Making measurements

As a species, we have been making measurements from our earliest civilisations. Ancient units of measurement tended to be based on the human body or the natural world. For example, Sumerians used the cubit as a measure of length, running from the elbow to the tip of the middle finger, while ancient Egyptians divided the night into 12 hours due to the appearance of 12 stellar constellations.

Today, we rely on measurements in many aspects of our daily lives. Whether it is weighing a newborn baby, checking the volume of fuel dispensed at a petrol station or following a recipe to bake a cake, measurements provide us with vital information.

Why study making measurements?

Chemistry, as a science, is rooted in both observation and accurate measurement. Antoine Lavoisier’s work required careful measurements of the weights, helping to confirm the principle of conservation of mass and disproving phlogiston theory.

Measurements provide us with precise and detailed information that helps us to more fully understand and interpret the world we observe. For our students, being able to make decisions about which apparatus to use, and to make accurate and precise measurements, allows them to develop important practical competencies in the laboratory and to develop understanding of how chemistry develops as a science.

Progression in ideas and skills

Formal assessment of practical skills, such as measurement, is achieved differently between nations. For example, in England, these skills are assessed indirectly in written papers at GCSE level, while in Wales, hands-on practical assessment contributes 10% to the final GCSE grade. Consider how your curriculum and schemes of work support your students’ skills development. Is there clear progression, with opportunities to practise at each level? Are there particular skills that are assumed to have been mastered at different levels? How do you support students who haven’t mastered the skills, for example those new to the school?

Individual results from this experiment can be quite variable and run the risk of confusing students. However, try plotting the results from all groups in the class on a graph of mass of magnesium against mass of oxygen. The resultant data and graph provide plenty of opportunities for a productive discussion about outliers, experimental error and mean values.

Practical problems and suggested solutions

Some measurements present particular challenges. Students need practice to be able to adjust the volume in a pipette so the bottom of the meniscus sits exactly on the mark. Remind them that their eyes should be at the same level as the meniscus when they take a reading from a burette to avoid parallax errors.

Intensely coloured solutions, such as potassium manganate(VII), cause additional problems as the bottom of the meniscus is difficult to see (see the image right). However, your students can use the top of the meniscus to take the readings, as well as holding a white tile or piece of paper behind the burette to ensure accurate readings. This kind of attention to detail illustrates the increasing level of care we expect students to take when making measurements.

Learners often struggle to measure the volume of a gas collected in an inverted measuring cylinder or burette at a particular time point. Change round the measurements and have them record the time when a particular volume is reached. In this way, their eyes can be lined up accurately. Demonstrate how to hold the timer in their sightline next to the apparatus, so they only need a small movement of their eyes to note the time.

Sometimes gas syringes are used to measure the volume of a gas. The movement of the piston inside the syringe can be erratic if it sticks in the barrel. Tap the syringe with a pencil to help alleviate this problem.

Access to sufficient electronic measuring equipment can also cause problems. For example, a limited number of balances can create bottlenecks in class experiments involving mass measurements. Inexpensive electronic balances, often marketed for weighing jewellery, can get around this problem. These balances usually read to two decimal places and make an excellent alternative to the traditional laboratory balance. When it comes to solving a similar problem in measuring elapsed time, you could try using a smartphone’s stopwatch function (if they’re allowed in your school).

Selecting equipment to make measurements

Part of the skill in making the right measurement is in choosing the right equipment. Provide your learners with a range of apparatus and ask them to select from this range. It’s a great way to get them thinking more carefully about their practical work.

Students typically measure the temperature of liquids or solutions to determine melting and boiling points, and in enthalpy change experiments. It is worth considering whether a temperature probe attached to a data logger, or a standalone digital thermometer, is an appropriate alternative to a traditional thermometer in these cases.

Both pH paper and pH meters can be used to measure the pH value of a solution. Each method has strengths and weaknesses. pH paper is cheap, but students often have difficulty in matching the observed colour to the key. pH meters are more expensive but give accurate readings if they have been calibrated against buffer solutions. When introducing the topic of measuring acidity, get learners to check values suggested by pH paper with a pH meter to clarify what particular colours mean.

The language of measurement

Learners often find the terminology used in relation to measurements confusing, as do teachers. This may be partly because in the past more than one word has been used to express the same idea, or the same word has been used by different people to mean different things, or because the scientific meaning of a word may be different from its everyday use. Words like accuracy, precision, uncertainty, error, reproducible and repeatable can all cause some difficulty. The term error can pose a particular problem because some students feel it implies they have made a mistake in their measurement.

Maths skills

Calculating percentage uncertainty associated with measurements is helpful for students deciding on the suitability of measuring apparatus and instruments. Usually, the uncertainty associated with an analogue instrument such as a burette is taken to be + or – half the smallest graduation. For digital apparatus, such as a balance, the uncertainty is assumed to be + or – the resolution of the device. Ask your students to calculate the uncertainties in different situations. This will help them to appreciate that these values depend both on the instrument used and the quantity measured. The OCR A-level chemistry practical skills handbook contains a useful section on uncertainties, relevant to all students.

Awareness of how to use significant figures correctly is important when making measurements using different apparatus in the same experiment. The choice of which value to use when a set of numbers is available on a stopwatch is a good topic for class discussion.

Some students find the concept of pH difficult to understand because it involves a logarithmic scale. Ask students to describe how much bigger the hydrogen ion concentration is in a solution of pH 2 compared to one of pH 5 to help reinforce this.

Synoptic approach