Making measurements

No comments

Help your students use appropriate apparatus to make and record a range of measurements

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.

It is the same in chemistry. Measurements provide us with precise and detailed information that helps us to more fully understand and interpret the behaviour we observe.

Progression in making measurements

Students are required to make and record measurements throughout their science education.

Progression in measuring skills
7–11 years (key stage 2)	11–14 years (key stage 3)	14–16 years (key stage 4)	16–18 years (key stage 5)
Take measurements using a range of scientific equipment with an increasing accuracy and precision, taking repeat readings when appropriate	Make and record observations and measurements using a range of different methods	Use appropriate apparatus to make and record a range of measurements accurately, including mass, time, temperature and volume of liquids and gases	Use appropriate apparatus to make and record a range of measurements (to include mass, time, volume of liquids and gases, temperature) Measure pH using pH charts, pH meter or pH probe on a data logger Set up electrochemical cells and measure voltages

The frequency with which the curriculum refers to the skill of making measurements raises some interesting questions:

Do we take the skill of making measurements for granted?
What progression do we expect to see in students’ skills over time?
What can we do to help students develop their measurement skills?
Is there a plan in place to improve students’ measurement skills?

Developing measurement skills

Measuring the volume of a liquid illustrates the development in measurement skills we expect to see in our students. The use of beakers, measuring cylinders, burettes and pipettes requires increasing levels of dexterity. Do students have an opportunity to practise using equipment when they meet it for the first time?

Antoine Lavoisier’s classic experiment of weighing mercury, heating it in air and weighing the resulting oxide, led the way in showing the importance of measurement in interpreting chemical behaviour. Students follow in Lavoisier’s footsteps when they burn magnesium ribbon in a crucible to find the formula of magnesium oxide. As an alternative to using fragile and expensive porcelain crucibles, try using metal bottle tops (pdf, CLEAPSS members only).

Antoine Lavoisier’s classic experiment of weighing mercury, heating it in air and weighing the resulting oxide, led the way in showing the importance of measurement in interpreting chemical behaviour. Students follow in Lavoisier’s footsteps when they burn magnesium ribbon in a crucible to find the formula of magnesium oxide.¹ As an alternative to using fragile and expensive porcelain crucibles, try using metal bottle tops.²

Individual results from this experiment can be quite variable and may run the risk of confusing students. Plotting the results from all groups in the class on a graph of mass of magnesium against mass of oxygen provides plenty of opportunities for a productive discussion about outliers, experimental error and mean value.

Weighing is not only useful for solids, but also for solutions. To introduce the principles of titration, try a microscale method that simplifies measurements by using mass rather than volume (pdf, CLEAPSS members only). Place the solution of unknown concentration in a small vial on top of a balance. Squeeze the bulb of a plastic pasteur pipette in a retort clamp to add drops of the known solution. This approach smoothly leads on to titrations using a pipette and burette with an increase in demand on student skills at each stage.

Weighing is not only useful for solids, but also for solutions. To introduce the principles of titration, try a microscale method that simplifies measurements by using mass rather than volume.³ Place the solution of unknown concentration in a small vial on top of a balance. Squeeze the bulb of a plastic pasteur pipette in a retort clamp to add drops of the known solution. This approach smoothly leads on to titrations using a pipette and burette with an increase in demand on student skills at each stage.

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. You may need to 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.

Reading the level of potassium permanganate in a burette

Source: Martyn Chillmaid / Science Photo Library

It can be easier to read the top of the meniscus of potassium manganate(VII), rather than the bottom

An intensely coloured solution such as potassium manganate(VII) poses a further problem. As it is difficult to see the bottom of the meniscus, it is better to read the position of the top instead. Hold a white tile or piece of white paper behind the burette to make readings easier to see. This kind of attention to detail illustrates the increasing level of care we expect students to take when making measurements.

Students often struggle to measure the volume of a gas collected in an inverted measuring cylinder or burette. It may be easier for them to record the time the liquid reaches a set mark on the cylinder or burette rather than trying to record the position of a moving meniscus at a set time.

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

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. Solving a similar problem in measuring elapsed time, most smartphones include a stopwatch function.

Selecting equipment to make measurements

Part of the skill in making the right measurement is in choosing the right equipment. Asking students to select from a range of apparatus you provide for them is an excellent way of getting them to think more carefully about their practical work.

Points to consider when choosing apparatus
Points to consider	Comments
Is the apparatus big enough?	Students may need to calculate the amount of product that will be produced
Is the apparatus too big?	The scale on the apparatus may not allow appropriate measurements
Is the measuring device accurate enough?	Students need to decide what level of accuracy is required before making their choice
Are there any chemical limitations to take account of?	A very soluble gas cannot be collected over water

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 is an appropriate alternative to a 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, it is helpful to get students to check values suggested by pH paper with a pH meter to clarify what particular colours mean.

The language of measurement

The terminology used in relation to measurements is often confusing to students and teachers alike. 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 have all been a cause of some difficulty. The term ‘error’ can pose a particular problem because some students feel it implies they have made a mistake in their measurement.

Correct use of precision and accuracy terminology

Source: Royal Society of Chemistry

A result or measurement is accurate if it is close to the true value. A precise result is one where the data points are close together (but there can be a random error). It is therefore possible to have a precise measurement that is not accurate.

The ASE/Nuffield publication The language of measurement provides a definitive list of the meaning of words associated with measurements together with case studies of their use in school investigations.

The ASE/Nuffield publication The Language of Measurement⁵ provides a definitive list of the meaning of words associated with measurements together with case studies of their use in school investigations.

Ask students to weigh a batch of chocolate bars as an engaging way of introducing them to ideas of true value, variability, outlier and measurement error. A dartboard is an effective analogy to develop ideas about accuracy and precision.

Ask students to weigh a batch of chocolate bars as an engaging way of introducing them to ideas of true value, variability, outlier and measurement error.⁴ A dartboard is an effective analogy to develop ideas about accuracy and precision.

Maths skills

When deciding on the suitability of measuring apparatus and instruments it is helpful for students to calculate the percentage uncertainty associated with measurements. 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 students to calculate the uncertainty in different situations to help them appreciate that this depends both on the instrument used and the quantity measured.

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.

Derek Denby is a retired UK chemistry teacher