Measurement
General Principles of Measurements
- Accuracy refers to how close a measured value is to the actual value, and is crucial in experimental procedures.
- Precision is how exact the measurements are. High precision means minor differences between repeated measurements.
- Use the appropriate measuring instrument for a given task for optimal accuracy and precision.
- Understand that every measuring instrument has a certain degree of uncertainty. For example, a ruler with a smallest scale of 1 mm has an uncertainty of ±0.5 mm.
The Use of Measuring Cylinders, Pipettes, and Burettes
- Use a measuring cylinder for volume measurements that do not require high precision. To read, place eye level with the cylinder’s waterline and note the bottom of the meniscus.
- Pipettes give a very accurate fixed volume measurement. Drain pipette by gravity; don’t blow out remaining liquid.
- Burettes are used specifically in titrations. To read a burette, note the bottom of the meniscus. Ensure the initial reading is noted before starting the titration process.
The Use of Balances
- Use a balance to measure mass. Ensure the balance is clean and level before use.
- When using a balance, handle with clean dry hands or gloves to prevent dirt and oil affecting the reading.
- If the sample is volatile or hygroscopic, use a closed container like a weighing bottle.
Reading Thermometers
- When reading a thermometer, ensure it is read at eye-level and avoid parallax error.
- Wait for the thermometer reading to stabilise before taking a measurement.
- Ensure the thermometer is clean and dry before use to prevent incorrect readings.
Dealing with Uncertainties and Errors
- Random errors affect precision; they are unpredictable and occur due to variations in observations.
- Systematic errors affect accuracy; they shift all measurements in a standardized way. Often, they result from faulty equipments or a flawed experimental design.
- Reducing errors: Ensure measuring equipment is clean and properly calibrated, conduct multiple readings and calculate an average, and implement careful and consistent experimental technique.
Units and Conversion
- Be able to convert between units, such as millilitres to litres.
- Understand the International System of Units (SI units), and use these for all scientific measurements to ensure standardisation.