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Activity

Sour food chemistry

Summary: 
Compare foods, some that are sour with two different tests: a pH indicator (purchased or home made), or by adding baking soda to look for gas bubble production.
Science content (2016 curriculum): 
Biology: Sensing, Organ Systems (4, 5, 6)
Chemistry: Atoms, Molecules (3-7)
Materials: 
  • white tray with wells e.g. ice cube or paint tray
  • For pH test:

  • test liquids: orange juice (pulp free), milk, lemon juice, vinegar, water (could also try yogurt). best if the liquids are not identified before the test
  • indicator dye: red cabbage dye or a purchased indicator dye (those that differentiate between pH 2, 4 and 6 are best)
  • For baking soda test:

  • candies, sour and regular
  • baking soda, a small scoop and water
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.

Indicator dye:
Add unknown liquids to wells of the tray.
Add the indicator dye, and look at the colour change. (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 liquid 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, predict which liquid is the least sour and most sour.
Taste the liquids to check. (Sometimes sugar is added to foods to offset the sour taste, so they may not taste as sour as predicted e.g. lemonade).

Baking soda bubbles:
Add a small scoop of baking soda to a well of the tray. Add water and mix together.
Drop a candy into the baking soda solution.
Bubbles will indicate that the candy has a sour coating, as the baking soda and the H atoms of the coating undergo a chemical reaction, making bubbles of carbon dioxide: HCO3 + H -> CO2 + H2O

Why foods taste sour:
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 taught: 
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