Of Poinsettia and Gingerbread – Christmas Chemistry Experiments

Title image credit: Maurice Snook, ACS (2011).

Why not try some Christmas chemistry with your science and chemistry students during the last week before Christmas? The easy-to-do Christmas experiments in this article can be used to shake things up a little just before you break up for turkey roast and minced pie. With the instructions come suggestions about what previous knowledge can be discussed together with these experiments.

1 Orange Peel Flamethrower

This experiment is very easy and can be done by the students or as a demonstration. It is suitable for ages 11 to 16. It can even be done with or demonstrated to primary children if you trust them with candles.

All you need is a candle and some orange peel. First, you have to light the candle. Then fold your orange peel, the shiny, orange side should face the candle. As you squeeze the peel, oils from the peel will squirt into the flame. The oils ignite and produce beautiful sparks in the candle flame. In addition, this experiment smells very nice and Christmassy.

This short practical or demonstration can be used to discuss the flammability of different organic substances. The oil burning in the flames is a fat, which can be used to recap carbon chemistry. The combustion of the oil can be linked to oxidation reactions and exothermic/endothermic reactions.

2 Poinsettia pH Paper and Indicator

2.1 Background

This experiment is suitable for ages 11 to 18. It is adapted from a procedure by A.M. Helmenstine (2017).

The poinsettia flower originates form warmer climates. Nevertheless, many people use them as a decoration in their house during the winter holidays. Their red leaves contain substances that change colour when they are in contact with an acid or a base. For this reason, poinsettias are one of the natural pH indicators such as turmeric and red cabbage.

2.2 Procedure

You will need a poinsettia flower, a beaker, water, scissors, filter paper, a bunsen burner or a heating plate, a tripod, a funnel, a pH meter or universal indicator paper, 0.1 M HCl (hydrochloric acid), vinegar (dilute acetic acid), baking soda solution (10 g/ 1 dm3)  0.1 M NaOH (sodium hydroxide) and any other acids or bases you would like your pupils to test.

With scissors cut a few petals of a poinsettia plant into very small pieces and put them into a beaker. Add just enough water to cover the petal pieces and boil with Bunsen flame or heating plate for a few minutes until the water has taken the colour of the petals. Then, filter the liquid with a funnel and filter paper into a conical flask. This solution can already be used as a pH indicator solution.

To make pH paper, some of the solution needs to be poured onto a petri dish. Afterwards, pH paper is placed onto the petri dish to soak in the indicator solution. The filter paper has to dry and can finally be cut into pH strips.

The pH paper and indicator solution can be tested against different acids and bases, such as 0.1 M HCl (hydrochloric acid), vinegar (dilute acetic acid), baking soda solution (10 g/ 1 dm3) and 0.1 M NaOH (sodium hydroxide).

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2.3 pH Chart Challenge and Links to Previous Knowledge

The exact colour range for pH values can vary for different poinsettia plants. Students can be challenged to make their own poinsettia pH chart. For this, they will have to measure the pH of the test solutions above with a pH meter or universal indicator paper which already has a colour chart. They can then match their poinsettia indicator colour to the pH meter or universal indicator chart.

Obviously, you can link this practical to indicators and natural indicators as well as everything the pupils already know about acids and bases.

3 Thermal Decomposition of Sodium Bicarbonate and its Importance for Gingerbread

3.1 Background

This experiment is suitable for ages 14 to 18. A-level and more able KS4 students could even be challenged to plan their own investigation and experiment to answer the question. The question is: What happens to sodium bicarbonate when it is heated in the oven during the baking process?

Sodium bicarbonate (NaHCO3) is an important part of many gingerbread and cookie recipes. Sodium carbonate is also the major ingredient in baking powder which is often used instead of sodium bicarbonate. The task of sodium bicarbonate is to release gases when heated. These gases form bubbles and are trap

ped inside the dough during the baking process. This is important since the gas raises the cake and provides the “fluffiness” in cakes and cookies.

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This experiment can be adapted and shortened by omitting the calculations and deciding which reaction equation is right. This would still demonstrate that sodium bicarbonate loses mass when heated during baking as it releases water and carbon dioxide. The importance of this for baking can still be discussed and stressed with students.

3.2 Procedure

Pupils are provided with three possible reaction about what could happen during the reaction and they have to find out what is happening:

  1. NaHCO3 -> CO2 + NaOH
  2. 2 NaHCO3 -> H2O + 2 CO2 + Na2O
  3. 2 NaHCO3 -> H2O + CO2 + Na2CO3

For the experiment, pupils need to weigh in and write down an exact amount of sodium bicarbonate. 2 to 3 g are suitable. The scales should be as exact as possible for this task. The sodium bicarbonate is put into a crucible. NOTE: It is important that the students write down the weight of the empty crucible as they will have to weigh the thermal decomposition product inside the crucible. They should also mark their crucible with their name.

The crucible should be heated for 15 to 20 minutes at 180 degrees Celsius. This can be done in any oven. (Maybe your food department can help you out, if you do not have any oven and the crucibles have been thoroughly cleaned before the experiment.) It is also possible to heat the crucible over a bunsen flame for about 20 minutes.

More able pupils and A-level students can be asked to decide themselves at which temperature and for how long they want to heat their sample. (Having been told about the use of sodium bicarbonate in baking powder, they should be able to tell that a normal baking temperature and time for cookies should be sufficient.)

After heating the sample, students need to weight it again and write down the new mass.

3.3 Calculations

This part can be omitted. If you do, you should provide your pupils with the information about chemical reaction directly and discuss the importance of it for baking.

Their task now is to figure out which reaction is taking place during the thermal decomposition of sodium bicarbonate. They need to be given the information that the gasses formed are carbon dioxide (CO2) and water (H2O).

A-level and more able KS4 students can try to have a go at these calculations themselves. In other classes, I would do the calculation together with the class. Or at least model one of the three possible calculations,

The reaction actually happening is the third one from the list above:

2 NaHCO3 -> H2O + CO2 + Na2CO3

Molar masses (M):  NaHCO3: 84 g/mol;  H2O: 18 g/mol;  CO2: 44g/mol;   Na2CO3: 106 g/mol

Key equations:  substance amount = mass/molar mass     n = m/M

mass = substance amount x molar mass    m = n x M

The mass of sodium bicarbonate before the experiment is sodium bicarbonate NaHCO3, let us assume it was 2.0 g. This means we had 0.024 mol of sodium bicarbonate in the beginning (n = 2/84). For two mole of sodium bicarbonate, one mole of sodium carbonate, Na2CO3, is formed. This means we have 0.012 mol sodium carbonate which equals 1.272 g (m = 0.012 x 106).

Does this mass, 1.272 g, match the mass that we have after the experiment?

You can do the calculation also for sodium hydroxide (NaOH; M = 40 g/mol) and sodium oxide (Na2O; M = 62 g/mol) to show that it must be sodium carbonate which is formed. The calculated masses for sodium hydroxide and sodium oxide will not match the mass from the experiment after heating!

For sodium hydroxide, we would have a mass of 0.48 g (m = 0.012 x 40) and for sodium oxide 1.488 g (m = 0.024 x 62), if 2.0 g (= 0.024 mol) sodium bicarbonate are weighed in before the experiment.

3.4 Gingerbread and Links to Previous Knowledge

This experiment can be used to discuss thermal decompositions and endothermic reactions. As seen with the calculations, conversions between mas and amount of substance can be practiced and revised. In addition, the conversion of mass can be discussed.

My experience is that pupils really like the link to everyday life which is baking, especially during Christmas time. This aspect should really be stressed when doing this experiment.

When teaching this experiment earlier, I provided my students with gingerbread recipes when they left after the session. The students really appreciated this little give-away and I can recommend if you want to do this experiment.

4 Silver Christmas Decorations with Tollen’s Reagent

4.1 Background

This experiment is most for A-level students if they are to conduct it themselves. For younger ages, it is better as a demonstration. It is essentially Tollen’s test and demonstrates how reducing sugars reduce silver ions to silver. The method is adapted from the Royal Society of Chemistry and the Nuffield Foundation (2015).

4.2 Procedure

You will need bottles that should be as small as possible. (Small booze bottles are useful.)  These bottles need to be cleaned thoroughly and rinsed with purified water before the experiment. Further, 25 cm3 beakers, funnels, pipettes, silver nitrate (AgNO3, s), potassium hydroxide (KOH, s), glucose (dextrose) (= reducing sugar), ammonia solution, (NH3, aq) and concentrated nitric(V) acid, (HNO3, aq) and purified water are needed.

The following solutions need to be prepared, but NOT mixed before the experiment. The solutions should suffice for about 10 experiments.

  1. 5 g of silver nitrate in 500 cm3 of purified water to make a 0.1 M solution
  2. dissolve 11.2 g of potassium hydroxide in 250 cm3 of purified water to make a 0.8 M solution
  3. dissolve 2.2 g of glucose in 50 cm3 of purified water.

The following instructions are for a 50-cm3-bottle, the amounts will have to be adjusted for larger or smaller bottles. Place 15 cm3 of the silver nitrate solution in a 25-cm3-beaker. In a fume hood, add a few drops of ammonia until the brown precipitate dissolves. The colourless complex ion, [Ag(NH3)2]+ is formed.

Now, pour 7.5 cm3 of the potassium hydroxide solution into the 25-cm3- beaker and a dark brown precipitate of silver(I) oxide (Ag2O) is formed. Then, add a few more drops ammonia solution till the precipitate dissolves again.

The formed clear solution is called Tollen’s reagent. Pour this solution into your small bottle using a funnel and add 1 ml of the glucose solution with a pipette. Screw the cap on the flask and swirl the solution so that the whole of the inner surface of the flask is wetted. The solution will turn brown. Continue swirling until a silver mirror forms after 2 minutes.

4.3 Christmas Tree Decoration and Links to Previous Knowledge

After the experiment, wash the solution down the sink with plenty of water. Rinse out the flask well with water. A string can be added around the neck of the flask, so it can be hung up in a Christmas tree at home.

Obviously, this experiment can be used to link to learning about reducing sugars, but also to redox reactions. It can even be used to discuss noble metals and why silver is easily reduced considering the electrochemical series.

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References

  1. Poinsettia pH Paper and Indicator: M. Helmenstine (2017), https://www.thoughtco.com/poinsettia-ph-paper-604229, (25 November 2017)
  2. Silver Christmas Decorations with Tollen’s Reagent: Royal Society of Chemistry and the Nuffield Foundation (2015), Learn Chemistry, http://www.rsc.org/learn-chemistry/resource/res00000822/a-giant-silver-mirror-experiment?cmpid=CMP00004158 (25 November 2017).
  3. Images: com (25 November 2017)

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