MadSci Network: Chemistry

Re: Why does cotton staurated with salt water conduct elec. for a few seconds?

Date: Thu Jan 4 00:56:52 2001
Posted By: Vernon Nemitz, , NONE, NONE
Area of science: Chemistry
ID: 976761348.Ch

Greetings, Ryan:

Let me start by attempting to portray your experiment:

             |     |            |
             |     |            |
             |     |__________  |
             |                  |
      [wire] |       [probes]   | [saturated thread]
             |      __________  |
             |     |            |
             |     |            |
             |     |            |
          [light bulb]

The probes would let you contact the thread at two spots, so the
completed electrical circuit could be something like this:
  (1) From the battery to 1st probe;
  (2) through the probe to the thread;
  (3) through the thread to 2nd probe;
  (4) through that probe to the light bulb;
  (5) through the light to the final wire;
  (6) through the wire back to the battery.

Let's assume you have just-now touched the probes to the thread.  Take
time out to remember that an electric current is a flow of electrically
charged particles.  The particles can be individual electrons, or they
can be atoms or molecules that have either lost or gained electrons (so
they are no longer electrically neutral particles).

Now ask yourself, "What ARE the charged particles in the thread?"  You
have probably discovered that the dry thread does not conduct any
electricity, so this means there are very few charged particles in it
that are able to flow.  You have also probably noticed that the thread
does not conduct when it is saturated with pure water.  Finally,
if you carefully dried a thread that had been saturated with very salty
water, so that the thread became completely encrusted with salt, and used
that dry thread in your experiment...this too would fail to conduct.  
Therefore, the salt and water TOGETHER are necessary, to provide the
charged particles that can flow.  How?

Well, suppose you thought about a variation of the experiment:

             |     |
             |     |
             |     |_______________
             |                    |
      [wire] |       [probes]     | [more wires extend probes]
             |      __________    |
             |     |         |    |
             |     |      |--|----|--|
             |     |      |          | [beaker of salt water]
          [light bulb]    |__________|

PLEASE, if you decide to do this, be sure there is plenty of
ventilation (do it outdoors)!

The two experiments have salt water in common.  Using the beaker, though,
will allow your light bulb to glow far longer than when you used the
saturated thread.  This is simply because there is lots more salty water
in the beaker than in the saturated thread.  So...what is it about salt
water, that conducts electricity?

The answer is related to what happens to salt when it dissolves in water.
By itself, salt consists of atoms of chlorine mixed with atoms of sodium
-- and each atom of chlorine has stolen one electron from a sodium atom.
Both the chlorine and the sodium "ions" are charged particles, and thus
are potentially able to flow in the form of an electric current.  BUT,
the solid substance of salt does not contain mobile particles!  They
cannot flow!

However, just add water, and the clusters of chlorine and sodium ions can
break apart.  Molecules of water surround each ion, and each ion becomes
about as mobile as the water molecules themselves.  NOW they can flow!

In the saturated thread, there is enough water present that in spite of
thread-fibers being in the way, sodium and chlorine ions can still flow.
The main reason that your light bulb glows for only a short time is:
The ions can only move so far, in the cramped crannies among the thread
fibers.  ALSO, some of the ions are "used up".  You may have noticed
a "swimming pool" odor near your saturated thread, about the time your
light bulb went out....

As has been indicated, there are ions all along the distance between the
two "probe" points.  In either thread or beaker, under the influence of
the battery, some of the ions will move toward one probe, and some will
move toward the other.  Their motion IS the essence of electric current,
in that segment of the experimental setup.  But what happens when the
charged particles reach the probes?

The probes are made of wire, and inside wires the primary particle that
represents electric current is the electron.  The battery can literally
push electrons away from itself along one wire, and pull electrons toward
itself along another wire.  In the diagrams above, electrons flowing from
the battery along one wire must eventually reach the salt water.  (So
does the LACK of electrons, along the other wire, also reach the salt
water, as the battery pulls on them.)

As mentioned earlier, each chlorine atom has stolen an electron from a
sodium atom.  Each chlorine ion therefore has excess electrons, and is
naturally attracted to the probe-wire that is feeling the lack.  In the
meantime, the robbed sodium ions are naturally attracted to the probe-
wire that has been filled with electrons by pressure from the battery.

In the beaker is a vast reservoir (compared to the saturated thread) of
ions that can flow toward the probe-wires.  As the ions encounter the
wires, "electrochemical reactions" take place, and one of the results is
that the battery pulls the stolen electrons off the chlorine ions.  This
causes chlorine gas to form (is is the source of swimming-pool odor).  In
small quantities chlorine is used to kill small life-forms like germs; in
large quantities it is able to kill large life-forms like humans.  BEWARE
OF IT!  (Use lots of ventilation.)

At the other probe-wire, the sodium ions do not grab any of the free
electrons that have accumulated there.  Instead they induce the water
itself to break apart slightly, as might be listed in this slightly
fanciful portrayal:
  (1) H2O is a typical description of the water molecule
  (2) HHO is an equivalent description; there are still two hydrogens
      and one oxygen being listed.
  (3) HOH is yet another equivalent description.
  (4) H+ & OH- is what we get if the HOH breaks apart.  There is a
      hydrogen ion on the left, and a hydroxide ion on the right.

Instead of the sodium ions taking the freely available electrons from
the nearby probe-wire, the hydrogen ions do that!  This happens because
the hydrogen ions have greater affinity for those electrons than do the
sodium ions.  The sodium ions remain in the water as ions; hydrogen gas
forms.  You might be happy to know that hydrogen gas is nontoxic.
However, it IS explosively flammable (especially in the presense of
chlorine and daylight!), so you should beware of it, too!

The relative paucity of ions in the saturated thread means that they can
only flow towards their electrochemical destinations in limited amount.
After they finish moving, the fact that they HAVE finished moving means
that electric current is no longer flowing!  Which is the essence of the
Answer to your Question.

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