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Egg-cellent Chemistry: Teach Key Concepts by Decorating Eggs

Heather Haley
Product Developer


Background

Unfertilized chicken eggs are a familiar product purchased from the grocery store. They are graded based on their appearance and classified by weight. Most eggs at the grocery store are uncooked (raw) and have a thin mineral structure on the exterior—the eggshell. Eggshells are composed of calcite (a crystalline form of calcium carbonate) and have small pores for gas exchange. An outer organic layer, the cuticle, surrounds the eggshell and limits the movement of particles, water, and bacteria through pores in the shell. The cuticle is made of proteins (85–90%), polysaccharides (4%), and lipids (3%).

Decorating eggs is a fun and memorable way to illustrate scientific concepts and make real-world connections. The 2 activities that follow (Egg-cellent Dyes and Painting Eggs with pH) are appropriate for either the kitchen at home or the laboratory at school.


Safety

Perform both activities in accordance with established laboratory safety practices, including appropriate personal protective equipment (PPE). Ensure that students understand and adhere to these practices. Know and follow all school district guidelines for the disposal of laboratory wastes. Read the activity-specific cautions below before performing the activities.

Handling eggs carries a risk of exposure to harmful Salmonella bacteria, which can grow on the exterior as well as the interior of eggs. To reduce the risk of exposure to bacteria, boil the eggs thoroughly until the yolk and white are firm. To reduce risk of growing bacteria, keep the eggs refrigerated at or below 40° F (4° C) until needed for the activities.

Food coloring and dyed eggs can stain hands, clothing, upholstery, and wood. To prevent staining your hands with food coloring, use disposable gloves.


Activity 1: Egg-cellent Dyes

Topics covered: biochemistry, chemical bonding, pH


Background

A limited number of chemical dyes are certified for use to color food, drugs, and cosmetics (Fig. 1). Grocery store food coloring contains FD&C dyes and is often used to color eggs. The absorption of food dyes on eggshells is pH dependent. The dyeing process involves the formation of salt linkages between positively charged atoms in the cuticle and negatively charged atoms in the dye. Proteins in cuticle contain amino groups (-NH2). Under acidic conditions, amino groups are protonated and become positively charged (green -NH3+). The positively charged amino groups are attracted to the negatively charged sulfonate, or carboxylic, groups (yellow) found in dyes and form a salt linkage (Fig. 2).


Figure 1  Seven FD&C (food, drug, and cosmetic) certified dyes.



Figure 2  Salt linkage between cuticle proteins and FD&C Blue 2.


Materials


Procedure

  1. To prevent staining your hands with food coloring, put on disposable gloves.
  2. Prepare a saturated solution of sodium bicarbonate (baking soda) by combining 9 g of baking soda and 100 mL of distilled water in a labeled 250-mL container.
  3. Use a marker to label 3 containers: “red acidic,” “red neutral,” “red basic.”
  4. Repeat the preceding step for colors blue, green, and yellow.
  5. Pour 80 mL of distilled water into the 12 empty containers.
  6. Use the12 containers of distilled water and the listed additives to prepare 12 solutions for dyeing eggs.




  1. Use a spoon to gently place an egg in each of the 250-mL containers. Rinse the spoon with water between uses.
  2. After 10 minutes, use a clean spoon to gently remove eggs from each 250-mL container. Rinse each egg under running water and dry it gently with a paper towel.
  3. Compare the color intensity of eggs dyed in acidic, neutral, and basic conditions.



Extension activities

Prepare another 12 containers to test how varying amounts of vinegar, drops of dye, or time in dye solution affects the color intensity of eggs.


Related products

Note: AP® is a trademark registered and/or owned by the College Board®, which was not involved in the production of, and does not endorse, these products.


Activity 2: Painting Eggs with pH

Topics covered: acids and bases, pH


Background

Aqueous solutions fall into 1 of 3 categories—acidic, basic, or neutral—depending on the number of hydronium ions (H3O+) and hydroxide ions (OH–) present. If the solution has an excess of hydronium ions, it is classified as an acid. Acids have pH values less than 7. If the solution has an excess of hydroxide ions, it is classified as a base. Bases have pH values greater than 7. If the solution has an equal number of hydronium and hydroxide ions, it is considered neutral. Neutral solutions have pH values of exactly 7 (Fig. 3).



Figure 3  Relationship between ion concentration and pH
(adapted from a chart in Armchair Chemistry by Hubert Alyea).


Organic compounds can be used to measure pH if they change color in a predicable way (as seen in litmus paper, pH paper, and acid-base indicators). Red cabbage leaves naturally contain one such organic compound, an anthocyanin named cyanidin. In neutral solutions (pH = 7), cyanidin is purple. In acidic or basic solutions, the molecular structure of cyanidin is altered, thus changing the way it reflects light. Cyanidin appears fuchsia (bright pink) at pH = 3 and blue at pH = 11 (Fig. 4).



Figure 4  Cyanidin at varying pH values.


Materials

*Prepare concentrated red cabbage juice by combining a head of chopped red cabbage, 2 qt of distilled water, and 15 mL of vinegar in an 8-qt stockpot. Bring contents to a boil. Reduce heat and stir occasionally as the mixture simmers for 30 minutes. Use a colander and an appropriate heat-resistant container to separate chopped cabbage from extracted juice. Return extracted juice to the stockpot and boil until the juice concentrate reaches 1-½ qt.


Procedure

  1. To prevent staining your hands with cabbage juice, put on disposable gloves.
  2. Pour 80 mL of prepared cabbage juice concentrate into a 250-mL container.
  3. Use a spoon to gently place an egg in the container of cabbage juice.
  4. Allow the egg to remain submerged in cabbage juice for 1 hour. Egg will appear blue because its shell is made of a basic substance (calcium carbonate).
  5. Use a spoon to gently remove the egg from the cabbage juice and rinse it under running water.
  6. Gently dry the colored egg with a paper towel.
  7. Prepare a saturated solution of sodium bicarbonate by combining 0.9 g of baking soda and 10 mL of distilled water in a labeled 250-mL container.
  8. Pour about 10 mL of lemon juice in a labeled 250-mL container.
  9. Dampen 1 side of a clean cotton swab with a small amount of lemon juice and use it to lightly brush pink-colored designs on the egg surface. Use the other end of the cotton swab to pat the brush designs dry.
  10. Dampen 1 side of another clean cotton swab with a small amount of sodium bicarbonate solution and use it to make green designs on the cabbage juice-colored eggs. Use the other end of the cotton swab to pat the brush designs dry.



Extension activities

When you are finished dyeing eggs, use your cabbage juice to find the pH of household liquids. See Red Cabbage Juice: A Homemade pH Indicator!


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