Evaluation and Conclusion Revision Notes for VCE SSCE Chemistry

Evaluation and Conclusion

Introduction

After conducting an experiment, it's essential to evaluate the method used and draw evidence-based conclusions. This process involves critically examining your experimental procedure, identifying strengths and weaknesses, and relating your findings back to your original hypothesis and research question. When evaluating your method, focus on comparing your results to accepted or literature values, as this helps identify areas for improvement.

infoNote

The evaluation and conclusion phases are where scientific learning truly occurs. By critically examining what worked and what didn't, you develop the analytical skills essential for scientific inquiry and improvement.

Evaluating the method

Purpose of evaluation

The method used in your controlled experiment should be thoroughly evaluated to determine whether it was appropriate and effective. Even well-designed methods may produce unexpected results, making evaluation crucial for scientific learning and improvement.

Key considerations when evaluating

When evaluating your experimental method, consider these important questions:

What were the sources of systematic errors?
What were the sources of random errors?
Was the method valid (did it measure what it aimed to measure)?
Was the experiment repeated to ensure reliability?
Was the data precise (were repeated measurements consistent)?
Would a larger sample or more variations in the independent variable strengthen your conclusion?
What improvements could be made for future experiments?

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Worked Example: Evaluating Two Methods for Measuring Gas Volume

Consider an experiment measuring the volume of carbon dioxide gas produced when calcium carbonate reacts with hydrochloric acid. Two different methods could be used:

Method a - Water displacement method: In the water displacement method, the gas is collected in an inverted measuring cylinder filled with water. Whilst this method is valid (it aims to measure the volume of $\text{CO}_2$ gas produced), it is not accurate. Carbon dioxide is slightly soluble in water, meaning some gas will dissolve rather than being collected. This causes the measured volume to be consistently lower than the true value, creating a systematic error.

Method b - Electronic balance method: The electronic balance method provides an improvement. By measuring the mass loss from the reaction vessel, you directly measure the mass of carbon dioxide that escapes. This method is generally more accurate because it avoids the problem of gas dissolution.

Identifying errors

Understanding different types of errors

Most practical investigations contain errors, which can be classified into two main categories:

Systematic errors:

Caused by problems with part of the method or equipment
Produce results that are consistently higher or lower than the accepted value
Identified when your results consistently don't match expected values
Often method-related rather than technique-related

Random errors:

Affect the precision of results
May be due to lack of experience with equipment
Cause results to vary unpredictably between trials
Become evident when insufficient trials are conducted

chatImportant

Mistakes are different from errors. Mistakes are blunders that produce outliers and should not be included as valid results. Always distinguish between genuine errors (which affect all measurements systematically or randomly) and one-off mistakes that should be excluded from your data analysis.

What to do about errors

Minimising random errors:

Repeat trials multiple times
Calculate the average of your results
Gain more experience with the equipment
Use more precise measuring instruments

Minimising systematic errors:

Take care when using equipment
Calibrate instruments properly
Consider alternative methods if necessary
Address design flaws in the experimental setup

Using qualitative observations

Your qualitative observations often provide important clues about systematic errors. For example, if you observe a yellow, sooty flame whilst burning a hydrocarbon, this indicates incomplete combustion is occurring. Such observations help you identify and explain methodological issues.

infoNote

Don't overlook qualitative data during your evaluation. Observations about color changes, unusual smells, unexpected physical states, or other sensory information can provide valuable insights into what went wrong (or right) in your experiment.

Acknowledging contradictions

When analysing data, acknowledge any contradictions or unexpected results. Rather than ignoring results that don't fit your hypothesis, investigate the reasons why they occurred by carefully examining your method. Sometimes, unexpected findings lead to new research questions and hypotheses, which can extend your investigation further.

Improving the method

For every methodological weakness you identify, suggest a specific improvement. The following table shows examples from an experiment investigating how carbon chain length affects the heat of combustion of alcohols:

Methodological weakness	How this may have influenced the results	Suggestion for improvement
The simple calorimeter was not insulated and did not have a lid	Once the water was heated above room temperature, heat may have been lost to the surroundings through the metal can and from the water surface. This would cause the calculated heat of combustion to be smaller than the true value (systematic error)	Make a lid for the can using one or more layers of aluminium foil. Loosely cover the sides of the can with foil to provide non-flammable insulation
The alcohol continued to evaporate from the wick after the flame was extinguished	The mass loss of alcohol would be greater than correct, decreasing the calculated heat of combustion	Snuff the flame using a metallic snuffer/cap rather than blowing it out. Remember to weigh the spirit burner with the snuffer before and after each trial
The three spirit burners had wicks of different lengths	Longer wicks produce larger flames that may transfer heat with different efficiencies. More efficient heat transfer means less heat loss as the experiment finishes more quickly. Different wick lengths introduce random uncertainty	Measure and record the wick length before starting to burn the alcohol in each trial. Try to match that length for subsequent trials

Drawing evidence-based conclusions

Purpose of a conclusion

A conclusion is a summary of your investigation that allows readers to understand what you did, what results you obtained, and how valid those results were. A good conclusion links your collected evidence to your aim and hypothesis, using your results as evidence to provide a justified response to your research question.

Components of a conclusion

Your conclusion should include four key components:

1. Restatement of the aim

Transform your aim from future tense to past tense to introduce your conclusion. Instead of "To measure...", write "Three different alcohols were measured..."

2. Statement of your results

Present the mean values from multiple trials and include results from all parts of the experiment. Keep this section concise whilst being comprehensive.

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Example Statement of Results:

"The heat of combustion of the three alcohols was calculated from the average of three trials for each alcohol and was found to be $550 \text{ kJ mol}^{-1}$ for methanol, $1020 \text{ kJ mol}^{-1}$ for ethanol and $1650 \text{ kJ mol}^{-1}$ for propan-1-ol."

3. Comparison to accepted values or hypothesis

Compare your results quantitatively to accepted values. Calculate percentage differences to show how close your results were to literature values.

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Example Comparison to Literature Values:

"These three values were considerably lower than the literature values of $726 \text{ kJ mol}^{-1}$ for methanol, $1360 \text{ kJ mol}^{-1}$ for ethanol (VCE Chemistry Data Book) and $2021 \text{ kJ mol}^{-1}$ for propan-1-ol (NIST Chemistry webbook). The percentage differences for these values were $24.2\%$ , $25.0\%$ and $18.4\%$ respectively."

4. Brief explanation of discrepancies

Suggest what may have caused your results to differ from expected values.

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Example Explanation of Discrepancies:

"It is likely that these experimental values are low because heat energy was lost to the surroundings during the experiment due to lack of insulation on the simple calorimeter and the lack of a lid."

Language requirements for conclusions

Past tense and third person:

Write conclusions in the past tense and third person, as though someone else conducted the work.

chatImportant

Incorrect: "I found that the heat of combustion of ethanol was less than it should be"

Correct: "The heat of combustion of ethanol was less than was expected"

The conclusion can be easily introduced by converting the aim into the past tense and writing it in the third person.

Complete conclusion example

The table below shows how to construct a complete conclusion for an alcohol combustion experiment:

Part of the report	Example of what might be written
Aim	To measure and compare the heat of combustion of three different alcohols, methanol, ethanol and propan-1-ol, using a spirit burner and a simple calorimeter
Conclusion: Restatement of the aim	Three different alcohols, methanol, ethanol and propan-1-ol, were burnt in a spirit burner and the thermal energy released was calculated using the change in temperature of the water in the calorimeter. These values were then used to calculate the heat of combustion
Statement of your results	The heat of combustion of the three alcohols was calculated from the average of three trials for each alcohol and was found to be $550 \text{ kJ mol}^{-1}$ for methanol, $1020 \text{ kJ mol}^{-1}$ for ethanol and $1650 \text{ kJ mol}^{-1}$ for propan-1-ol
Comparison to accepted values	These three values were considerably lower than the literature values of $726 \text{ kJ mol}^{-1}$ for methanol, $1360 \text{ kJ mol}^{-1}$ for ethanol (VCE Chemistry Data Book) and $2021 \text{ kJ mol}^{-1}$ for propan-1-ol (NIST Chemistry webbook). The percentage differences for these values were $24.2\%$ , $25.0\%$ and $18.4\%$ respectively
Brief explanation of discrepancies	It is likely that these experimental values are low because heat energy was lost to the surroundings during the experiment due to lack of insulation on the simple calorimeter and the lack of a lid

bookmarkSummary

Key Points to Remember:

Evaluate your method by considering validity, precision, accuracy, and reproducibility. Identify both systematic and random errors.
Systematic errors cause consistent bias in results (too high or too low), whilst random errors reduce precision and cause inconsistent variation.
For every weakness identified, suggest a specific, practical improvement to the method.
Structure your conclusion by restating the aim, presenting results, comparing to accepted values, and explaining discrepancies.
Write conclusions in past tense and third person to maintain scientific objectivity and proper academic style.

Evaluation and Conclusion (VCE SSCE Chemistry): Revision Notes