Different strokes with starch
Jyotsna Vijapurkar, Aakanksha Sawant, Swapnaja Patil
The starch test is an easy-to-do test, commonly included in school textbooks (generally from grades 6 through 9). All one has to do is add a few drops of iodine to test if something contains starch; if it does, it turns blue. This test is usually done on potatoes or rice or cereal flours. Even if the books say that students can use any food item for the test, the figure alongside this activity often depicts the test being done on a potato. Doing the test this way, with predictable outcomes, is rather dull and boring, and indeed barely even a test in the real sense! More importantly, opportunities for a host of elements of a scientific inquiry are missed – exploring, generating questions, guessing what the answers might be, figuring out how to find answers to new questions that may arise, and perhaps doing further tests to answer those questions. How can these elements be included in this experiment? By simply presenting it in a slightly different way, and including items other than potato and rice which, as students have already been taught, are sources of starch. Here we share how we have been doing this test with groups of teachers, to demonstrate the power of a well conducted experiment.
Making it truly investigative, i.e., doing the experiment to find out the answer
We begin by asking teachers about the various carbohydrates they have had for a recent meal and jot them down on the board, marking those that they think contain starch (all carbohydrates are not starch!). We then add some more items, ones that the class may not be sure about (and neither were we, until we did the experiment). Our list includes bananas (ripe) and plantains (raw); brinjals; sugar; pulse and cereal flours; boiled eggs. So we start by asking for a guess of which contain starch, which don’t, keeping count of the ‘ayes’ and ‘nays’ on the board next to each item. Everyone has to make a guess, or explicitly state they are not sure. Now the whole class is eager to find out the result of the test.
Often, as they attempt a prediction, the reasoning serves to bring out alternate conceptions that they may harbour. A lively discussion ensues as other participants and observers also get involved in trying to solve the ‘puzzle’.
Uncovering long-held, textbook-generated misconceptions
Recall the time when you were in school and did a lesson on ‘Food and Nutrition’. Which category did dal fall into? Protein, right? This leads students to believe that dal contains nothing but proteins, and similarly, cereals (and potato) contain nothing but starch.
To the surprise of most participants, dal (any dal) flour does turn blue. So strong is the impression carried over from the nutrition lessons, the conjecture commonly put forth to explain this anomaly is that the powder must be adulterated (cereals generally are cheaper than pulses). So how can we figure out if this is indeed the reason for the blue colour? Well, pound some dal in front of the class and then add iodine. Watch it turn blue again, and watch the response. A good time to point out that no more than about 25% of most dals is protein; the rest is carbohydrates (plus some moisture, fibre, fat and minerals).
Similarly, we had someone who once said brinjals wouldn’t turn blue, because they are vegetables and have vitamins and minerals. You should do the test with brinjals, which show some very interesting results. We are still trying to figure those out!
Making connections
Another characteristic of inquiry is to help make connections to create a coherent picture, instead of learning in fragments as is generally done in schools. Take the case of onions, and the discussion around it: “Maybe, because it is a plant part that is for storage, it does have starch?” “No, because in onions, carbohydrates are stored as sugars.” We suggest, “Let’s do it and see!” When we add iodine, there is one part that turns blue. Why does it? Now the discussion moves to parts of the onion and which part is a modification of what – something that would normally only be covered in a botany class. Another case – after some time has elapsed after adding iodine to even an item like sugar that contains no starch, the iodine turns darker. We were once asked that a solution of sugar be used instead since it was indeterminate, with confusing colour change, with crystals. Just asking “Why is iodine kept in dark bottles?” launches a discussion on oxidation of iodine. The seeds of brinjal turn quite dark after the vegetable is cut. So how do you know if it is because of starch reacting with iodine? One needs a control! Take two freshly cut slices and add iodine on one.
Summing up: the teacher’s act
To be honest, we knew what happens to brinjal or onions, because we had learned it in a previous presentation of this experiment. However, we put up an act of not being sure ourselves. We accepted all questions and encouraged answers, thus increasing engagement and participation in the activity for a long time. We did not give, but got them to arrive at explanations. Participants have stayed behind beyond the scheduled time, for as long as 30 minutes, to discuss the observations made and, more importantly, the pedagogy adopted. Experiments that were not conclusive were refined and repeated by the teachers and researchers, to resolve disagreements. The goal of this entire exercise was to demonstrate how teachers can adopt inquiry practices in their classrooms. Even a simple and common experiment which doesn’t require sophisticated equipment can be made challenging, engaging and conceptually rich, all at the same time.
As an observer remarked at the end, “I never knew the starch test could be so interesting!” Indeed, for a couple of days after this demonstration, we ourselves bring new stuff to test.
The authors are at the Homi Bhabha Center for Science Education, Tata Institute of Fundamental Research, Mumbai. They can be reached at jvijapurkar@gmail.com, aakankshasawant1992@gmail.com, swapnajapatil17@gmail.com.
