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Sound Writing

Subsection 5.18.3 Materials and Methods or Experimental Sections

Materials and methods sections (in biology) or experimental sections (in chemistry) contain a straightforward explanation of how you carried out your lab experiment, addressing any methodological choice that may have impacted your results. They should be written with enough detail that a trained scientist in the field could duplicate the experiment. This should include details like chemical concentrations and number of replicates. However, you don’t need to explain standard techniques that someone with in-field knowledge would understand. Also avoid giving a historical narration of what you did: for example, “Professor Saucedo brought us the solution and we began to pipette it” is unnecessary detail. If the logic behind a specific method isn’t obvious, include a brief explanation. The style and content of materials and methods or experimental sections is similar between biology and chemistry, but chemistry has a more standardized subheading structure for distinguishing between experiments and general information.

Note 5.18.14. Style for Materials and Methods/Experimental Sections.

In terms of style, the Materials and Methods/ Experimental section is written in past tense, because the experimental work took place in the past. Stylistic preference for active or passive voice can vary.

Example 5.18.15. Materials and Methods Section: Biology.

In order to measure the photosynthetic activity of Elodea canadensis, many 10 cm sprigs were dark adapted (deprived of light for a minimum of four hours). One glass bottle was then filled with 500 mL of 0.1% sodium bicarbonate solution. The initial oxygen content of the solution was measured using an oxygen meter (Extech Model 407510) while stirring for 60 seconds. Two 10 cm sprigs of the dark-adapted Elodea canadensis were then placed into the bottle and capped. The bottle was placed in front of a bank of lights on a light table and left at a specified light intensity for 90 minutes. Light intensities were manipulated by changing the distance from a lightbulb. The specified light intensities utilized within all lab sections were 0, 10, 30, 50, 80, 140, 200, and 380 \(\mu\text{mol photons m}^{-2}\text{s}^{-1}\) and were measured with a light meter (Li-Cor LI189). After 90 minutes, the Elodea canadensis was removed and the final dissolved oxygen of the solution was measured in the same way as before (same procedure as initial reading). Seven independent trials were performed at each light intensity. The average changes in dissolved oxygen content were normalized as rate per hour (mg/L-hr) for each light intensity and were analyzed by standard error.
This is a great example of using an appropriate level of detail. It includes everything another scientist might need to know to replicate the experiment, like the light intensity measurements and time periods. Details such as the specific instruments used should be included paranthetically. However, it doesn’t include details that might appear in the lab handout but another scientist could do differently or figure out on their own (for example, how to cut sprigs of Elodea canadensis, how to use a stir plate, or the bottle sizes used for the experiments). One weakness of this example is the underdeveloped data analysis section, which comes at the end of the paragraph. The phrase “analyzed by standard error” is unclear and lacks detail. For longer reports the data analysis section of the methods should be its own paragraph or even an independent subsection, although this depends on the number of statistical analyses conducted. It should be clear to the reader how data were processed and what groups were compared in the statistics.
This section is written in past-tense passive voice, which is common for materials and methods sections. Another appropriate style would be past-tense active voice. The first sentence, re-written in active voice, could read like this: “In order to measure the photosynthetic activity of Elodea canadensis, we dark-adapted many 10 cm sprigs by depriving them of light for a minimum of four hours.”

Example 5.18.16. Experimental Section: Chemistry.

General. All reagents and solvents were obtained from commercial suppliers and used without further purification. All reactions were performed at room temperature and atmospheric pressure unless otherwise noted.
Unknown Cation Determination. Solutions of 0.10 M sodium sulfate, sodium phosphate, sodium chromate (10 drops each), and sodium molybdate (20 drops) were added to individual wells in a 24-well plate, followed by 10 drops of a solution of unknown salt (0.50 g) in deionized water (5 mL). The resulting solutions in the well plates were gently stirred by rotating the plate in circles. The wells were examined against a black background for the appearance of precipitates. A flame test was performed by dipping a wire loop into the solution of unknown salt, holding the loop in a Bunsen burner flame, and noting the color of the resulting flame.
Unknown Anion Determination. Solutions of bromine, chlorine, and iodine were prepared in three separate test tubes and mixed vigorously. The bromine solution was orange and composed of 10 drops of 6M KBr solution, 20 drops of hexane, 2 drops of 6M \(\text{H}_2\text{SO}_4\) and 3 drops \(\text{H}_2\text{O}_2\) (3%). The yellow chlorine solution was prepared with 10 drops of (5%) NaClO solution, 20 drops of hexane, and 5 drops 6M HCl. Finally, the iodine solution was purple and composed of 10 drops 6M KI solution, 20 drops of hexane, 2 drops 6M \(\text{H}_2\text{SO}_4\) and 3 drops \(\text{H}_2\text{O}_2\) (3%). A small amount of the unknown salt, covering just the tip of the spatula was added to each test tube. The mixtures were shaken vigorously until the halide ion mixed with the halogen in the hexane layer. The color of the hexane layer either changed or remained the same depending on if a reaction occurred. By comparing the colors of previous mixed solutions, the unknown anion was identified through color matching.
In chemistry, this section is called the experimental section. It should be divided using subheadings into a “general” section and a section for each experiment, with the subheadings bolded and each followed with a period like in the example above. Each section should be about one paragraph in length. This is an example of how structure in chemistry often is a bit more rigid than in biology. Similar to the biology example, this example demonstrates an appropriate level of detail to repeat the experiment but nothing extra like the details of standard techniques or what type of glassware the author used. For chemistry, it’s important to include the concentrations and exact quantities of reagents and their physical state (gas, aqueous, etc.). Also include details like the source of reagents and temperature and pressure, as they can impact outcomes.
This section is written in past tense passive voice. This is conventional for chemistry experimental sections. It is also conventional to avoid starting sentences with numbers.