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Dissolving Salt
Name: staci
Status: other
Age: 30s
Location: N/A
Country: N/A
Date: Around 1999
Question:
HI, I AM SENDING THIS E-MAIL FOR MY DAUGHTER WHO IS 10
YEARS OLD AND IS DOING A SCIENCE PROJECT. SHE NEEDS TO DO HER RESEARCH
REPORT ON "DOES SALT DISSOLVE IN WATER?" I JUST CAN'T SEEM TO FIND ENOUGH
INFORMATION ON THIS. FROM ONE OF YOUR ARCHIVE ARTICLES ABOUT DISSOLVING
IT SEEMED TO SUGGEST THAT IT DOES. I READ A SMALL ARTICLE SAYING THAT
SALT DOES NOT DISSOLVE IN WATER AND THAT IF ALL THE WATER WAS EVAPORATED
THE SALT WOULD BE LEFT BEHIND. PLEASE CLARIFY THIS FOR US AND SEND ANY
INFORMATION YOU MAY HAVE THAT CAN HELP HER EXPLAIN WHY THIS DOES OR DOES
NOT OCCUR. THANK YOU.
Replies:
Salt DOES dissolve in water. That doesn't mean that a chemical change has
taken place however. A chemical change is where bonds are broken and NEW
BONDS FORM causing a rearrangement of atoms into new molecules. Dissolving
is a physical process where the substance retains its own physical
characteristics but just changes state or shape etc. When salt is put in
water, the sodium and chloride atoms are pulled apart by the water. They
disappear. But if the water is taken away, the sodium and chloride atoms
rejoin with each other.
Van Hoeck
Hi Staci...i'm going to try to explain to you about
your question so you can help your daughter...
see...let's try a simple experience:
take a glass of water, pure, clean tap water and
put a teaspoon of sugar in it. Mix well. You will be able
to see that the water remains clear, and you have
now a solution of sugar in water, if you taste the
water now is slightly sweet... the sugar dissolved
in the water.
Now keep putting again one teaspoon after another
teaspoon of sugar in the water. After 2 or 3 teaspoons
the water will no more be clear, but turbid.
That happens because every liquid (we call them
solvents) has a definite capacity to dissolve some
amount of each substance. Less than that amount we have
a solution and the liquid remains clear; more than
that amount the excess don't dissolve and remain
floating to begin with making the liquid cloudy.
Now if you put more sugar again the excess will
go to the botton of the cup.
If you evaporate the liquid, the solid will not evaporate
and will remain all at the botton of the cup.
(do not try to heat the solution to hasten the
evaporation because the sugar changes upon
increasing of temperature and you will turn it
into a syrup)
You can do that with salt and many other substances
because the water dissolves a lot of them.
The experiment will be slower because the salt
is more soluble in water than the sugar is.
But after putting some amount of salt in
water you will find that it not solving anymore
and it stays at the botton of the glass.
The sea water tastes salty just because there
are a large amount of salts dissolved in it.
Now look, when a substance (as the sugar, or
the salt) dissolves in water making a solution
that doesn't changes their properties and the
substance can be recovered, there still are
the sugar (or salt) and the water, or as the
chemists say solute and solvent.
When one makes a solution what
happens is a physical fact, because there
are no chemical changes. But if for example
you heat the sugar solution making a syrup,
now you have done a chemical change and it is not
possible to recover the sugar.
With the salt solution what happens is different
because the salt is not affected by the heating,
and you can recover it.
OK? Ask again to NEWTON!
Mabel
(Dr. Mabel Rodrigues)
Dear Ms.
Salt does indeed dissolve in water, but there is a maximum
concentration that the salt can have. When the salt concentration
reaches its maximum value, the salty water is said to be "saturated."
If you take salty water and heat it enough to evaporate all of
the water, the salt will be left behind, it's true. But this
doesn't have anything to do with whether the salt is dissolved or not.
The reason this happens is that salt particles are very strongly
attracted to one another, but water molecules are less strongly
attracted to one another. So when you heat the salty water, you
break the attractions between water molecules but not the
attractions between salt particles, and the water molecules fly away
(evaporate) leaving the salt particles behind.
If you were to use a blowtorch, you could continue to heat the
salt particles to very high temperatures and make the salt melt,
and eventually it would evaporate too.
I hope this helps.
Best Regards,
Prof. Topper
Dept of Chemistry
The Cooper Union
New York, NY
Yes, salt dissolves in water. That does not mean, however, that the salt
disappears when this happens. The salt and water are mixed together in the
solution. When the water evaporates, the salt is left behind, because water
forms a gas much more easily than salt does.
So this article you read - what evidence did it offer to claim that salt
does not dissolve in water? This would require some pretty astonishing
proof, given that most of the water on earth (the ocean) has salt dissolved
in it.
Richard Barrans, Ph.D.
Assistant Director
PG Research Foundation, Darien, IL
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