Measurement and scientific units

A measuring cylinder has graduations for measuring the volume of a liquid, so it is the most suitable instrument. A thermometer measures temperature, not volume. To obtain a correct reading, keep your eye level with the liquid meniscus. Exam tip: match each instrument with the physical quantity it measures.

‘The plant is very tall’ is vague because the word ‘very’ does not give a measurable value. It should be made precise by recording the plant’s height in a suitable unit, for example, 25 cm. The other options already include numerical measurements of time, temperature, or volume. Exam tip: In scientific observations, use numbers and standard units wherever possible.

The governing concept is parallax error, which occurs when a scale is viewed from an angle rather than at eye level. In a measuring cylinder, the observer should place the eye horizontally level with the liquid meniscus and read the appropriate part of the meniscus, usually its bottom for water-like liquids. Looking from above changes the apparent position of the meniscus and can make the recorded volume too high or too low. Therefore option A is correct. A chemical change concerns formation of a new substance, controlling an independent variable concerns experimental design, and random sampling concerns selection of specimens; none describes this reading problem.

The governing concept is instrument resolution, meaning the smallest difference that an instrument can distinguish reliably. Thick ruler markings occupy more space and make the exact position of an object’s end difficult to identify, especially when two lengths differ only slightly. The ruler may still measure length and its unit remains unchanged, but the observer cannot report fine differences with good precision. Thus option A is correct. The object itself is not changed by the ruler, units do not disappear, and the roles of dependent and independent variables are determined by the investigation, not by the thickness of scale marks.