|WHAT DOES FLOATATION HAVE TO DO WITH THE DENSITY
OF SALT WATER?
|Question Date: 2009-04-09|
A density is the amount of material (its mass)
in a given volume. For example, the density of
pure water is 1 kg/L. That means that each liter
of pure water has a mass a 1 kg. When minerals
dissolve in water, they separate into ions
(positively or negatively charged atoms or
molecules). For example NaCl (salt) dissolves to
form Na+ and Cl- ions. There can be a lot of Na+
and Cl- and other ions dissolved in saltwater and
each of these ions has a greater mass than water.
What that means is, if you have extra ions in
water (instead of just pure water) those ions
take up part of the volume of water and make it
heavier. If you have a liter of pure water, it
will have a mass of 1 kg. If you have a liter of
salt water, you will have a lot of pure water
plus some other ions that have a greater mass than
water, making the whole liter of saltwater have a
mass greater than 1 kg. So ions increase the
density of saltwater.
When ions get dissolved in water, they float
along with the water molecules, but they don't
increase the volume very much. However, density
is mass divided by volume, so since we've added a
lot of mass (the ions) without changing the
volume very much, we get water that has a higher
An important concept when thinking
of floating is Archimedes' Principle. It states
that objects more dense than water will sink,
while objects less dense will float. This makes
some sense - a rock would sink in water, but
styrofoam wouldn't. If we were to increase the
density of water (by adding ions), we could make
it easier for something to float in the water.
Most ionic substances are denser than water,
and as a result, they increase the density of the
solution they are dissolved in. Obviously, this
affects what floats and what doesn't.
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