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Activity

Sour food molecules

Summary: 
Dye is added to unknown liquids. Depending on the colour the dye turns, the sourness of the food can be predicted. (The dye detects the concentration of loose hydrogen atoms, or acidity, or pH, of the liquid.)
Science content (2016 curriculum): 
Biology: Sensing, Organ Systems (4, 5, 6)
Chemistry: Atoms, Molecules (3-7)
Science topic (2005 curriculum connection): 
Life Science: Human Body (grade 5)
Physical Science: Chemistry (grade 7)
Materials: 
  • five dropper bottles, with coloured walls so that the contents cannot be seen, filled with: orange juice (pulp free), milk, lemon juice, vinegar, water (could also try yogurt)
  • dropper bottle with indicator dye: red cabbage dye (picture 1) or a commercial indicator dye (picture 2); those that differentiate between pH 2, 4 and 6 allow for finer separation
  • white tray with wells e.g. ice cube or paint tray
  • marker
Procedure: 

Introduction:
What is it that makes some foods taste so sour?
Foods are sour when they have a high concentration of loose hydrogen atoms. (Atoms are tiny particles that make up us and everything we see around us. Atoms link together to make molecules.) Do an experiment to predict how sour some foods are without tasting them.

Make a numbered cup for each food bottle, or label each well of the tray. Add a few drops of each food to its own numbered cup/well. Add a drop of dye to each cup/well of food.
The color of the dye shows the concentration of loose hydrogen atoms in the food.
(With red cabbage dye, high concentration of hydrogen atoms turns the dye pink; low concentration of hydrogen atoms leaves it purple, and medium concentration of hydrogen atoms turns it pinky-purple. With commercial pHydrion pH 1-10 indicator dye, high concentration of loose hydrogen atoms turn the dye orange; medium concentration of hydrogen atoms turn the dye yellow; low concentration of hydrogen atoms turn the dye green. )

The food with a high concentration of loose hydrogen atoms is the most sour.
The food with a low concentration of loose hydrogen atoms is the least sour.
By looking at the color of the dye, which number food is the least sour? Which number food is the most sour?

Look at the key to find out what each of the foods are. (Vinegar, Water, Milk, Lemon juice, Orange juice). Does the sourness of these foods match what you predicted using the dye?

Vinegar and lemon are very sour because they have a high concentration of loose hydrogen atoms. They are also said to have a low pH.
Apples and oranges are just a little bit sour because they have an intermediate concentration of loose hydrogen atoms.
Milk and bananas are not sour because they have a low concentration of loose hydrogen atoms. What other foods can you think of that are not sour? They have a higher pH.

In each case the loose hydrogen atoms interact with receptors on your tongue, and depending on their concentration your brain perceives the food as sour or not.

Notes: 

Other foods to add: buttermilk, orange juice (though colour may mess it up).

Grades tested: 
Gr K
Gr 1
Gr 2
Gr 3
Gr 4
Gr 5
Gr 6
Gr 7
Teacher: 
Scott Malin
Self guided
Teaching site: 
New York Hall of Science
Tyee Elementary
Activity originally developed and delivered: 

New York Hall of Science