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Floating eggs in salt water

Density is a 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 confused the two, density and weight are actually two different measurements. Weight is defined as the mass of an object times the force of gravity. In our world where we have gravity forcing things downward, a denser object will be heavier too. Clear as mud? Well, let’s do this experiment and at least make it as clear as, err… salt water.

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. Yes, the volume increases a little bit when we add the 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 into ions 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 oxygen that it contains (remember H20). The saltwater has the additional mass of the sodium and chlorine atoms that dissolve into the water. In other words, the salt water that we create has more stuff in it and is much denser than ordinary water.

So how can we prove that the saltwater is denser than ordinary water? By understanding that the higher the density of a fluid, the easier it is for things to float in it. You can think of the salt water as having more particles with which to hold up the floating object.

  1. Fill two glasses with water
  2. Add 6 tablespoons of salt to one of the glasses of water and mix well.
  3. Place an egg in each of the glasses.
  4. Notice that the egg will float in the saltwater but sinks in the ordinary tap water. That is because the saltwater is denser than the regular water.
This same principle applies to people swimming in oceans or salt water 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 “floaty” they are when they swim in the Dead Sea (by the way, the high salt content makes it hard for things to live in the Dead Sea water and hence the name, “Dead” sea).

You might not know this but skinnier or more muscular people are denser than fatter people. Some championship swimmers for instance, will sink in salt water while heavier people will still float. Make a note of that fact – you can use it next time you and Dad go swimming.

Supplies: Salt, Eggs