A Guided Introduction to Experimental Design and Optimization

For students not used to creating their own protocols, the process of guiding them to do so can be arduous. Many students are used to being given a list of steps to follow, and the slightest deviations from the classroom norms are met with tremendous amounts of hesitation. After all, the act of composing a list of steps does not come naturally to many students but can be gradually introduced. At Acera, we frown on those “cookbook” style labs and seek to introduce the skills of invention and scientific inquiry into our lab projects. The introduction of those tactics to some students, however, is incredibly challenging, so the goal of our November experiment was to teach the basics of experimental design by having students alter the ratio of 2 chemicals to see how it changed the overall chemical reaction.


Potassium Nitrate, or “salt peter”, is a well known oxidizer or donor of electrons. When it encounters a reducer, such as dextrose or sucrose, and a flame, it produces lots of non-toxic smoke. Increasing the sugar slows the reaction to the point at which only a trickle of smoke that lasts up to an hour emerges. On the contrary, changing the nitrate concentration increases the rate of reaction and the smoke output. I asked students to design an experiment to test which ratio of potassium nitrate and dextrose produces the most smoke. The experimental process is simple: Combine a 5 part ratio of nitrate:dextrose with 1% sodium bicarbonate in a 600 mL tempered glass beaker and heat while stirring with a strong wooden stick at 250-300C for 15-30 minutes until the sugar is melted, dark brown, and paste-like in consistency. The resulting mixture is then scraped onto aluminum foil and allowed to cool. Finally, it is brought outside and the teacher can safely light the dried paste on a metal baking sheet while the students watch from a minimum of 5 meters back.


It is important to note that this is a very strong reaction, and there are many safety considerations. We mandate that students wear lab coats, a face shield or goggles, and two pairs of gloves during the heating. Also, when the time comes, the students must not light the reactions. The ignition and resulting smoke can emerge very quickly. It is also recommended that the teacher practice and demo this lab extensively in advance to gain familiarity with the overall process.


So, the question is, why perform such an extreme reaction when more mild ones can be substituted? After all, there are pretty easy ways for students to work with chemical ratios, including baking soda and vinegar. Simply put, the students want to work with something that is a little bit more reactive and will naturally pay closer attention when they are working with something that they perceive is “dangerous.”


Obviously, safety is a top priority, and our students have undergone extensive safety training that was detailed in a prior blog entry. The training was to prepare students for adverse events that may happen when working with chemicals that are beyond the norms of traditional school science. Having this lab tech training means that they can do things that are a little bit out of the realm of normal. A more dramatic reaction will really help them to understand the nature of the chemicals, the energy transfers involved, redox reactions, and the importance of experimental ratios in guiding research. Finally, the students will think it’s cool!


This is an important point, and once they start seeing the different reactions, they will attempt to optimize their ratios and experimental design to create even more smoke producing reactions. The streamlining of protocols is a very important skill for scientists to have and can be used for all future experiments. For the teacher, it is also crucial to remind students to keep a running log of their results and adaptions to their protocols. After all, the purpose of this lab is to optimize an experimental ratio and work on lab report writing skills.


Another facet of this experiment is allowing students to collect their own lab results. They know that they need to find the reaction that produces the most smoke, but the students are able to come up with different means of getting that information. Some measured duration, others took pictures and isolated the smoke hue, while still others measured the meters away from the reaction in which the smoke plume was visible. No matter what method the students used, they all collected a set of data points that they could use to compare the different ratios.

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Once the students had completed the ratios or exhausted their lab time, the conclusion was completed and a number of different topics were addressed. I started the post lab by asking the students what things the conclusion should contain. They mentioned discussing which reaction was the strongest and why, on a chemical basis, the reaction behaved the way it did. The student reactions were varied. They responded with a variety of tie ins to chemistry including energy generation, the science of redox reactions, and predicting reaction products between the nitrate, dextrose, and sodium bicarbonate.


From here, there are a number of jumping off points for future discussions including redox reactions, energy, and even the environmental effects of CO2/CO. As for the lessons taught in this lab, experimental design is something that can be worked on and perfected throughout the year. The more the students get exposure to true inquiry, the easier it will get. Like all skills, it needs to be reinforced and practiced to become second nature.


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