Chemical Experiment Report Discussion Phrase Collection | How to Write About “Errors,” “Yield,” and “Points for Improvement”

Important Points in the Discussion Section of a Chemistry Laboratory Report

The discussion is not simply a section for repeating the results. It is the section where you explain what the results obtained in the experiment mean, how they compare with theoretical or literature values, and, when there is a discrepancy, what may have caused it.

To write a good discussion, keeping the following three points in mind can make it easier to organize your writing.

• Write based on the results.
• Explain the connection between cause and result specifically.
• Consider whether the measured value would become larger or smaller.

Writing only “there was an error,” “the experiment failed,” or “there was an operational mistake” is insufficient. By explaining which operation affected which value and in what way, you can make the discussion more convincing.

Basic Expressions Useful in Discussions

First, the following table summarizes basic expressions frequently used in chemistry laboratory reports. They can be used when describing results, suggesting causes, making comparisons, and writing about improvements.

In a report, expressions such as “is considered,” “there is a possibility,” and “suggests” make it easier to write scientifically without making overly definitive statements.

Expressions for Writing About “Error”

When discussing error, rather than simply writing “an error occurred,” explain what caused it and which value was affected. Error sources can be organized more easily by dividing them into categories such as equipment, operation, sample, environment, and calculation.

Expressions for Indicating Causes of Error

• One possible reason why the measured value did not agree with the theoretical value is 〇〇.
• The difference between the experimental value and the literature value is considered to be attributable to 〇〇.
• One possible cause of the variation observed in the measured values is the effect of 〇〇.
• Because 〇〇 was insufficient, an error may have occurred in the result.
• Due to the operation involving 〇〇, the measured value may have been overestimated.
• Due to the effect of 〇〇, the measured value is considered to have been underestimated.

Expressions for Errors Caused by Equipment

• If the line of sight was not aligned with the scale when reading it, a reading error may have occurred.
• Because the smallest scale division of an instrument has limitations, the measured value contains a certain degree of uncertainty.
• If the zero point of the electronic balance was offset, a systematic error may have occurred in the mass measurement.
• If the pH meter was insufficiently calibrated, all measured values may have shifted either higher or lower.
• An error may have occurred in the measured temperature due to the reading position or the precision of the thermometer scale.

Expressions for Errors Caused by Experimental Operations

• The amount recovered is considered to have decreased because part of the sample was lost during transfer.
• Part of the precipitate may have adhered to the filter paper or equipment during filtration and could not be completely recovered.
• If the endpoint was exceeded during titration, the titrant volume may have become larger than the actual value.
• If drying was insufficient, moisture may have remained in the sample, causing the measured mass to be larger than the actual value.
• If the heating temperature was too high, side reactions or decomposition may have progressed, affecting the amount or purity of the product.

Expressions for When the “Yield Is Low”

When the yield is low, it is easier to write the discussion by separately considering whether the desired product was not formed sufficiently or whether the product that had formed was lost during experimental operations. Possible causes include incomplete reaction, side reactions, losses during filtration or recrystallization, and operations before and after drying.

Expressions for Describing Causes of Low Yield

• One possible reason for the low yield is that the reaction did not proceed to completion.
• The formation of the desired product may have decreased because a side reaction occurred.
• The loss of part of the desired product during filtration or transfer is considered to be one cause of the reduced yield.
• During recrystallization, part of the desired product may have remained dissolved in the mother liquor, reducing the recovered amount.
• Part of the desired product may have dissolved in the washing solution during washing, resulting in a lower yield.
• Because the reaction conditions were not appropriate, formation of the desired product may not have proceeded sufficiently.

Expressions for Connecting Low Yield to the Result

• These factors are considered to have reduced the amount of product that could actually be recovered, resulting in a lower yield.
• Because part of the desired product was lost during the operation, the actual yield is considered to have been lower than the theoretical yield.
• If the reaction was incomplete, the theoretical amount of the desired product would not have been formed, resulting in a lower yield.
• If by-products were formed, part of the starting material may have been consumed to form substances other than the desired product, resulting in a lower yield.

Expressions for When the “Yield Is Too High”

When the yield exceeds 100% or is much higher than expected, substances other than the desired product may have been included in the measurement. Typical causes include insufficient drying, contamination with impurities, insufficient washing, and residual solvent or moisture.

• One possible reason why the yield exceeded 100% is that the sample was insufficiently dried.
• If water or solvent remained in the sample, the measured mass would be greater than the actual mass of the desired product.
• If impurities were not removed and remained in the sample, the mass of substances other than the desired product may also have been included in the measurement.
• If washing was insufficient and components of the mother liquor remained, the apparent yield may become high.
• A high yield alone does not indicate that the purity of the product is high.

Expressions for Writing About “Purity”

When discussing purity, evaluate it by combining results such as melting point, color, crystal appearance, spectra, chromatography, and yield. A high yield does not necessarily mean that the purity is high.

When the Purity Is Considered High

• Because the melting point was close to the literature value and the melting point range was narrow, the purity of the sample is considered to be relatively high.
• Because the coloration of the crystals became weaker after recrystallization, some of the colored impurities are considered to have been removed.
• Because a peak corresponding to the desired product was observed in the spectrum and impurity-derived peaks were small, the desired product is considered to be the main component.
• If only the main spot was observed by TLC, the sample is considered to be relatively close to a single component.

When the Purity Is Considered Low

• Because the melting point was lower than the literature value and the melting point range was broad, impurities may have remained in the sample.
• Because coloration remained in the crystals, colored impurities may not have been completely removed.
• Because multiple spots were observed by TLC, components other than the desired product may have been present.
• Because the yield was high while the melting point was low, the sample may have been weighed while containing impurities or residual solvent.

Expressions for Writing About “Variation in Measured Values”

When values obtained from repeated measurements do not agree, consider the causes of the variation. Possible causes include random error, reading error, differences in experimental operations, and sample inhomogeneity.

• One possible cause of the variation observed in the measured values is an error in reading the scale.
• Because repeated measurements gave similar values, the reproducibility of the measurement is considered to have been relatively high.
• Because there were large differences among the measured values, the experimental conditions may not have been completely consistent between measurements.
• Because the endpoint was determined visually, differences in judgment may have caused variation in the titration volume.
• If the sample was not mixed uniformly, the measured value may have differed depending on the portion sampled.
• Temperature changes may have altered the measurement conditions and caused variation in the measured values.

Expressions for “Comparison with Theoretical and Literature Values”

In chemistry laboratory reports, experimental values are often compared with theoretical or literature values. Whether the values agree or differ, it is important to consider the reason.

Expressions for When the Values Agree

• The experimental value was close to the literature value, suggesting that the desired product was likely obtained.
• The measured value generally agreed with the theoretical value, suggesting that the experimental procedure was performed relatively appropriately.
• Because the relative error was small, the reliability of the measured value is considered to be relatively high.
• Because repeated measurements gave similar values, the reproducibility of the experimental results is considered to be high.

Expressions for When the Values Differ

• The experimental value was lower than the literature value, suggesting that impurities may have been present in the sample.
• Possible reasons why the experimental value did not agree with the theoretical value include errors in measurement procedures or sample preparation.
• The difference from the literature value may be attributable to experimental conditions that did not completely match those used for the literature value.
• Because theoretical values assume ideal conditions, actual experiments may show differences due to variations in temperature or concentration.

Expressions for Writing About “Graphs”

In experiments using calibration curves, titration curves, reaction rate graphs, temperature change graphs, and similar plots, discuss the trends that can be read from the graph. When writing about graphs, explain not only that “the relationship was linear” or that “a point deviated,” but also what this means.

When a Linear Relationship Was Obtained

• As the concentration increased, the absorbance also increased, and an approximately linear relationship was observed between them.
• Because the calibration curve showed linearity, the measured values are considered to follow the relevant law within this concentration range.
• Because the plotted points were generally distributed along the regression line, the reproducibility of the measurements is considered to have been relatively high.
• The concentration of the unknown sample could be determined using the obtained regression line.

When Points Deviated from the Line

• One possible reason why the plotted points deviated from the regression line at higher concentrations is that the measurement range was not appropriate.
• Possible reasons why some measurement points deviated significantly include errors in concentration during sample preparation or instrument reading errors.
• The variation in the graph is considered to be attributable to uncertainty in the measured values or insufficient mixing of the sample.
• For measurement points that deviate significantly from the regression line, operational errors or differences in measurement conditions need to be examined.

Expressions for Writing About “Improvements”

When writing about improvements, rather than simply writing “be more careful,” explain what should be improved and how doing so would reduce a particular error. Matching each improvement with its corresponding cause makes the discussion more convincing.

Improving Measurement Procedures

• When reading a scale, aligning the line of sight with the scale can reduce reading errors.
• Performing multiple measurements and calculating the average can reduce the effect of random error.
• Calibrating the measuring instrument before use can reduce systematic error.
• Recording the measured value after it has stabilized can reduce variation.

Improving Sample Handling

• Carefully transferring the sample can reduce losses caused by adhesion to equipment or spilling.
• Thoroughly drying the precipitate after filtration can reduce mass errors caused by moisture.
• Using only the minimum necessary amount of washing solution can reduce losses caused by dissolution of the desired product.
• Thoroughly mixing the solution before measurement can prevent local variation in concentration or pH.

Improving Reaction Conditions

• Allowing sufficient reaction time can reduce yield loss caused by incomplete reaction.
• Proper temperature control can help prevent side reactions and decomposition.
• Accurately measuring the amounts of reagents can reduce errors caused by discrepancies in reaction quantities.
• Avoiding rapid heating may help prevent bumping and decomposition.

Useful Discussion Expressions for Specific Experiments

This section summarizes useful discussion expressions for common topics in basic chemistry experiments.

Titration Experiments

• Because the endpoint was determined visually, individual differences may have occurred in the timing used to judge the color change.
• If titrant was added beyond the endpoint, the titration volume would become larger than the actual value, causing an error in the calculated concentration.
• Parallax when reading the burette scale may have introduced a reading error into the titration volume.
• Because repeated titration values were similar, the reproducibility of endpoint determination is considered to have been relatively high.

Recrystallization

• Recrystallization is considered to have removed some impurities into the mother liquor, improving the purity of the sample.
• Part of the desired product may have remained dissolved in the mother liquor, reducing the recovery rate.
• Because the melting point became closer to the literature value, the purity is considered to have improved through recrystallization.
• Although the yield decreased, the narrower melting point range suggests that purification was effective.

Filtration

• Part of the precipitate may have adhered to the filter paper or equipment and could not be completely recovered.
• Because the filtrate was cloudy, part of the precipitate may have passed through the filter paper.
• Using a large amount of washing solution may have dissolved part of the precipitate and reduced the recovered amount.
• Insufficient drying may have left moisture in the precipitate, causing the measured mass to be larger than the actual value.

Melting Point Measurement

• Because the melting point was lower than the literature value, a small amount of impurities may have remained in the sample.
• Possible causes of the broad melting point range include contamination with impurities and an excessive amount of sample.
• If the heating rate was too high, errors may have occurred in determining the beginning and end of melting.
• Because the melting point range became narrower after recrystallization, the purity of the sample is considered to have improved.

pH Measurement

• If the pH meter was insufficiently calibrated, all measured values may have shifted either higher or lower.
• Insufficient cleaning of the electrode may have allowed contamination from the previous sample, affecting the measured value.
• If the value was read before it stabilized, variation may have occurred in the pH measurement.
• Dissolution of carbon dioxide from the air may have shifted the pH of the solution toward the acidic side.

Distillation

• Because the main fraction was obtained within a temperature range close to the literature value, it is considered to contain a large amount of the desired component.
• Because the distillation temperature did not remain constant, multiple components may have distilled together.
• If the heating was too strong, low-boiling and high-boiling components may have distilled simultaneously, reducing separation efficiency.
• If cooling was insufficient, part of the vapor may not have condensed, reducing the recovered amount.

Expressions That Should Be Avoided

In chemistry laboratory reports, expressions that are too vague or make unsupported definitive statements should be avoided. Rewriting expressions such as the following with more specific content makes them more appropriate for a discussion section.

A Basic Structure for Writing a Discussion

If you find it difficult to write a discussion, organizing it according to the following structure can make the process easier.

Result → Estimation of Cause → Effect on the Result → Improvement

Using this structure makes it easier to write a discussion based on experimental results rather than simply giving impressions.

Checklist Before Writing the Discussion

Before submitting your report, it is useful to check the following points.

• Are you doing more than simply repeating the results?
• Have you compared the results with theoretical or literature values?
• Have you written specific causes of error?
• Have you explained which operation affected which result?
• Have you considered whether the measured value would become larger or smaller?
• Have you distinguished between causes of low yield and causes of excessively high yield?
• Have you avoided confusing purity with yield?
• Have you described trends that can be read from graphs?
• Do the improvements correspond to the identified causes?
• Have you avoided ending the discussion with only statements such as “the experiment failed” or “a mistake was made”?

Summary

In the discussion section of a chemistry laboratory report, it is important to explain errors, yield, purity, variation in measured values, improvements, and related factors specifically. Rather than simply writing “there was an error” or “the yield was low,” you need to consider what caused the result, which value was affected, and how it was affected.

When writing a discussion, organizing it in the sequence of “result,” “cause,” “effect,” and “improvement” can make the content easier to structure. For example, when the yield is low, separating the possibilities of whether the desired product was not formed or whether it was lost after formation can lead to a more specific discussion.

The example expressions introduced in this article are only references for writing a discussion. In an actual report, adjust the expressions according to your own measured values, observations, and events that occurred during the experiment, and write a discussion supported by evidence.