Re: Are objects more bouyant in salt water?

Yes, they do.

There is a very famous principle in Physics, known as Archimedes Principle, which states that the amount of liquid displaced by a floating object has the same mass as the object.

Imagine that you have a cork that is floating half under the water, and half sticking out. The half that is under the water is occupying space that would be occupied by water if the cork wasn't there. That volume of water -- which is half of the volume of the cork -- has been 'displaced'. It would weigh the same as the whole cork. We say that the cork has only one half of the density of the water, because the cork occupies just twice as much space as the same mass of water.

Now let us get back to the subject. Salt water is just a little bit more dense than fresh water. A liter of fresh water weighs 997 gram, but a litre of sea water weighs 1023 gram (at 25 deg C in each case). If an object weighing 1023 gram were floating in the sea, it would displace just 1 litre of water. But if it were floating in pure water, when it sank to the 1 litre displaced level, it would have displaced only 997 gram of water. It would need to sink just a little bit further to displace the extra 26 gram to match its mass. So objects float lower in fresh water than in sea water.

Ships have a marking on their hull to show where the water level should be when they are carrying a maximum safe load. It is called the 'plimsoll line'. But if you look at the hull of a ship, you will find there is not a single line, but a series of four or five lines! The reason is that the density of water is different for fresh water and for salt water, and also different for cold water and for warm water -- so that with a given load, the ship will float higher in some ports than in others. It will float lowest in warm, fresh water -- Panama Canal, New Orleans -- a little higher in fresh cold water -- London, Great Lakes -- higher still in warm salt water -- Colombo, Singapore -- and highest of all in cold salt water -- Southampton, New York.

The Dead Sea in Palestine/Israel has a very high salt content, and so it has very dense water. It is said that people swimming in the Dead Sea can read a book while floating on their backs. I had hoped to bring you a picture of some such thing from the Web, but failed to find it. I know that when I was younger than you now are I was told that it was easier to float in salt water than fresh water, and was disappointed to find that it was not: in the sea I kept getting swamped by waves, which were not a problem in the quiet inland waterhole where I usually did my swimming!

John.