Actually -- it does mix -- but as you have
observed, not easily.
Salt water with a substantial amount of salt
dissolved in it is chemically and physically quite
distinct from fresh water. First, salt water has
substantially higher density (enough that
in very salty water like the Dead Sea or The Great
Salt Lake, it is very difficult to dive -- human
bodies float substantially higher in the water).
Secondly, salts like Sodium Chloride dissociate
when dissolved in water into Sodium and chlorine
ions surrounded by water molecules called ion
complexes, which share the heavy ionic potential
of the ions. So when you mix the two, (this is
easy to see if you dissolve some food coloring
into one of them), the salt water immediately
sinks to the bottom (so it only contacts
freshwater on a small surface) and at this
boundary, there is a requirement for thermal
energy if the salt water is fairly
concentrated, slowing the diffusion of the
two. However, if you wait a long while, you will
see that they do mix-- this can be hastened by
heating or by vigorous stirring... Such mixing
will not occur if two liquids are immiscible --
like oil and water.
The reason salt water and fresh water tend to
separate is because their densities are
different. Density is a useful idea in
science. It means how much "stuff" is in a
certain amount of space. A can of air has less
mass than a can of water that's the same size. So
the can of water has a higher density. Salt water
weighs more than the same amount of fresh water.
This means that fresh water will "float" on top of
salt water. This happens when water from rivers
flow into the sea.
There are some interesting experiments you can
do with salt water. Ask your teacher or parents to
help you try this: Take a grape and put it in a
cup of water. Usually it will sink. Now try adding
salt (or sugar) to the water. How much do you
have to add before the grape floats? You
haven't changed the density of the grape (you can
weigh and measure it to make sure), so why does
it float when you add salt or sugar to the water?
In fact, if you pour salt water into fresh
water, it does not separate ! It mixes. This is
what happens when a river (fresh water) meets the
sea (salt water).
Take two glasses, each half full of water. Put
in a teaspoon of salt into one of the glasses and
stir. Then take a small sip. It will be salty !
Now pour the liquid in this glass into the glass
with fresh water, and take a small sip. It will be
less salty. You will find that the teaspoon of
salt is now perfectly distributed (dissolved)
through the entire amount of water.
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