To analyze the results of the Biuret test for protein, let's carefully evaluate the control samples and the mystery food sample.
### Step-by-Step Analysis
1. Understanding the Controls:
- Positive Control (Gelatin): Gelatin is known to contain protein. When Biuret solution, which is used to test for the presence of proteins, is added to a solution containing protein, the expected result is a color change to pink/purple. This acts as the positive control, verifying that the Biuret test can correctly identify protein.
- Negative Control (Water): Water is known to contain no protein. When Biuret solution is added to pure water, there should ideally be no color change, and the solution should remain blue. This acts as the negative control, verifying that the Biuret test does not falsely indicate the presence of protein when it isn't there.
2. Observed Results:
- Gelatin (Positive Control): The observed color is pink/purple, which is the expected result and indicates that the positive control worked correctly.
- Water (Negative Control): The observed color is pink/purple. This is not the expected result. The water should have remained blue to indicate the absence of protein, but it instead showed a color that suggests protein presence.
- Mystery Food Sample: The observed color is blue, indicating the absence of protein.
3. Identifying the Error:
- The key observation here is that the negative control (water) showed a pink/purple color instead of remaining blue. This discrepancy indicates an error in the test.
4. Possible Cause of the Error:
- The negative control showing a protein presence color (pink/purple) may suggest several potential issues:
- Contamination: The Biuret reagent might be contaminated. If the reagent itself has been contaminated with protein or another substance that causes a color change, it could lead to false results.
- Procedural Error: There might have been a procedural mistake during the experiment, such as using equipment that was not properly cleaned, leading to cross-contamination between samples.
- Improper Reagent Storage: If the Biuret reagent was not stored correctly, it might have degraded or gotten contaminated, leading to false-positive results.
### Conclusion
The observed color of the negative control (water) as pink/purple indicates a significant error. Because the negative control should have shown no color change and remained blue to confirm the absence of protein. The error suggests that the Biuret test is not valid in this instance. The likely cause of this error could be contamination of the Biuret reagent or a procedural issue during the experiment.
This means that the results for the mystery food sample cannot be trusted until the source of the error is identified and rectified.
