Reading exercise: Antibiotics and bacteria resistance

The following text is written for pupils with low reading ages to help them study the topic of antibiotics and bacteria resistance.

Before antibiotics

Think back to the last time you cut yourself. Can you imagine that cut becoming infected with bacteria – so seriously infected that you would die?

Before the discovery of antibiotics, there was nothing anybody could do. Bacteria could kill 80 percent of people with infected wounds.

Who would have thought a mouldy plate would lead to this?

In 1928, Alexander Fleming, a doctor at London’s St. Mary’s Hospital, found that a mould on a discarded plate had antibacterial properties. This mould was ‘penicillin’. Penicillin is an antibiotic.

Antibiotics kill bacteria and slow down their growth. A bacterium consists of one single cell and antibiotics disturb their cell functions. Antibiotics do not work against viruses because a virus consists of a DNA fragment instead of a cell.

Human life expectancy increased rapidly by eight years when antibiotics were first introduced in the 1930s.

Bacteria resistance

Within four years of penicillin being introduced onto the market, bacteria resistance was being reported. Bacteria resistance means that an antibiotic no longer kills the bacteria.

Today bacteria resistance against commonly used antibiotics is increasing rapidly around the world and a growing problem.

Active Reading Exercise: What is an echo?

The following active reading exercise includes a short test and tasks suitable for students aged 11 to 14 when studying waves and sound.

When waves hit a surface, they are reflected. This means they bounce off the surface and come back. For example, light is reflected by the surface of a mirror.

When you are high up in the mountains and call out in a loud voice. Your sound waves will be reflected by a mountain surface nearby and you can hear it coming back after a few seconds. This is called an echo.

You can measure the distance from where you stand to the mountain surface and time how long it takes until your hear the echo. With this information you can calculate the speed of sound.

This principle is used by animals like dolphins and bats for navigation. They send out sound waves and listen for their echo. This helps them to work out how far away predators or food are. When used like this echoes are called sonar.

Sonar is also used by submarines for navigation. Submarines send our sound waves and listen for the echo to know their own position. They can also detect other submarines and ships.

Tasks:

  1. Box the words in the text you do not know.
  2. Highlight what happens to waves when they hit a surface.
  3. Highlight the word that describes the reflection of sound waves.
  4. Circle the two types of waves that are mentioned in the text.
  5. a) Underline the animals that use sonar.
  6. b) Underline how submarines use sonar.

*Extension: Explain what a dolphin needs to know to work out the distance to a fish when using sonar.

Which liquid dissolves candy canes fastest?

  1. Get four beakers. Fill one beaker with cold water, one with hot water, one with oil and one with vinegar. The hot water can be taken from the kettle.
  2. You need to put one candy cane in each beaker at the same time and start your timer.
  3. Record your observations.
  4. Record the time when the first candy cane has dissolved. BUT, do not stop the timer.
  5. Record the time when the 2nd and 3rd candy cane have dissolved too.
  6. Write your conclusion and state which liquid dissolves the candy cane fastest and which is the slowest.

Orange peel flamethrower

Here you can see the orange peel flamethrower in action.

The instructions are the following:

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.

So, go a head and surprise your family and friends with some amazing chemistry at Christmas dinner.

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).

Poinsettia-1024x679

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.

christmas-cookies-2918172_960_720

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)

How to make your own soap – A science practical for homemade christmas presents

Chemistry pratical or Christmas present? This can be both…

A tang of science

Following my last article about the chemistry of soap, I have become interested in making my own soap. From my research I have put together a practical/experiment that can be used in science lessons for students to make their own soap. The available scents with this recipe are coconut and cocoa. You can of course also try this yourself if you want to make some nice christmas presents.

The experiment works via the saponification reaction between fats from vegetable oils and sodium hydroxide from lye. The students will learn about saponification, how soaps work and how they can be made industrially.

1. Introduction

Humans have been been making and using soap for at least 2300 years. In all that time the manufacturing process has not changed much. Fats from animals or plants are reacted with sodium hydroxide in a reaction called saponification where soap is produced and glycerol is a…

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The Chemistry of Making Jam

It is early autumn. Your fridge and freezer are overflowing with raspberries, black berries and other berries you have collected over the summer. What to do to make room? Make jam! Besides preserving fruits and berries, it is also great fun with some interesting chemistry involved. You can see the result of my first attempt to make jam from black berries I picked during late summer in the image above.

What do you need to make jam? You will need equal amounts of fruit and sugar in weight. I will refer to a recipe used for 1 kg of fruit and 1kg of sugar. The task of the sugar is to bind water and draw it away from the fruit. With no water available, no mould will be able to grow on jam which causes the preserving effect. As you can see, it is important to use enough sugar, but too much of it will lead to the formation of sugar crystals. In addition, acidity is needed, so the jam can set properly. Fruits naturally contain some acids, but more will have to be added for certain fruits like berries to reach the perfect pH for jam to set, which is at 2.8 to 3.3. To reach this pH-value, you can either add citric acid or use jam sugar where citric acid is already contained. I chose another method and added the lemon juce pressed from one lemon which naturally contains a lot of acid.

What happens after mixing together fruit, sugar and acid? The mixture is first heated and stirred until all the sugar has dissolved and the boiling point is reached. Then continue to boil gently for 15 to 20 minutes. What happens now is that the pectin molecules, which consist of long chains of sugar molecules linked together, react with each other to form a network of long chains. This reaction happens at 104.5 degrees Celsius and it gives jam its gel-like texture. Fruits contain pectin, but in different amounts. For example black berries that I chose to make my jam do not contain enough pectin to form a gel. In this case, jam sugar can be used, which contains pectin additionally to the citric acid. It is also possible to add one fruit that is high in pectin to the jam making process, which could be an apple or a citrus fruit. Instead, I decided to add four table spoons of grated lemon zest, which also contains pectin. In fact, pectin added to jam sugars is made from the peels of citrus fruit.

The jam is ready to pour into jars, when enough pectin molecules have reacted to form the gel-network. You can test when this point is reached by taking some of the jam up with a cold spoon and letting it cool. Then observe how the jam moves when you move the spoon around. It should just stick to the spoon and stay in one place, when the pectin network has set sufficiently. Make sure, to wash your glass jars with soap and hot water before using them. You can even heat them in an oven at 160 degrees Celsius for sterilization. To create a vacuum inside the glas jars, put them into a pan with hot water before filling them. Pour the jam into the jars when it is still hot, then screw the lit on and remove them from the hot water bath. As the glass jars cool, the vacuum will slowly come on. This is yet another precaution, to make the jam last as long as possible.

Making jam proved to be great fun for the chemist, the food enthsiast and the environmental activist inside me. It is a great way to preserve locally grown berries and fruits for the winter which is  much more environmentally friendly than buying berries in winter that have come all the way Chile or New Zealand. It is also better than having jam from a supermarket of which you have no idea where it comes from and under which conditions it has been been made. In addition, I found that opening a glass of my own home-made jam is far more exciting and satisfying.

What are Ionic Liquids and what are they good for?

Image: An ionic liquid tested for the application in lithium-ion batteries at Chalmers University of Technology Gothenburg and Uppsala University (Sweden).

Ionic liquids are basically salts with low melting points of under 100 degrees Celsius. Some might pause now because in school we are normally taught that salts have very high melting points. This is true for most – let us say – ”common” salts as for example table salt with the scientific name sodium chloride (NaCl). Sodium chloride has a melting point of 801 degrees Celsius which I guess can be considered a high melting point.

Salts consist of positive and negative ions that attract each other electrostatically which results in the formation of ionic bonds. This is how they form the compounds that we know as salts. Sodium chloride contains positive sodium ions and negative chloride ions. The strong attraction between the positive and negative charges causes the high melting points we observe in most salts.

Nevertheless, salts with low melting points and ionic liquids do exist. Why is that? The melting points of a salt are generally lower, the larger the ions are that it is made up from. For example, when changing the positive ion ( also called cation) in sodium chloride from sodium to the bigger rubidium ion, the salt rubidium chloride is obtained which has a melting point of 715 degrees Celsius. The melting point of rubidium chloride is 86 degrees Celsius lower than that of sodium chloride because the rubidium ion is bigger. When choosing even larger postive ions, for example the organic cation 1-ethyl-3-methyl-immidazolium, a salt called 1-ethyl-3-methyl-immidazolium chloride (a real tongue breaker) is obtained which has a melting point of only 89 degrees Celsius. This salt is an ionic liquid which melts under 100 degrees Celsius. We arrived at this ionic liquid simply by exchanging the positive ion (cation) with larger ones.

Also, the negative ions (also called anions) – the chloride ion in our case – can be exchanged with larger ones. When switching from the chloride ion to the organic anion bis(trifluoro methane sulfonyl) imide (another tongue breaker), the melting point can be lowered even more. When putting together the anion bis(trifluoro methane sulfonyl) imide with the cation 1-ethyl-3-methyl-immidazolium a salt with a melting point of -15 degrees Celsius is obtained. This salt (I will spare you the name) is a room temperature ionic liquid (RTIL). This is indeed a salt which is liquid at room temperature.

So, why do chemists make liquid salts? I admit a salt that melts at a lower temperature than water is pretty cool, but are they good for anything? To answer this question, we have to first look at the properties. The most important properties of ionic liquids are that they can conduct electricity due to their mobile ions and that they are stable even at high temperature which also means that they are non-flammable and cannot easily catch fire. Especially, their good high-temperature staibility makes ionic liquids interesting for many applications, for example as high-temperature lubricants in machines or as solvents for chemical reactions in the industry. Due to the conduction of electricity, ionic liquids are also tested in solar cells and batteries.

I have talked to Manfred Kerner, a PhD student in the Applied Physics Group at Chalmers University of Technology in Gothenburg (Sweden) who is working on the application of ionic liquids in batteries. Kerner is trying to use ionic liquids as the electrolyte (= the liquid placed between two battery electrodes) in lithium-ion batteries. One big problem lithium-ion batteries face is the high flammability of commonly used electrolytes which is a part of the reason for many youtoube videos showing laptops on fire. Ionic liquids can be a solution to this problem because they are stable at high temperatures and do not catch fire, while they are able to conduct electricity. These properties are especially useful for high-temperature lithium-ion batteries which run at temperatures higher than room temperature and are popular in hybrid electric vehicles.

Nevertheless, Kerner admits that there are still some hurdles to overcome until ionic liquids can be broadly applied in batteries. One example is their high cost. Another is that ionic liquids strongly attract water from air which damages the electrode materials of lithium-ion batteries. For this reason the use of controlled water-free environments such as gloveboxes is necessary. In addition, ionic liquids are not stable in contact with all the possible battery electrode materials which limits their use.

Despite these drawbacks some ionic liquids are already on the market as electrolytes for batteries according to Kerner. However, they are so far mainly limited to research applications. Nevertheless, some day in the future we might find ionic liquids in our own laptop or car batteries.

Can we see atoms?

Image credit: Pixabay 2017.

photos of atoms

Background

There is a large number of physical and chemical techniques available today for the analysis of substances. They provide the opportunity to gain information about the properties of atoms, molecules and ions. Infrared spectroscopy, for example, measures the vibrations of molecules. Mass spectrometry, which is widely known from TV crime dramas, gives information about the weight of molecules or ions. This information about mass can then be used to identify poisons, drugs and other substances.

However, most techniques cannot give us a direct picture of what atoms, molecules or ions really look like. Normally, conclusions about their looks are drawn from measured properties. Imagine you are drawing a picture of a house while somebody is telling you about its appearance, but you cannot see it yourself. This is how physicists and chemists use the information from their measurements to figure out what atoms, molecules and ions look like. The measurements are giving them information about substances, which corresponds to the information about the house, such as colours; size or where doors and windows are.

But are there any methods that can provide us with – let us say – a photo of an atom? The main problem is that atoms are extremely tiny. One atom in an apple is as small as the apple itself is compared to our entire Earth. Nevertheless, there are a few methods that can indeed capture pictures of atoms with the help of quantum mechanics and other cool science. A few of these methods are listed below.

Transmission electron microscopy (TEM)

Atoms cannot be seen under normal light microscopes. The reason is that the wavelength of visible light is much larger than atoms themselves are. To be able to see a sample in a microscope, the wavelength has to be smaller than the sample itself. For example, light waves are smaller than cells which is why cells observed in light microscopes. Transmission electron microscopes use electrons instead of visible light. Thanks to the wave-particle duality, electrons can behave as both waves and particles. As waves, they have a much smaller wavelength than light which makes it possible to see atoms in a transmission electron microscope. Besides the use of electrons, the working principle of a TEM is the same as that of a light microscope.

Scanning tunneling microscopy (STM)

Scanning tunneling microscopy is another microscopic technique. It is based on a quantum mechanical phenomenon called ”tunneling”. Electrons can ”tunnel”, or in other words transtition, from an atom on the tip of an extremely sharp needle to atoms on a sample surface. Needle and surface have to be at a very short distance from each other to enable tunneling. The probability of tunneling increases when the gap between the needle and the surface gets smaller. This means that electron tunneling is more likely to occur when the needle tip is above the center of an atom as compared to the tip being above the space between two atoms. In consequence, a topographic picture of the atoms on the sample surface can be obtained.

Atom probe tomography (APT)

Atom probe tomography is a very powerful technique that can provide three-dimensional images of a sample’s atomic structure. In APT, the magnification is caused by a highly curved electric field instead of electron properties like wavelength or tunneling. For this technique, atoms have to be removed from the sample surface and turned into ions, in other words they have to be ionized. To obtain three-dimensional information, the atoms are removed from the sample layer by layer, which means that, unlike the previous two methods, this technique is destructive.

The Isolation and Detection of Starch – A Practical for Science Lessons

1 Goal

In this lab you will isolate starch from potatoes and investigate if different food samples contain starch. This is done with the help of Lugol’s solution (iodine/ potassium iodide solution).

2 Introduction

Starch is an organic compounds that belongs to the carbohydrates. Carbohydrates are an energy storage for both plants and animals. Starch molecules are very long and the building blocks repeat themselves. They form long chains and belong to the so-called poly saccharides. The two building blocks of starch are amylose which forms spiral chains and amylopectin which forms branched chains. Both are built up from glucose rests which is why the chemical formula can generally be written as (C6H10O5)n. Starch can be found, e.g. in root crops and grains.

The presence of starch can be detected with with the help of Lugol’s solution which is a mixture of iodine and potassium iodide dissolve din water. Potassium iodide is added to increase iodine’s solubility in water. Iodine molecules (I2) are stored in the spiral chains of amylose when Lugol’s solution (brown solution due to the iodine) is added to the starch. This storage compound (”iodine starch”) causes a blue-black colour. A schematic of the compound is shown to the right in the figure above.

3 Materials and Chemicals

  • Two Potatoes
  • Different other foods: Flour, bacon, cheese, apples, pasta and rice are recommended
  • Lugol’s solution (= iodine/potassium iodide solution)
    – Preparation: Dissolve 10 g potassium iodide in 100 ml of distilled water. Then slowly add 5 g iodine crystals, while shaking. Finally, lter and store in a tightly stoppered brown glass bottle.
  • Knife and Spoon
  • Several Test Tubes with Gummy Plugs
  • Mortar
  • One 250 ml-Beaker and one 800 ml- or 1000 ml-Beaker
  • Heating plate
  • Linen cloth
  • Funnel
  • Two Bowls
  • Grater

    4 Implementation

    4.1 The isolation of starch

    First, the potatoes need to be cleaned and peeled. Thereafter, they are grated and put into a bowl. 500 ml of water are added and the mixture is stirred thoroughly with a spoon for at least five minutes. A linen cloth is placed in a funnel and the mixture is pressed through into a large beaker (800 ml or 1000 ml). A part of the grated potatoes, for example the cellulose, will stay behind in the linen cloth. The liquid in the large beaker needs to stand and rest for approximately ve minutes. Then more water is added, approximately 100 – 200 ml. The finely dispersed, solid starch particles will slowly settle at the bottom of the beaker. Afterwards, the
    water is decanted (= poured off). Then 100-200 ml of water are added again to
    the large beaker and decanted when the solid starch particles have settled on the
    bottom a second time. This cleaning step is repeated until the starch particles have
    a completely white colour. Afterwards, the starch is dried in a at bowl in air and
    at room temperature.

    4.2 The detection of starch

    The foods are crushed in a mortar and and small amounts of each are put into their respective test tube. The test tubes are filled up to a third with water and shaken vigorously. In case not all the food particles are suspended, the test tubes are heated in a water bath (water bath = a 250 ml-beaker filled with water and the test tube inside is heated carefully on a heating plate, the test tube is afterwards cooled under owing, cold water). Then one drop of Lugol’s solution is added to each test tube and the test tubes are shaken with a gummy plug on top.

5 Questions for Discussion

1. What is observed macroscopically when iodine is built into starch molecules? What happens when no starch is present?

2. Which function does starch have?

3. In which foods do you expect to detect starch? In which foods should there be no starch?

4. Do the results in the table match your expectations? If not, why could they be different?

References

Image retrieved from: Petra Mischnick, Skolan för kemivetenskap , Kungliga Tekniska Högskolan, Stockholm, 11 January 2013 (https://www.kth.se/che/archive/arkiv/molnov-1.272910, 30 August 2017).