|MadSci Network: Chemistry|
Greetings, Ryan: Let me start by attempting to portray your experiment: [battery] | | | | | | | |__________ | | | [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: [battery] | | | | | |_______________ | | [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.
Try the links in the MadSci Library for more information on Chemistry.