This activity relates the solubility of gas to the boiling of eggs.
Students observe the color of the yolk of two boiled eggs. One of the eggs is left in hot water after it is boiled and the other is placed in cold water immediately after boiling it. The first of these yolks appears to have a grayish green coating because hydrogen sulfide gas is present and reacts with the iron in the yolk to produce iron II sulfide (which is the gray-green color). The second yolk appears yellow because no iron II sulfide is formed. Instead, the hydrogen sulfide gas moves out of the egg into the cool water surrounding it. This is because the gas is more soluble in cold solution than in hot.
Solubility of gas
30 minutes of class time.
- 2 eggs per group
- 2 beakers (or a small pot and a bowl)
- hotplate (or stove top burner)
The yolk of an egg contains iron. The white of an egg contains sulfur in the bonds of the protein. When you cook an egg the sulfur is released from the protein in the form of hydrogen sulfide gas. The smell of cooking egg is due to the hydrogen sulfide gas. Gas is less soluble in hot solutions and is more soluble in colder solutions. As you boil an egg, the egg begins to heat up faster on the outside causing the hydrogen sulfide gas to move toward the center of the egg where it is cooler and more soluble. When the hydrogen sulfide gas reaches the yolk it immediately begins to react with the iron in the yolk to form the iron (II) sulfide, which is a green color. To prevent the yolk from turning green you need to get the hydrogen sulfide gas to move away from the center so it can't react with the iron in the yolk. To do this you need to make the outside of an egg cooler than the inside so that the hydrogen sulfide gas will migrate outwards.
The green compound is not harmful in anyway but most people would prefer a bright yellow yolk to a green one.
- Put two eggs in a beaker or pot of water and boil for 15 minutes. (Start timing after the water begins to boil.)
- At the end of the 15 minutes, remove the container from the heat source. Remove one egg and place it in a container of cold water. Leave the other egg in the hot water that it was boiled in.
- Let the eggs sit in the water for about 30 minutes. Add ice to the cold water container if necessary to maintain a cool temperature.
- Peel both eggs and remove the yolks. Try to remove the yolks whole. Observe the results.
- What is the formula for the gas that reacts with the iron in the yolk to produce the green color?
- What is the formula for the green compound that is formed on the yolk?
- Write the balanced chemical equation for the formation of green compound on the surface of an egg yolk.
- Why does putting the egg in cold water prevent the formation of the green color around the yolk? (Explain in terms of solubility.)
- If you need to boil eggs for an event at your home, will you place the eggs in cold water after boiling them? Why or why not?
This activity can be done in the lab in groups or as a class activity/demonstration where only a few eggs are boiled. I have also used this as a take home activity that requires students to do this with a parent/guardian at home and get a parent signature at the bottom of the page that states that the student completed the activity and explained the answers to all of the questions to the parent/guardian. I have enjoyed a good parent response to this assignment.
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I have modified this activity from one that was presented many years ago at a Michigan Science Teacher Association Conference. I wish that I had the name of the woman that presented it.