Cambridge IGCSE Marine Science Practical Skills 4: Primary productivity and nutrient investigations

This chapter maps the three marked Topic 4 activities in Cambridge 0697: testing foods for starch, reducing sugar, protein and lipid; investigating energy release by burning food; and investigating light intensity and photosynthesis in an aquatic plant or macroalga. The theory hub owns nutrient functions and energy pathways.

Prepare comparable food samples

Crush equal food masses with equal distilled-water volumes, mix for a fixed time and filter or allow solids to settle where a clear extract is needed. Label every tube. Use separate clean droppers so reagents and samples do not contaminate one another.

Include a known positive control to show that the reagent and conditions work and a negative control, such as distilled water, to establish the baseline. Record the initial appearance, reagent, treatment and final observation. “Positive” alone loses the colour evidence.

Four food tests

For starch, add iodine solution. A positive result changes orange-brown to blue-black.

For reducing sugar, add Benedict's solution and heat the tube in a hot-water bath. A positive result changes from blue towards green, yellow, orange or brick-red depending on amount and conditions. Do not heat a sealed tube or point it at anyone.

For protein, add biuret solution. A positive result changes blue to lilac or purple. The reagent is alkaline, so use eye protection and small quantities.

For lipid, add ethanol to the sample, shake, then add the ethanol extract to water. A cloudy white emulsion is positive. Ethanol is flammable and must be kept away from ignition sources.

Colour depth can support a semi-quantitative comparison only when sample mass, extraction volume, reagent volume, heating and observation conditions are controlled. A colorimeter and calibration standards may strengthen an unfamiliar extension, but the basic specified tests remain qualitative.

Burning food to heat water

Measure a known water mass or volume in a suitable container and record its initial temperature. Determine the food sample's mass. Ignite it below the container at a fixed distance and allow it to heat the water while stirring safely. Record the maximum water temperature and the final food mass if some remains.

Energy transferred to water can be estimated by:

$$Q = mc\Delta T$$