How is saltwater able to conduct electric charge between two wires?

The salt (NaCL) water solution is an electrolyte solution which is, essentially, a conductive solution.

The conductivity of the salt water is due to the presence of both positively and negatively charged ions. These ions in solution are free to accelerate in the presence of an electric field and thus, like the free electrons in a metal conductor, are able to participate in an electric current (not to be confused with an electron current).

When there is an electric current through the salt water, there are actually two contributions: (1) the positive sodium ions drifting in the direction of the electric current and (2) the negative chlorine ions drifting the opposite direction.

While it may seem that the oppositely directed ion currents should cancel they, in fact, add. The flow of negative ions contributes to an electric current in the opposite direction due to the negative sign of their charge.

At the interface between the metal conductor and salt water, there are reactions that either remove electrons or add electrons to the conductors thus completing the path for charge to flow around the circuit.

From the online GenChem Textbook section on Electrolysis:

Figure 1 An electrolytic cell. The battery pumps electrons away from the anode (making it positive) and into the cathode (making it negative). The positive anode attracts anions toward it, while the negative cathode attracts cations toward it. Electrical current is carried by electrons in the wire and electrodes, but it is carried by anions and cations moving in opposite directions in the cell itself. Since the anode can accept electrons, oxidation occurs at that electrode. The cathode is an electron donor and can cause reduction to occur.

It is important to note that electric current is simply defined as the flow of electric charge and this definition does not depend on the species of charge carrier.

There is, in fact, just one electric current in the circuit while there are three (or more) different species of charge carriers along the circuit's path: (1) electrons in the metal conductors, (2) positive sodium ions in the salt water, (3) negative chlorine ions in the salt water, and (4+) if the voltage source is a battery, ions in the battery's electrolyte.

Most materials do not change when a current flows through the material. The material somehow allows charge (in most cases electrons) to pass through the material without affecting the material. Metals are very good at this because some of the electrons in a metal are very loosely bound to the atoms.

Pure water consist mostly of H20 molecules. They offer very little option for electrons to pass through the fluid.

Salt water consists (mostly) of H20 molecules, Na+ ions and Cl- ions. When two electrodes are put in such a fluid with some potential difference the Na+ ions will be attracted by the negative electrode and will move (somewhat more) to that electrode. Likewise for the Cl- ions and the positive electrode.

When the voltage (potential difference) is large enough the Cl- ions at the positive electrode will give an electron to the electrode, and recombine to Cl2 molecules (or react with the electrode). You will be able to see and/or smell this. The situation at the negative side is more complex, but at that side the net result will be that the Na+ will help the H20 to get converted to H2 and OH-. You will see bubbles of H2 gas here.

The net effect is that NaCl and water become Cl2 gas and H2 gas. This is a chemical reaction. It is quite different from normal electron conduction:

• it will last only while the is NaCl (or rather: Cl- ions) left
• it requires some minimum voltage

Without the NaCl a comparable process can take place, using the small amount of H2O molecules that are dissolved into H+ and OH- ions. Because this amount is very very small pure water will conduct much much less than salt water.

In some sense these electrochemical conduction is more like charging a battery than pushing current through a wire. In a wire (or other resistive material) all energy from the electrical current*voltage is converted to heat. With electrochemical conduction some energy goes into the endothermic (= energy consuming) chemical reaction. With a lot of extra work (in this case, oa. capturing the gasses and keeping the close to the electrodes) what you get is a charged battery: it can supply current when you connect its two leads.

So a very brief answer to your question could be: because salt water behaves like a (badly engineered) accu.

Just to add, since none of the other answers have pointed it out, but electricity CAN jump through air. Or even rubber or other insulators, with the right voltage and current. Ever see a spark? A air gap between two wires can be bridged, and it happens accidentally (spark when you plug something into an outlet, when you connect jumper cables to a car, static electricity) or intentionally (car spark plugs, electric lighters, Jacob's Ladders). At a certain voltage/current, normally insulating material like rubber and glass can conduct electricity, at a break down voltage. This is the Dielectric strength of the material. Most things can be made to conduct if you juice them enough. Salt Water is just relatively easier to do.