Measurement and scientific units

Mass measures the amount of matter in an object and is commonly expressed in grams (g) or kilograms (kg). Therefore, 100 g is a measurement of mass. In contrast, 100 mL measures volume, 25°C measures temperature, and 10 cm measures length. Exam tip: Identify the unit written with a number to determine the physical quantity.

A stopwatch is used to measure a time interval, so it is the correct answer. A thermometer measures temperature, whereas a measuring cylinder measures the volume of a liquid. Exam tip: match each instrument with the physical quantity it measures.

Option C is a precise observation because it gives the plant’s height as a measurement: 25 centimetres. A number with a standard unit allows the observation to be checked and compared. “The plant is tall” is only a general description, since tall can mean different things to different people. Exam tip: When possible, make an observation precise by including a number and the correct unit.

To measure length, one end of the object is placed at the zero mark of the ruler and the reading at the other end is noted. Therefore, the zero mark is correct. Starting directly from a middle or last mark will not give the length unless the difference between the two readings is calculated. Exam tip: If the zero edge of a ruler is damaged, start from a clear mark and subtract the initial reading from the final reading.

Centimetres are suitable units for measuring the length of a pencil, so option C is correct. Litres and millilitres measure volume, while grams measure mass; none of these can measure length. Exam tip: identify the physical quantity first, then select its appropriate unit.

In option C, the yellow colour of the solution is an observation, while the volume of 40 mL is a measurement because it includes a numerical value and a unit. The other options are only subjective descriptions and do not give a measurement. Exam tip: Look for a number with a suitable unit to identify a measurement.

Option B specifically asks how long salt takes to dissolve, so it requires measuring time. Comparing the time in hot and cold water can show the effect of temperature on the rate of dissolving. Options A, C, and D ask only about observable properties and do not require a time measurement. Exam tip: Questions containing “how long” or “how much time” require time to be measured.

A difference between two measurements may result from random error, a faulty instrument, or a variation in the measurement method. Therefore, check the instrument, including any zero error, and repeat the measurement correctly. Choosing one value without checking is not a scientific procedure. Exam tip: When readings disagree, repeat the measurement and look for consistent readings.

Option C asks how long wet paper takes to dry in sunlight and shade, so the time can be measured using a clock or stopwatch. It is therefore a measurable and testable question. Option A involves observing the colour of paper, not measuring time. Exam tip: Questions containing “how long” usually require measurement of time.

Keeping the eye directly in front of the mark makes the line of sight perpendicular to the scale and reduces parallax error. When viewed from an angle, the mark may appear shifted, producing an incorrect reading. Exam tip: Keep your eye level with the mark whenever reading a scale or measuring cylinder.