The difference in density and weight (and how salt impacts the density of water)
Although we often confuse the two, density and weight are two different measurements. 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. Weight is defined as the mass of an object times the force of gravity. In our world where gravity forces things downward, a denser object will be heavier too. Clear as mud? 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 volume, we can take ordinary water, add salt, 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 ordinary water consists of the mass of the hydrogen and oxygen that it contains (remember H20). The saltwater has the additional mass of sodium and chlorine atoms that dissolve into the water. In other words, the salt water we create has more stuff and is much denser than ordinary water.
Prove saltwater is denser than ordinary water with this floating eggs in saltwater experiment
So how can we prove that 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 saltwater as having more particles with which to hold up the floating object.
- Fill two glasses with water.
- Add 6 tablespoons of salt to one of the glasses of water and mix well.
- Place an egg in each of the glasses.
- 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.
Swimming in saltwater
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.
For instance, 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.
Experiment Supplies
Supplies: Salt, Eggs
Image Credits
Proportion of salt to sea water (right) and chemical composition of sea salt via Wikimedia Commons by Hannes Grobe with usage type - Creative Commons License. November 2, 2008Graphic breakdown of water salinity, defining freshwater, brackish water, saltwater, and brine water via Wikimedia Commons by Peter Summerlin with usage type - Creative Commons License. February 9, 2011
Eggs in salted water and in pure water from Six Weeks Of Vacation (1880) via Six Weeks of Vacation by Paul Poire with usage type - Public Domain. 1880
