The Mystery of the Floating Egg
Students place a fresh egg in a beaker of tap water. It sinks. They stir in 40 g of salt and the egg rises until it floats with about a quarter of its shell above the surface. A student claims, "The salt made the egg lighter, so it floated." Another says, "The salt made the water denser, so the egg could float." The teacher asks them to test the ideas. They take the egg out, dry it, and find its mass is unchanged at 58 g. They then place the same egg in a second beaker of plain tap water and it sinks again. Finally, they gently add fresh water to the salty beaker and watch the egg slowly sink back down.
1. Which claim is supported? Use the mass result and the fresh-water test to justify your answer.
Read the worked answer
The density claim is supported. The egg's mass stayed at 58 g, so the salt did not make the egg lighter. The egg also sank in plain water, showing its own mass and volume did not change. What changed was the water: dissolving salt adds mass to the water without adding much volume, so the salty water is denser than plain water. When the liquid's density is greater than the egg's average density, the egg floats. The fresh-water test confirms this: adding plain water lowers the liquid's density, so the egg sinks again. The buoyant force depends on the liquid's density, not on the egg's weight changing.
2. Explain why the egg floated with part of it above the surface rather than staying fully underwater.
Read the worked answer
An object floats when the upward buoyant force equals its weight. The buoyant force equals the weight of the liquid pushed aside. If the egg were fully underwater in the salty water, it would push aside more salty water than its own weight, giving an upward force larger than its weight, so it rises. As it rises, part of the egg comes out of the water and it pushes aside less liquid. It settles where the weight of the displaced salty water equals the egg's weight - that is, partly above the surface. This is why about a quarter of the shell showed above the water.