Measurement and Apparatus Exam Questions

(a) Pipette (25 cm3 pipette) [1]

(b) Gas syringe (or measuring cylinder inverted over water/collecting over water) [1]

(c) Electronic/digital balance (2 d.p. balance) [1]

(d) Thermometer [1]

Examiner note: A measuring cylinder is not precise enough for titration work — a pipette delivers an exact fixed volume. A beaker is never an acceptable measuring instrument because it has only approximate graduations. For gas volume, a gas syringe is more accurate than collecting over water if the gas is slightly soluble.

(a) The meniscus is the curved surface of a liquid in a container, caused by the attraction between the liquid and the container walls [1]

(b) The eye should be level with / at the same height as the bottom of the meniscus (to avoid parallax error) [1]

(c) The student should record 23.0 cm3 (not 23 cm3) — the decimal place shows the precision of the measurement and indicates the reading has been taken to the nearest 0.5 or 1.0 cm3 [1]

Examiner note: Reading from above makes the volume appear lower; reading from below makes it appear higher. This is parallax error. For water and aqueous solutions, read from the bottom of the meniscus. For mercury (used in some thermometers), read from the top of the meniscus because mercury curves upward.

(a) A thermometer (to measure temperature) [1] and a polystyrene cup or beaker (to hold the reaction mixture) [1]

(b) Any two from:

• Use a polystyrene cup (insulated container) to reduce heat loss to the surroundings [1]
• Stir the mixture to ensure even distribution of heat [1]
• Record the starting temperature before adding the magnesium [1]
• Use a lid/cotton wool plug to reduce heat loss from the top [1]
• Repeat the experiment and take an average to improve reliability [1]

Examiner note: A polystyrene cup is a much better insulator than a glass beaker. If the question asks for improvements, suggesting a lid and stirring are the most commonly credited answers. Recording the start temperature is essential for calculating the temperature change.

(a) 0.05 cm3 (half a division on most burettes, which are graduated in 0.1 cm3 intervals) [1]

(b) A burette allows precise measurement of variable volumes, enabling the exact endpoint to be determined / a burette is more precise than a measuring cylinder [1]

(c) Volume = 26.40 - 1.50 = 24.90 cm3 [1]

Examiner note: Burette scales read from top to bottom (0 at the top, 50 at the bottom), the opposite of a measuring cylinder. A common error is reading the scale the wrong way. The titre (volume delivered) is calculated by subtracting the initial reading from the final reading. Always record burette readings to 2 decimal places.

• Accuracy refers to how close a measurement is to the true/accepted value [1]
• Precision refers to how close repeated measurements are to each other / the degree of reproducibility [1]
• A measurement can be precise but not accurate (e.g., consistently measuring 25.2 cm3 when the true value is 25.0 cm3 due to a systematic error) [1]

Examiner note: Repeating experiments improves reliability (precision) but does not improve accuracy if there is a systematic error (e.g., a faulty instrument). To improve accuracy, use calibrated instruments and correct technique. To improve precision, repeat measurements and calculate a mean.

(a) A gas syringe gives a direct reading of gas volume / works for gases that are soluble in water [1]

(b) Any one from: make sure the syringe moves freely / check for leaks / do not exceed the maximum volume of the syringe / lubricate the barrel [1]

(c) Mean = (45 + 44 + 52) / 3 = 47 cm3 [1]. The result 52 cm3 is anomalous / an outlier. It should be discarded and the mean recalculated as (45 + 44) / 2 = 44.5 cm3 for a more reliable result [1]

Examiner note: An anomalous result is one that does not fit the pattern. It should be identified, excluded from the mean, and the experiment repeated if possible. Simply averaging all three values without noting the outlier loses the second mark. Suggest possible causes: a leak, different mass of zinc, wrong concentration of acid.

• A systematic error causes all measurements to be shifted in the same direction (all too high or all too low) by a consistent amount [1]
• Example: a balance that is not zeroed correctly, or a thermometer that reads 1 degree too high [1]
• A random error causes measurements to scatter unpredictably above and below the true value and can be reduced by repeating measurements and averaging [1]

Examiner note: Systematic errors cannot be reduced by repeating the experiment — they affect every reading equally. They are reduced by calibrating instruments and improving technique. Random errors can be reduced by taking more measurements and calculating the mean. Identifying the type of error in a practical scenario is a common Paper 4 question.