Investigating Vitamin C Loss in Vegetables
A student is planning a practical investigation for the BGCSE Food and Nutrition coursework. The hypothesis is: 'Boiling vegetables for a longer time reduces their vitamin C content.' The student will use fresh green peppers, which are known to be high in vitamin C. The school laboratory has a burette, DCPIP solution (a blue dye that decolourises when vitamin C is added), and standard vitamin C solution. The student has access to a kitchen for boiling samples. The student needs to produce reliable results and present them appropriately. The teacher has warned about controlling variables and the importance of repeating measurements.
1. Outline a detailed procedure for the investigation, including how to control variables, how to measure vitamin C content, and how to ensure reliability and safety.
Read the worked answer
Procedure: 1) Prepare samples: take equal masses (e.g., 10 g) of fresh green pepper. Boil one sample for 2 minutes, another for 5 minutes, another for 10 minutes, and keep one raw as control. Use the same volume of water (e.g., 100 ml) and same starting temperature (boiling). 2) After boiling, immediately cool samples and blend each with a fixed volume of distilled water (e.g., 50 ml) to extract vitamin C. Filter to obtain clear juice. 3) Titrate the juice against DCPIP solution of known concentration. For each sample, fill a burette with the juice and add dropwise to a known volume of DCPIP until the blue colour just disappears (endpoint). Record the volume used. 4) Repeat each titration at least three times and calculate mean volume. 5) Use a standard curve made with known concentrations of vitamin C solution to convert titre volume to concentration. 6) Control variables: same mass of pepper, same volume of water, same boiling time (except independent variable), same extraction method, same DCPIP concentration, same observer. 7) Safety: wear goggles, use care with boiling water and glassware, DCPIP is a dye (avoid skin contact). 8) Reliability: repeat titrations, use fresh DCPIP, calibrate burette. Present results in a table and graph of vitamin C concentration vs boiling time.
2. Explain how the student should analyse the data and what conclusions could be drawn. Include how to deal with anomalous results and how to evaluate the method.
Read the worked answer
Analyse by calculating mean titre volumes for each boiling time, converting to vitamin C concentration using the standard curve. Plot a line graph of concentration against boiling time. Look for a trend: as boiling time increases, vitamin C concentration is expected to decrease. Identify anomalies (e.g., a point far from the trend) and exclude them from mean if justified, or repeat. Calculate percentage loss relative to raw control. Conclusion: support or refute hypothesis. Evaluate method: sources of error include inconsistent extraction, oxidation during blending, inaccurate endpoint detection, and variation in pepper samples. Suggest improvements: use a colorimeter for endpoint, standardise extraction time, use same batch of peppers. Discuss reliability by comparing repeats and calculating range. Mention that vitamin C is water-soluble and heat-sensitive, so longer boiling leads to more leaching and destruction. Relate to food preparation advice: steam or stir-fry to preserve vitamin C.