Osmosis & Food Tests
Science Practicals: GCSE, topic 1.1.4
Investigating Osmosis
This experiment shows the effect of water concentration on osmosis.
Measure change in mass
- Cut discs of raw potato and measure their mass.
- Put discs in different concentrations of sugar or salt solution.
- After 30 minutes, measure the mass of each disc again.
Calculate percentage change in mass
- Subtract the final mass from the initial mass of each disc.
- Divide this number by the initial mass and then multiply by 100 to give a percentage change in mass.
- The concentration in which the potato's mass changed the least is closest to the water concentration of the potato (because the least water has moved into or out of the potato by osmosis).
Testing for Carbohydrates, Lipids and Proteins
Reagents can be used to test for the presence of various food substances. The first step is to grind up the food and add distilled water to dissolve some of the food. You can then test for the food substances:
Starch
- To test for starch, add iodine solution.
- It will turn blue-black if starch is present.
Sugar
- To test for sugar, add Benedict’s reagent and heat for about two minutes.
- It will turn any of green, yellow or red if sugar is present.
- The colour depends on the concentration.
Proteins
- To test for proteins, add Biuret solution.
- It will turn mauve or purple if proteins are present.
Lipids
- To test for lipids, add Sudan III.
- If lipids are present, a red-stained oil layer will float on the water surface.
Test yourself on Osmosis & Food Tests
Investigating Osmosis
The change of mass in the osmosis experiment is worked out by which equation?
Select the correct pairs of reagents and substances being tested?
If we add iodine to starch then it will turn...
If we add protein to Biuret's solution then it will turn...