TEACH | Chemistry

The chemical chameleon demonstration produces vivid colour changes as a result of redox reactions. IanRedding/Shutterstock.com

Colourful chemistry: redox reactions with lollipops Use a lollipop to activate colour-changing redox reactions in this simple but eye-catching activity.

By Marisa Prolongo and Gabriel Pinto

Teaching oxidation-reduction (redox) reactions is part of from a lollipop is used as the reducing agent. When glucose all secondary-school chemistry curricula. In this article, we is added to a solution containing OH– ions, the groups describe a vivid colour-changing demonstration to illustrate in the glucose donate electrons, giving rise to groups: a chain of redox reactions, whereby electrons are transferred between different compounds and ions. The activity is –C(H)(OH)– + 2OH– à –C(=O)– + 2H O + 2e– suitable as a teacher demonstration, or older students could 2 (secondary alcohol) (ketone group) carry out the experiment themselves. In our experiment, glucose is added to a Oxidising and reducing agents solution together with sodium (NaOH), so electrons from glucose (C6H12O6) are first donated to – A redox reaction is any chemical reaction in which a permanganate ions (MnO4 ). The oxidation products of the molecule, atom or ion loses or gains electrons, altering its reducing sugar are mainly glucuronic acid (C6H10O7,), plus . An oxidising agent gains electrons (and is some arabinonic acid (C5H10O6) and formic acid (CH2O2). reduced in the reaction) and a reducing agent loses electrons If the lollipop is made from fructose, which is an isomer of

(and is oxidised in the reaction). In this experiment, glucose glucose, the main product is fructonic acid (also C6H10O7).

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In a series of redox reactions, electrons are donated continually from glucose to successive compounds of . At each step in the chain, a colour change is visible. Manganese is ideal for this experiment, as it has more stable oxidation states than any other transition metal (from +2 to +7), each of which has a different colour. You may be familiar with the classic ‘chemical chameleon’ demonstrationw1, of which this experiment is an adaptation. In the original version, you begin with a and glucose solution, which changes colour upon mixing with a spatula. By using a lollipop instead, the glucose is added more gradually to the solution, Marisa Prolongo which makes it easier to follow the colour changes. Using a miniature electric whisk means the lollipop is stirred faster than by hand. Figure 1: Materials required for the lollipop experiment

Materials Procedure Chemistry You will need the following materials The activity is suitable for a single Redox reactions (see figure 1): lesson. The experiment takes only about · 15 minutes and can be followed up by a Potassium permanganate (KMnO4) Ages 16–19 crystals set of discussion questions. · Spherical lollipop containing glucose The steps are as follows: The redox chemistry of (or other reducing sugar, 1. Fill the flask or beaker with 200 ml manganese is a fascinat- e.g. fructose) of distilled water. ing aspect of transition · 3–4 sodium hydroxide (NaOH) metal chemistry. This 2. Stir in the NaOH pellets with the pellets (approximately 0.5 g) simple practical exercise spoon until they have dissolved · helps students to famil- 200 ml distilled water completely. iarise themselves with the · 250 ml conical flask or beaker 3. Using the spatula, add a few variable oxidation states (glass or plastic) REVIEW potassium permanganate crystals of manganese and their · Spoon and spatula (not too many, or the colour will be respective colours. · Miniature electric whisk, too dark to see the changes). When Observing the different e.g. hand-held milk frother potassium permanganate (KMnO4) colours will elicit discus- · Adhesive tape is added to the alkaline NaOH sion and will be a point solution, it dissolves into potassium Safety note: of focus for understanding + – (K ) and permanganate (MnO4 ) what is happening in the A lab coat, gloves and safety goggles ions. should be worn. Teachers should redox steps in the reaction. 4. Attach the stick of the unwrapped follow their local health and safety lollipop to the mini electric whisk rules, in particular concerning the use Andrew Galea, chemistry using adhesive tape (see figure 1). of potassium permanganate and the lecturer, Giovanni Curmi disposal of the resulting solution. See 5. Insert the lollipop into the solution Higher Secondary School, also the general safety note on the and switch on the whisk to start Malta Science in School website and at the mixing. end of this print issue.

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As the lollipop dissolves into the 1. The first colour (purple) corresponds has an oxidation state of +5 and a – solution, you will observe colour to permanganate ions (MnO4 ). blue colour. changes for each redox reaction. The Manganese has the oxidation state – – MnO4 (aq) (purple) + 2e first two changes happen very rapidly +7 (figure 2). 3– → MnO4 (aq) (blue) (3–5 seconds), while further changes 2. The permanganate ions (MnO –) are 4 3. The ions (MnO 2–), take a little longer. Students can take then reduced to manganate ions 4 which have an oxidation state photos (e.g. with the camera of a (MnO 2–). The oxidation state of 4 of +6, are further reduced to mobile phone) at various time points manganese changes from +7 to +6, (MnO ), with to better compare and follow the and the colour changes from purple 2 an oxidation state of +4, causing changes in colour. A video from the to green (figure 4). authors demonstrating the experiment is a colour change from green to MnO –(aq) (purple) + e– w2 4 yellow-brown (figure 5). available in Spanish . 2– → MnO4 (aq) (green) MnO 2–(aq) (green) + 2 H O(l) + 2e– An intermediate blue stage occurs 4 2 → MnO (s) + 4OH–(aq) between steps 1 and 2 (figure 2 What happens in the (yellow-brown) 3). One explanation is that the experiment? mixture contains both the purple 4. Finally, when even more glucose – As the lollipop dissolves in the solution permanganate (MnO4 ) and the is incorporated into the solution, 2– containing manganese ions, at least five green manganate ions (MnO4 ), brown-black manganese dioxide different colours can be distinguished which combine to produce a blue (MnO2) forms a colloidal (as shown in figures 2–6), which solution. Another explanation is that suspension in alkaline solution, correspond to different oxidation states a part of permanganate is reduced which (if fairly dilute) can appear 3– of manganese. to (MnO4 ), which orange (figure 6).

Figure 2: The first colour in Figure 3: Before the colour Figure 4: Permanganate ions Figure 5: Manganate ions Figure 6: The final colour the chain of redox reactions changes from purple to are reduced to manganate are reduced to manganese change is to orange, when is purple, which corresponds green, there is a blue ions, resulting in a colour dioxide, causing the colour to manganese dioxide forms a to the permanganate ions. intermediate stage. change to green. change from green to yellow- colloidal suspension in the brown. alkaline solution. Marisa Prolongo

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Variations in colour Permanganate ions (MnO -) Manganate ions (MnO 2-) Manganese dioxide (MnO ) The food colours used in lollipops do 4 4 2 not have a big impact on the colours you see in this experiment, but some other factors do play a role. Once the reactions start, there are always mixtures of ions in the solution, resulting in mixtures of colours that are not always easy to interpret (see figure 7). Another factor is that the colour of manganese ions in solution is Nicola Graf generally different from the colour of their corresponding solid salts. This is because manganese ions form complexes with water due to Figure 7: Colours of the three main manganese compounds (top row) and example mixtures of these compounds as the redox reactions proceed (bottom row) the electron-accepting capacity of their atomic d-orbitals. In addition, the tendency for molecules to accept electrons varies with pH and temperature, so if you change these Electron configuration and transition metals variables or the quantities of the chemicals, the colours will vary, and the Electrons are arranged in energy levels called shells. Each shell is divided colour changes will occur at different into subshells, which are made up of orbitals. Transition metals have one times between experiments. or more electrons in their outermost d-orbital. The difference in energy between individual d-orbital electrons is relatively small, so all transition metal cations have a variety of ways of forming chemical bonds involving different numbers of d-orbital electrons. This is why transition Discussion metals have several oxidation states. To link the lollipop demonstration to When electrons absorb certain frequencies of electromagnetic radiation, the chemistry of redox reactions, ask they jump to a higher energy level. In many transition metals, the your students some of the following difference in energy between d-orbitals corresponds to the energy questions: of radiation of the visible light spectrum. For example, the d-orbital · In the experiment, what is the electrons of permanganate ions absorb electromagnetic radiation from reducing agent that donates the yellow part of the visible spectrum, but what we see as the colour electrons in the redox reactions? of permanganate ions is the colour complementary to yellow – that is, This depends on which reducing purple. We see the colour of the remaining wavelengths that were not sugar you use, but in our experiment, absorbed (fgure 8). the reducing agent is glucose

(C BACKGROUND 750 nm 400 nm 6H12O6). · What is the oxidising agent that accepts the electrons? The first oxidising agent in the 630 nm 430 nm reaction is the permanganate ions. After that, electrons are donated to manganate ions. · Does potassium permanganate in solution absorb the part of

visible light that corresponds to Nicola Graf the colour we see (green) or to its 590 nm 480 nm complementary colour (red)? Potassium permanganate absorbs electromagnetic radiation 560 nm from the red part of the visible spectrum, but what we see as the Figure 8: The colour of the solution is the colour that is complementary (i.e. opposite on the colour wheel) to the wavelength of light absorbed by d-orbital electrons. colour of manganate ions is the complementary colour, green.

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· Do you know of any other chemical Acknowledgements Resources elements that show different colours This article is based on a presentation For ideas on introducing redox reactions using in different oxidation states in everyday examples, see: at the Spanish Science on Stage festival solution? Voak H (2016) Redox resources. Science (Ciencia en Acción) in 2014. This work in School 36. www.scienceinschool.org/ Examples include: chromium was first carried out by students at IES (Cr O 2–, orange; CrO 2–, yellow) and content/redox-resources 2 7 4 Manuel Romero Secondary School in vanadium (V2+, violet; V3+, green; To try a colour-change experiment involving Málaga, Spain. We are grateful for the pH-sensitive plant dyes, see: VO2+, blue; VO 3–, yellow) 4 support of the Technical University of Shimamoto GG, Vitorino Rossi A (2005) An · What are the main uses of Madrid (Universidad Politécnica de artistic introduction to anthocyanin inks. manganese, for example in biology Madrid) for the projects ‘Promotion of Science in School 31: 32–36. or industry? experimental learning of chemistry’ www.scienceinschool.org/content/ Manganese compounds are used and ‘Chem-Innova’, and of the Spanish artistic-introduction-anthocyanin-inks in stainless steels and batteries, Royal Society of Chemistry (Real ______and as fuel additives and pigments. Sociedad Española de Química, RSEQ). Manganese is also an essential Marisa Prolongo is a physics and cofactor for many enzymes, such Web references chemistry teacher currently working as photosystem II in chloroplasts. w1 For variations of the experiment, visit at Principia, an interactive science However, in large amounts it is toxic the Science Brothers website and the museum in Málaga, Spain. She has to humans. Hobby Chemistry website. See: www. more than two decades of teaching sciencebrothers.org/the-chemical- chameleon/ and https://hobbychemistry. experience, including a variety of Variations of the experiment wordpress.com/2015/03/31/ student-centred learning methods and chemical-chameleon/ activities. This experiment can be performed in a w2 A video of the experiment is available in number of different ways. For example, Gabriel Pinto is a professor at the Spanish on YouTube and the IES Manuel Technical University of Madrid, Spain, rather than using a lollipop, you could Romero Secondary School website. See: where he has been teaching chemistry use a chewing gum containing sugar www.youtube.com/watch?v=1pE3U7UkIqc as the reducing agent; or instead of or http://iesmanuelromero.es/index.php/es/ to engineers for 32 years. He has been adding the glucose to a flask, you noticias/132-noticias-curso-2013-14/519- involved in a number of STEM outreach could add it to a plastic bottle and experimentos-de-ciencias programmes and activities for students. shake it to observe the colour changes Marisa and Gabriel are Scientix (see figure 9). Your students could use ambassadors and have taken part in their creativity to think of alternative many national and international science experiments. festivals and workshops presenting experiments for STEM education. Marisa Prolongo

Figure 9: An alternative experimental setup using chewing gum and a plastic bottle

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