ASK A SCIENTIST
Question: Why do you float better in salt water then you would in a pool?
Answer: Let's consider Archimedes' Principle. The principle states that the buoyant force (the force that helps you to float) is equal to the weight of the displaced fluid. It means that in order for you to float in water, your body will have to move out of the way a certain volume of water. Another consideration is that the water has to have a mass per volume ratio higher than your body; in other words, the water has to be denser than your body. The denser the water, the more you will float.
The next part of the answer deals with the difference between the density of water in a swimming pool and in salt water.
One difference that most of us have experienced (or heard of) is the distinct salty taste of the ocean water. The ocean is salty because there are a lot of chemicals dissolved in it. Most of these chemicals come from the erosion of the Earth's crust. The total salt content, also called salinity, of the ocean water varies from region to region. It is affected by several factors such as rain, snowfall, ocean currents and melting of ice. A good estimate of the salinity of the ocean is 36,000 parts per million (ppm).
The salinity in the swimming pool comes from the bromine and chlorine compounds which are generally added to the swimming pool to control the levels of bacteria and viruses. These chemicals are used to minimize the spread of diseases between swimming pool users. In a swimming pool the salinity is about 8,000 ppm. The salinity of the ocean is 4.5 times higher than the swimming pool!
This is important information when we consider floatation because the salinity will influence the density. If you were to measure the weight of one glass of ocean water and compare it with the weight of one glass of swimming pool water you would see that the ocean water is the heaviest one. Since density is determined by the ratio between mass and volume, the salty ocean water is denser than the swimming pool water.
So, now it is simple to conclude that the saltier the water, the easier it will be for you to float.
Ask a Scientist appears Thursdays. Questions are answered by faculty at Binghamton University. Teachers in the greater Binghamton area who wish to participate in the program are asked to write to Ask A Scientist, c/o Binghamton University, Office of Communications and Marketing, PO Box 6000, Binghamton, NY 13902-6000 or e-mail firstname.lastname@example.org. Check out the Ask a Scientist Web site at askascientist.binghamton.edu. To submit a question, download the submission form(.pdf, 460kb).