Derek Denby explains how to get the most out of lab work
For teachers in England and Wales, the introduction of new A-level chemistry specifications has brought practical work back into the spotlight. The straitjacket of assessed tasks is gone. Teachers have a wonderful opportunity to plan the practical work they want to do, ensuring students develop skills throughout their course. How can they make the most of this opportunity to get the best out of practical work?
The importance of objectives
There are lots of reasons for doing practical work: developing a skill in a particular technique; providing an opportunity to solve problems; helping develop conceptual understanding; or simply demonstrating chemical change. Teachers need to be very clear about the specific reason for choosing each experiment. This principle has been well documented in the Getting Practical1 initiative. Teachers must set precise and realistic objectives when planning experiments and check how well they have been achieved at the end of the session.
The objectives should always be shared with students so they know why they are doing the work.
The biggest danger is trying to squeeze too many outcomes from a single experiment. It is much better to use several sessions to build skills in a manageable way.2
The checklist in Box 1 gives some questions teachers can use to make sure they are aiming in the right direction.
Box 1: Experiment objectives checklist
- Are there too many of them?
- Are they precise?
- Are they realistic?
- Can students understand them?
- How will you know if they have been achieved?
Focusing on the task
There is a danger that practical work can become for some students a ‘painting by numbers’ activity. They perceive their job as simply following instructions to complete the task. Their focus is purely ‘hands-on’. But practicals can easily be made ‘minds-on’ by encouraging students to question what they do and make sense of what they observe (Box 2).
Box 2: Examples of minds-on questions
- Why did the solution change colour?
- Why are you using a burette rather than a measuring cylinder?
- Did you expect it to change like that?
- What are you going to do with these readings?
- Can you explain what is happening in the reaction vessel?
- What must be happening in terms of atoms and ions?
- Have you met this kind of behaviour elsewhere?
Students need to be engaged to make effective links between what they see and the conceptual ideas they are trying to develop.
Research suggests teachers should spend more time making explicit the ideas an experiment is designed to illustrate rather than assuming they will simply emerge from what the student has measured or observed.3 This means devising in advance questions students can use to check their understanding of experimental procedures and results. Asking students to discuss their experiments in groups is an effective way of making these vital links between observation and ideas. Yes, it takes time. But what is the point in spending a shorter time on experiments students get nothing out of?
Learning to investigate
Students often find it difficult to plan an investigation. Maybe that is because we don’t give them sufficient opportunities to develop this skill. Investigations don’t need to be complicated. Reducing information or limiting available equipment can transform a recipe driven activity into one in which students have to work out what to do themselves. In Box 3 an acid base titration, tests for anions and cations and the ideal gas equation form the basis of straightforward investigations for students.
Box 3: Some straightforward investigations
1. You are provided with 0.1 mol dm-3 sodium hydroxide solution, phenolphthalein indicator, distilled water, test tubes and dropper pipettes. Your task is to find the approximate concentration of ethanoic acid in the supplied vinegar using only these materials.
2. John James is a gardener. He stores a number of bottles of chemicals in a shed. The roof of the shed has developed a leak. Water has run over the bottles and has removed all the labels. John has sent samples of the solutions from the bottles to you for analysis. They are labelled A to F.
The chemicals John keeps in his shed are:
- Ammonium chloride, ammonium sulfate and ammonium nitrate for use as fertilisers
- Copper(II) sulfate for treatment of potatoes to prevent blight
- Iron(II) sulfate to eradicate toadstools from lawns
- Sodium bromide to remove green algae from ponds
You are provided with solutions of sodium hydroxide, barium chloride, silver nitrate and lead(II) nitrate. Your task is to use these solutions to help you identify the samples sent for analysis.
3. You have access to a tape measure, a thermometer, a copy of today’s weather forecast and a data book. Use these materials to find out the mass of argon in the room.
Variety in practical work
Practical work can easily become very repetitive (some might say boring) with students mostly working in pairs on a test tube scale following teacher produced handouts. This is particularly true for the most able students, a group often insufficiently engaged by practical work. Changing your approach from time to time can be stimulating.
Some experiments lend themselves to whole class activities in which results are pooled and used to come to an agreed conclusion. Finding the most cost effective brand of bleach is a good example. Instead of everyone carrying out the same iodine-thiosulfate titration, groups of students are given different bottles of bleach to analyse. They combine results so they can come to an overall judgement.
Students might be forgiven for thinking all chemistry experiments can be carried out in under an hour because they always fit neatly into one lesson. The preparation and purification of aspirin is an opportunity to carry out an extended two step synthesis over several sessions, developing some key practical skills along the way (see table below).
|Experiment||Practical skills developed|
|Converting methyl 2-hydroxybenzoate (oil of wintergreen) to 2-hydroxybenzoic acid (salicylic acid)|
- Devising a risk assessment
- Setting up glassware using retort stand and clamps
- Heating under reflux
- Filtering under reduced pressure
|Converting 2-hydroxybenzoic acid to impure 2-acetoxybenzoic acid (aspirin)|
- Devising a risk assessment
- Safely handling solids and liquids including corrosive substances
- Filtering under reduced pressure
|Purifying and analysing 2-acetoxybenzoic acid|
- Recrystallising a solid
- Thin layer chromatography
- Melting point determination
- Calculating percentage yield
Carrying out experiments on a reduced scale has clear benefits in safety and economy of materials. It also helps students develop the skill of careful observation. Bob Worley at CLEAPSS has documented some excellent examples of reduced scale experiments4, 5 that can easily replace more traditional methods (see Box 4).
Box 4: Microscale experiments
- Electrolysis in a Petri dish
- Percentage water of crystallisation in a beer bottle top
- Microtitration with a dropper pipette
- Cracking hydrocarbons using a Pasteur pipette
- Precipitating hydroxides on a laminated sheet
Small scale experimentation also offers the opportunity for students to work individually. Most of the time, students work in small groups, which can lead to some being passive observers in practical sessions. Asking students to work on their own from time to time is a really good way of making sure that everyone is actively involved in developing their skills.
Contextualising practical work
Contexts can bring practical work to life. It helps students appreciate that chemistry and the work that chemists do are an important part of the world around them. A simple change in the choice of experiment to illustrate a topic or idea can make the activity much more engaging. Some examples are shown in the table below.
|Standard experiment||Contextual alternative|
|Iron(II) sulfate–manganate(VII) titration||How much iron is in dried thyme?|
|Thiosulfate–iodine titration||How much sulfur dioxide is in a bottle of wine?|
|Effect of temperature on reaction rate using sodium thiosulfate and acid||Effect of temperature on the reaction between stewed rhubarb and potassium manganate(vii) solution|
|Paper chromatography of inks||What’s in artificial sweetener tablets?|
Integrating skill development
To satisfy the new A-level practical endorsement requirements, students must carry out a minimum of 12 practical activities and develop competence in a range of skills. Some teachers may treat these activities in isolation, to be undertaken and ‘ticked off’ at stages throughout the course.
However, meeting the requirements of the practical endorsement should be a consequence of practical activity, not the reason for doing it.
This is the most important aspect of effective practical teaching. Students develop experimental skills to be able to apply them, not for their own sake.
You should aim to integrate the skills a student needs within an overall scheme of practical work. So a student will recrystallise a solid, not because they have to satisfy the practical endorsement, but because they want to produce a pure sample of aspirin (see Table 1),6 and they will use a pH meter because they want to investigate the difference between strong and weak acids.
Derek Denby is an independent science education consultant based in the UK with a particular interest in practical work.
The Royal Society of Chemistry’s Developing Expertise in Teaching courses are designed to support you throughout your teaching career. Practical work is an integral part of all face-to-face workshops.
To find out more about the CPD for Teachers courses on offer, visit http://rsc.li/teacher-cpd
1. Getting practical. SCORE (pdf)
2. Education in Chemistry, January 2015
3. R Millar and I Abrahams, Sch. Sci. Rev., 2009, 334, 59 (pdf)
4. http://bit.ly/1MnZAxa (pdf)
6. Many of the examples of practical work in this article are taken from practical activities developed by Derek Denby and Emily Perry