Finding the elements in food
Every biological molecule in this chapter is built mainly from carbon (C), hydrogen (H), oxygen (O) and nitrogen (N). You can demonstrate that for yourself.
Activity 1 — showing experimentally that food contains water
Method
Materials: meat, egg shells, plant leaves, crucibles
- Crush the meat, egg shells and leaves separately and thoroughly.
- Put each into a separate crucible and heat until the water leaves and burning begins.
- While it burns, hold a glass plate above it.
- Use cobalt chloride to test whether the drops forming on the plate are water.
Observation
The cobalt chloride turns from blue to pink — so what formed is water (H2O).
Activity 2 — showing experimentally that biological molecules contain carbon
Method
Materials: several crucibles, plant leaves, a piece of fish, a little cooked rice
- Crush the fish, rice and leaves separately into pastes.
- Put each paste into a separate crucible and heat strongly.
- Rub the residue onto a sheet of white paper.
Observation
What remains is carbon.
Activity 3 — showing experimentally that biological molecules contain nitrogen
Method
Materials: two test tubes, sodium hydroxide solution, copper sulphate solution, egg white, a piece of fish
- Crush the fish well, add water, dissolve and filter it.
- Put 2 ml of egg white and 2 ml of the fish extract into separate test tubes.
- Add a larger amount of sodium hydroxide.
- Add a few drops of copper sulphate on top.
Observation
A purple colour appears in both test tubes.
The reasoning behind Activity 3
The last activity is the biuret test, which shows that protein is present. Since proteins always contain nitrogen, proving protein is there also proves nitrogen is there. It is indirect evidence, not direct.