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Sound Writing

Subsection 5.18.5 Discussions

The discussion synthesizes the findings of your experiment in the context of literature in the field. It should return to scientific concepts you introduced at the beginning of the lab report and clearly state how your results relate to them. The discussion should use the opposite structure of the introduction, beginning zoomed in on your specific results and then broadening in focus. It should clearly state how your results do or do not support your hypotheses. Then, it should zoom out and compare the results with current knowledge in the field. Be sure to compare the methods you used with other studies you examine. Don’t shy away from pointing out inconsistencies with other studies or potential errors in your work.
For undergraduate classes and especially at the 100 or 200 level, discussions for biology lab reports are much more broad than those for chemistry. Biology lab reports usually require more comparisons to outside published literature, while chemistry lab reports may focus more on clearly interpreting the results of the experiment. Biology lab reports also put a heavier emphasis on what questions are raised by the research and important directions for future work.

Note 5.18.22. Conclusions.

Some longer lab reports may also include a seperate conclusion, but this is unusual at the introductory level and not included here. The conclusion expresses the significance of your work, explaining how your results contribute to the field more broadly and potentially introducing ideas for further work. These same goals should be achieved in the closing paragraph of your discussion if a conclusion is not included.

Example 5.18.23. Discussion: Biology.

The results of the experiment supported the hypothesis that as the light intensity increases the amount of dissolved oxygen would also increase. The results also support the idea of a light saturation point with the plateau seen on the graph. As light intensity increases it reaches a point where adding more light will not increase the rate of photosynthesis because the chloroplasts have reached their saturation point. The overlapping of the standard error bars of the last four light intensities shows that there is no significant difference between the numbers, and the peak represents the amount of light E. canadensis can use most efficiently, which is 80 \(\mu\text{mol photons m}^{-2}\text{s}^{-1}\text{.}\)
Light saturation points have also been found in other studies on photosynthesis, mostly using photosynthetic algae. In the experiment referenced in the introduction, marine plankton algae were tested at different light intensities and had a saturation point after which more light no longer enhanced the rate of photosynthesis (John H Ryther, 1954). In addition, in a 1973 study Diner and Mauzerall measured photosynthesis in the algae Chlorella vulgaris and Phormidium luridium. Dissolved oxygen was measured in these algae after being tested in low light intensities and high light intensities using a repetitive-flash method. Both Chlorella vulgaris and Phormidium luridum reached similar saturation points around a light intensity of 180 \(\mu\text{mol photons m}^{-2}\text{s}^{-1}\text{.}\) The saturation point of the algae is notably higher than that of the tested E. canadensis at 80 \(\mu\text{mol photons m}^{-2}\text{s}^{-1}\text{.}\) The algae may have evolved higher light saturation points so that they can take full advantage of the light they get because their habitat, water, can limit the amount of light they can absorb. The E. canadensis does need to have this ability because they grow out of the water, unlike algae that is always surrounded by water and sometimes grows deep underwater. This study concluded that the C. vulgaris and the P. luridium reached saturation points where they could no longer increase their rate of photosynthesis because there was too much light for chloroplasts of their size to absorb (B. Diner, D. Mauzerall).
In order to address the importance of chloroplast size as a limiting factor for photosynthetic rate, another study investigated the potential of creating modified chloroplasts with continuous grana, the stack of thylakoid disks. Using the female thalli of Marchantia polymorpha grown on a petri dish the chloroplasts were isolated using a centrifuge at 2000 g for 1 minute. The chloroplasts were then modified and tested under very specific different light intensities. The saturation point was 43.2 \(\mu\text{mol photons m}^{-2}\text{s}^{-1}\) higher for modified chloroplasts, but a limit still existed (R Mache and S. Loiseaux, 1973). The experiment proved that there would always be a saturation threshold even with modified chloroplasts because larger chloroplasts still have a capacity limit and will regulate the photosynthetic rate. Further studies could compare chloroplast size and photosynthetic capacity between different kinds of photosynthetic organisms and seek to understand the evolutionary trade-offs of chloroplast size.
These three paragraphs explain one of the major results of the study, the existence of a light saturation point, and compare it to other published studies. The first paragraph makes it clear what the peak in dissolved oxygen production rate means. The second paragraph looks at two other studies that also found light saturation points. It’s important to include interpretation here. One strong statement in this example addresses the habitat difference between the study organisms, which could explain different values for light saturation. The third paragraph goes further by explaining a study that may help explain results observed in the lab. Finally, it presents ideas for further investigation in this direction.
In a longer lab report, multiple significant findings should be explained and compared to studies in this way. Think about what larger interpretive claims this leads you to: do the patterns line up? If not, how could the differences be explained or investigated? What further questions do these results raise?
When plants were kept in the dark, dissolved oxygen decreased over the 90 minute trial period. These plants used oxygen to convert sugars into usable energy through cellular respiration (Campbell et al 2009). In light independent settings the plant is required to use what is known as the Calvin cycle, where the plant converts carbon dioxide into glucose in the stroma, using what it had created from light dependent reactions to support itself through the light independent reaction (Campbell et al 2009). This process requires oxygen, explaining a loss in dissolved oxygen over time.
This paragraph addresses another result, the negative values for plants left in the dark. This interpretation is based on facts from the textbook. For a more advanced lab this result might also be compared to published studies, but because this phenomenon is universally understood and accepted it is appropriate to just quickly explain the science behind it here.
While patterns are clear, there was considerable variation between the three trials. This experiment did not control for temperature or time of day, which could have influenced the photosynthetic rate and created excess variation. The overall error is fairly minimal but does vary a lot with larger error bars at 140 and 200 \(\mu\text{mol photons m}^{-2}\text{s}^{-1}\) and small error bars at 30 \(\mu\text{mol photons m}^{-2}\text{s}^{-1}\text{.}\)
This paragraph addresses variables that were not controlled for by the experimental design. This is an important thing to address, and just saying “results may be due to human error” isn’t going to cut it in college. Think about any potential confounding variables.
This experiment supported existing literature on photosynthesis by showing that Elodea canadensis reaches peak photosynthetic efficiency around 80 \(\mu\text{mol photons m}^{-2}\text{s}^{-1}\text{.}\) Differences in photosynthetic efficiency between species may be explained by different chloroplast size, but their evolutionary purpose is not fully understood. Further research should be conducted to determine how photosynthetic efficiency and capacity are related to habitat.
The concluding paragraph of the discussion should remind the reader of the major results and their potential implications, which is especially important when the discussion is long enough that they might be overwhelmed. This last paragraph ties up what the author thinks is most important and shares a path forwards for this research. Thinking critically about what new research questions this work suggests is an important part of lab reports for biology!

Example 5.18.24. Discussion: Chemistry.

Determination of Unknown Gas 1
By comparing the calculated molar mass of Unknown Gas 1 to the molar masses of the gases given in Table 1, it was hypothesized that Unknown Gas 1 was Argon gas (\(\text{Ar}_{(g)}\)). This was because the calculated molar mass of the unknown gas, 39.97601g/mol, was very close to Argon gas’s ideal molar mass of 39.948g/mol. However, because the experiment dealt with real gases in real conditions, and the values given in Table 1 are for ideal gases, it was conceded that it was possible Unknown Gas 1 might have been carbon dioxide gas (\(\text{CO}_{2(g)}\)), which has an ideal molar mass of 44.009g/mol. Because both \(\text{Ar}_{(g)}\) and \(\text{CO}_{2(g)}\) are inert gases, the results of the flame test only confirmed that it was one of the two. To distinguish between which gas it actually was, the limewater test was used. Limewater [\(\text{Ca(OH)}_{2(aq)}\)], when mixed with \(\text{CO}_{2(g)}\text{,}\) forms a precipitate. The reaction for \(\text{Ca(OH)}_{2(aq)}\) mixed with \(\text{CO}_{2(g)}\) is given by:
\begin{equation*} \text{CO}_{2(g)}+\text{Ca(OH)}_{2(aq)}→\text{CaCO}_{3(s)}+\text{H}_2\text{O}_{(l)} \end{equation*}
Because the \(\text{Ca(OH)}_{2}\) formed a precipitate when it was added to the \(\text{CO}_{2(g)}\text{,}\) Ar(g) was ruled out as a possibility, and it was concluded that the identity of Unknown Gas 1 was carbon dioxide, \(\text{CO}_{2(g)}\text{.}\)
Determination of Unknown Gas 2
When the molar mass of Unknown Gas 2 was compared with the ideal gases’ molar masses in Table 1, it was hypothesized that the identity of Unknown Gas 2 was \(\text{Ar}_{(g)}\text{,}\) with a possibility of it being oxygen (\(\text{O}_{2(g)}\)). Although \(\text{Ar}_{(g)}\) was hypothesized to be Unknown Gas 1, it was proven to be \(\text{CO}_{2(g)}\) instead, so \(\text{Ar}_{(g)}\) was still a possibility (a term of the experiment was that none of the unknown gases would be the same as each other). Unknown Gas 2’s molar mass was found to be 35.9581g/mol, which is about equidistant between \(\text{Ar}_{(g)}\)’s ideal molar mass (39.948g/mol) and \(\text{O}_{2(g)}\)’s molar mass (31.998g/mol). \(\text{Ar}_{(g)}\) was selected as the more likely candidate because of the comparison between Unknown Gas 1 and \(\text{CO}_{2(g)}\)’s molar masses. The calculated molar mass of the unknown gas ended up being roughly 4g/mol less than the ideal molar mass of \(\text{CO}_{2(g)}\text{.}\) This is most likely because of the difference between actual and ideal gases (see Sources of Error). So, \(\text{Ar}_{(g)}\) was more likely to be Unknown Gas 2 because its ideal molar mass is roughly 4g/mol more than the calculated molar mass of Unknown Gas 2. The actual identity of Unknown gas was confirmed with the flame test. If the gas had been \(\text{Ar}_{(g)}\text{,}\) the inert reaction would have snuffed out the flaming matchstick. However, if the gas had been \(\text{O}_{2(g)}\text{,}\) which supports combustion, the gas inside the flask would have combusted. The flame test showed that Unknown Gas 2 was an inert gas, supporting the hypothesis that it was \(\text{Ar}_{(g)}\text{.}\) To definitively confirm the gas’s identity and rule out \(\text{CO}_{2(g)}\) as a possibility (despite the fact that \(\text{CO}_{2(g)}\) had already been confirmed as the identity of Unknown Gas 1, and no gas could be used twice), the limewater test was applied to Unknown Gas 2. When the dropper of \(\text{Ca(OH)}_2\) was added to the flask and swirled, no precipitate formed, confirming that Unknown Gas 2 was Argon gas, or \(\text{Ar}_{(g)}\text{.}\)
Determination of Unknown Gas 3
The experimental molar mass of Unknown Gas 3 (42.5765g/mol) was compared with the ideal molar masses of the gases given in Table 1, and from that it was hypothesized that Unknown Gas 3 was propane gas, or \(\text{C}_3\text{H}_{8(g)}\text{.}\) This hypothesis was supported by the fact that the first two unknown gases’ actual molar masses were consistently less than the gas they were identified as. So, because \(\text{C}_3\text{H}_{8(g)}\)’s ideal molar mass is 44.097g/mol and Unknown Gas 3’s experimental molar mass was 42.5765g/mol, \(\text{C}_3\text{H}_{8(g)}\) was likely to be the identity of Unknown Gas 3. The other possible identities of Unknown Gas 3 were \(\text{CO}_{2(g)}\) and \(\text{Ar}_{(g)}\text{,}\) because they had the ideal molar masses closest to Gas 3’s molar mass, but they were deemed unlikely to be the gas because they had both already been identified as Unknown Gases 1 and 2, respectively, and no gas would be used twice. The flame test for Unknown Gas 3 confirmed the hypothesis that Gas 3 was \(\text{C}_3\text{H}_{8(g)}\text{.}\) \(\text{C}_3\text{H}_{8(g)}\) supports combustion, and so if the gas had been \(\text{C}_3\text{H}_{8(g)}\) the gas inside the flask would have caught fire when exposed to a flaming matchstick. This is exactly what happened, and so definitively ruled out \(\text{Ar}_{(g)}\) and \(\text{CO}_{2(g)}\) from the list of possibilities, as both are inert gases and would have caused a flaming matchstick to be extinguished. So, Unknown Gas 3 was identified as propane gas, \(\text{C}_3\text{H}_{8(g)}\text{.}\)
These three paragraphs walk the reader through an interpretation of the results for each unknown gas. Subheadings make for excellent organization. The author explicitly lays out how the data presented in the results leads to the conclusions drawn by the author. Take the last four sentences of Unknown Gas 3 as an example. First, the author asserts a conclusion: that the flame test confirmed the identity of the gas. Second, they pull in outside information supporting the conclusion, which is that \(\text{C}_3\text{H}_{8(g)}\) is combustible. Third, they describe the result, that the gas caught fire. Fourth, they bring in additional outside information (that \(\text{Ar}_{(g)}\) and \(\text{CO}_{2(g)}\) are inert) to confirm their conclusion. Finally, they re-assert the conclusion so it is clear to the reader.
An important distinction from the biology example is the way outside information has been used. In the biology example, results are compared to similar published studies to look at larger trends. Here, the outside information used is molar mass and flammability information that is known for the gasses in question. These data are not cited because they are chemical facts drawn straight from the lab handout. While this lab teaches important chemical concepts and techniques, it is far from representing novel knowledge in the field, and drawing comparisons to contemporary chemical literature (or even literature from the 1970s) would be forced and inaccurate. Thus, the emphasis is much narrower: instead of demonstrating context in the field, the author just shows how they used their results to draw their conclusions. In higher-level chemistry classes where experiments are more novel, more outside literature will be required. However, discussions at an undergraduate level should still avoid the kind of broad statements that might occur in a biology paper.
Sources of Error
While all the tests led to conclusive results, there was still the possibility of error during experimentation, which could have affected the values and qualities recorded. A possible source of error in the experiment is the equation used to calculate the molar mass of the unknown gases. The ideal gas law was used, which is given as: PV=nRT, or PV=gRTMW, Where P is the atmospheric pressure, V is the gas volume, n is the number of moles of the gas, R is the universal gas constant, T is the temperature, g is the mass, and MW is the molar mass (also known as the molar weight). The ideal gas law assumes that the temperature and pressure are constant, and that no forces act upon the gases except for the negligible ones created by the gas atoms colliding momentarily. It is nearly impossible to ensure perfectly constant temperature and pressure, and to remove all external forces, so the calculated molar mass of the unknown gases could not have been equal to the ideal values given in Table 1. By possibly giving a molar mass value closer to the ideal value of a gas that was not the actual gas, the predictions of the identity of the unknown gases could have been less accurate. However, this source of error was compensated for. It was recognized that the calculated molar masses of the unknown gases were consistently less than the ideal molar masses, and so the predictions were based on the ideal gases with molar masses slightly larger than the experimental molar masses.
The discussion is also a space to discuss potential sources of error in your experiment. This is most important if your results were inconsistent, like if your percent yield is very low or over 100% or (in this example) if flame test results and molar mass didn’t lead to one conclusive identification. If you are comparing to outside sources and your results disagree with the literature, it’s important to discuss what differences in methodology could have led to different results. Novel results can be valid, but should be assessed critically! In a college course, “human error” as an explanation is insufficient: be specific and thoughtful about what could have impacted your results.
The three gases identified in this experiment were carbon dioxide (\(\text{CO}_{2(g)}\)), argon (\(\text{Ar}_{(g)}\)), and propane (\(\text{C}_3\text{H}_{8(g)}\)). These three gases are fairly similar in that they are heavier and denser than air, yet all have their own unique properties that make them identifiable from each other. Carbon dioxide forms a precipitate when mixed with limewater, propane is combustible, and argon is a non-reactive inert gas. All of these gases are present in earth’s atmosphere naturally, though in mostly very small quantities. By isolating these gases and working with them individually, it was possible to examine their separate properties with negligible interference from the rest of the atmospheric gases
The closing paragraph summarizes the report by restating the most important results. Because this experiment is more about learning techniques and chemical properties than contributing to research, there’s less of an emphasis on future research than in the biology example. Ideas for future research become more significant in upper division Chemistry courses.