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Tamary Massena
“How Salty Does Water Have to Be for an Egg to Float?”
September 10, 2013


Why would an egg float in water with a lot of salt in it, but not in plain tap water? What happens to salt (sodium chloride or NaCl) molecules when dissolved in water? Why does adding salt to water increase its density?

Did you know that if you put an egg in a cup of tap water, it will sink to the bottom? But, if you add enough salt, the egg will float back up to the surface! Why? Because the density of the egg is higher than the density of tap water, so it sinks. Density (ρ), as shown below in Equation 1, is the mass (m) of a material per unit volume (v). For example, the density of freshwater under standard conditions is approximately 1 gram (g) per cubic centimeter (cm3). In other words, if you filled a 1-cm x 1-cm x 1-cm box with freshwater, the water inside the box would have a mass of 1 g. Adding salt to the water increases the density of the water, because the salt increases the mass without changing the volume very much. With enough added salt, the saltwater solution density is higher than the egg's, and the egg will then float, as shown in Figure 1 below. The ability of something, like the egg, to float in water or some other liquid is known as buoyancy. Equation ...

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