale starts at absolute zero (0 K), the point where all atomic motion ceases. While degrees Celsius are commonly used to express temperature, it is not the SI unit. One Kelvin is equal in magnitude to one degree Celsius, but the scales are offset, with 0°C being equal to 273.15 K. |
Liquid-in-glass thermometer Digital thermometer |
The thermometer bulb must be fully immersed in the substance and given enough time to reach thermal equilibrium with the substance before a reading is taken. Read temperature of a liquid-in-glass thermometer at eye level to avoid parallax error. |
Length |
The distance between two points. |
Meter (m) |
Ruler Measuring tape Meter stick Vernier calipers Stage Micrometer - a microscope slide that has a ruler etched on it |
Use the most accurate measuring instrument for the task. Minimize parallax error by reading measurements straight on. Repeat measurements to calculate an average, which can help reduce random errors. |
The following example shows different ways to express the same unit.
- Oxygen consumption (milliliters per gram per hour)
- Oxygen consumption (ml/g/h)
- Oxygen consumption (ml g-1 h-1)
Metric prefixes are used to indicate multiples or submultiples of a base unit in the metric system. They allow for concise representation of very large or very small quantities.
