Fill two glass bowls with water to the same level. Put a fresh egg that you just bought into one bowl and an old egg you bought a week or so ago into the other bowl. The fresh egg sinks to the bottom of the bowl. The old egg is suspended or may even float near the surface of the water. Fresh eggs only contain the white and yolk; the weight of these and the egg shell is heavier than the upward force in the bowl of water, so the egg sinks to the bottom. However, old eggs start to decompose and as they do, gases are produced. The more gas inside the egg, the more buoyant the egg becomes when it’s placed in water. The upward force is greater than the gravitational pull, so the egg is suspended or floats, depending on the build up of gas.
Add a tablespoon of salt to the water that contains the fresh egg. Give the water a gentle stir until the salt dissolves. Keep adding further tablespoons of salt, one at a time. Watch how the egg starts to become more buoyant. Eventually the egg rises to the surface of the water. This is because salt makes water denser, so the upward force is greater than the downward force and the egg floats.
Boil a pan of water then turn off the heat. Crack open an egg and drop it gently into the hot water. Watch it sink to the bottom. Wait while the egg starts to cook. The white of the egg expands as it cooks; it also contains tiny air bubbles, which means it has a greater surface area and is more buoyant. It displaces more water and the upward force is greater, so the egg starts to rise to the surface.
Archimedes principle is all about displacement. A boat floats because the material it’s made from is formed into a large surface area, so its weight is evenly distributed. However, the more weight you load into the boat the lower the boat is in the water. Eventually the weight inside the boat is such that the boat sinks. Crack open an egg into a bowl. Keep the two halves of the eggshell; you may need to crack open two eggs to get two fairly equal-sized shells. Place the two halves of the egg into a bowl of water; they both float. Now pour water into one of the shells. Watch as it gets lower in the water. The weight of the water you’re pouring in means that as the shell gets lower in the water, more water is displaced. Eventually the upward force is less than the downward force, so the shell sinks.