“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 1: ρ=m v ρ = Density in whatever units are used for mass and volume. m = Mass in grams (g), kilograms (kg), or any other unit of weight. v = Volume in centimeters cubed (cm3), meters cubed (m3), or any other unit of volume
“How Salty Does Water Have to Be for an Egg to Float?” Theory
Density is measure of how much matter takes up a certain amount of space or volume. The more matter you can pack into a certain space, the denser it is. Although we often confuse the two, density and weight are actually tow different measurements. Weight is the mass of an object times the force of gravity forcing things downward, a denser object will be heavier too. Since density is defined as how much matter takes up a certain amount of volume, we can take ordinary water, add salt to it, and make it denser. The volume also increases a little bit when we add salt but the mass increases by a much bigger factor. This is because rather than just floating around and taking up space, the salt dissolves which are attracted to the water molecules and bind very tightly to them packing more matter into the space. The mass of the ordinary water consists of the mass of the hydrogen and the oxygen that it contains. The salt water has the additional mass of the sodium and the chlorine atoms that dissolve into the water. In other words the saltwater that we create has more stuff in it and is much denser than ordinary water. The higher the density of a fluid, the easier it is for things to float in it. The saltwater has more particles with which to hold up the floating object. This same principle applies to people swimming in oceans or saltwater lakes. The average person will sink in ordinary water but can float like a boat in saltwater. The Dead Sea is 33% salt by mass. People find it very odd how light they feel in the Dead Sea (Corda, C. 2002) .
Ocean water is very different from fresh water. Ocean water is very salty. It contains many different kinds of dissolved minerals or salt. The amount of dissolved salts in the ocean is referred to as salinity. The salt dissolved in the ocean was once a part of rocks on the land. Rivers eroded the rocks and carried bits of salt and other minerals into the ocean. This action resulted in the accumulation of salt in the ocean water to be extremely dense. Density is a term used to compare the mass of the same amount of two different substances. The density of a material decides whether it will sink or float. Materials which are less dense than a given liquid will float. For example, an egg floats in mercury because the egg is less dense than mercury. On the other hand if an egg is…