Salt water is a powerful tool for teaching students about the different densities of objects and solutions and how they impact each other. While an egg is denser than plain water, adding salt to the water causes the water to become denser. Likewise, an egg that formerly sank in plain water will float in salt water due to the increased density.
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Fill two glasses with roughly two cups of plain water. In one glass, add 100 millilitres of salt and mix to dissolve the salt in the water. Carefully place a raw egg (in its shell) into the cup of plain water. Show the children how it immediately sinks to the bottom. Remove the egg from the cup of plain water and gingerly place it into the cup of salt water. Show the children how the egg floats in the glass.
Layers of Water
Show the students how where the salt is in the glass impacts the floating of the egg. Fill a glass half full of plain water. Mix water and salt together in a separate cup, using approximately four tablespoons of salt and two cups of water. Place a napkin over the top of the plain water cup, pressing down the napkin so that it rests just on top of the water. Gently and slowly pour the salt water into the cup over the napkin and then carefully remove the napkin from the cup. Do not mix the cup. Place the egg into the cup and watch as it floats in the middle of the cup, where the salt water meets the regular water.
In the first phase of the experiment, fill a cup with plain water and carefully place a coin, an egg and a bottle cork into the water. Observe as the egg and coin sink to the bottom and the cork floats. Remove all three objects from the water and add two tablespoons of salt to the water. Stir to mix the water and salt together. Replace the coin, egg and cork into the salt water solution. Observe the three objects, as in this phase, the cork and egg will float at the top while the coin sinks. Remove all three objects from the cup and pour out half of the salt water solution so the cup is half full of salt water. Lay a paper napkin into the cup on the surface of the salt water. Slowly pour plain water into the cup until the water level is nearly full and carefully slide out the napkin. Do not mix the water at this point. The napkin helps preserve separate layers of salt water and plain water. Add the three objects, the coin, the egg and the cork, to the cup. Observe as the coin sinks, the cork floats and the egg is suspended in the middle of the cup. The experiment demonstrates how different densities in the water solution and the objects impact each other and change what happens to the egg in the cup.
Add salt to the bottom of a tall glass until it is 1/5 full of salt. Pour a small amount of water slowly into the glass, adding only enough to moisten the salt. Place an egg in the glass on top of the layer of salt. Carefully pour more water in the glass, pouring it down the sides, as suggested by the Steve Spangler Science website, so that it fills the glass with disturbing the salt layer. By pouring it slowly, you help keep the salt and the water from mixing right away. Seal the glass by covering the top with a piece of plastic and securing it in place with a rubber band. Observe the glass over the next several days and weeks. As the salt slowly mixes with the water, the egg will likely slowly rise higher and higher in the glass.
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