In which of the following scenarios is an ionic substance conductive? (A) When ground into a powder, (B) when in the gas state, (C) when cooled with liquid nitrogen, (D) when dissolved in water, or (E) when just below its melting point.
Ionic substances are those that are composed of cations and anions. Cations are ions that are positively charged, while anions are ions that are negatively charged. In an ionic substance, these ions come together to form lattices of alternating ions.
Now what about the second part, conductive? If you imagine an incomplete circuit containing an electrical cell, a negative electrode, and a positive electrode, a material is conductive if when it bridges the gap it allows current to flow. Now in the case of a metal, electrons flow from one end to the other because metals have delocalized electrons that can carry charge.
However, the ions in an ionic substance are fixed in a lattice. When an ionic substance is put into the gap, no current flows. Electrons cannot jump from ion to ion. There may be limited interactions between the surface ionic material and the electrodes. But overall, no current flows from one end to the other.
For conduction to occur, we need some form of mobile charge carrier. Therefore, for an ionic substance, the ions must be mobile. Ions must be able to move from one plate to another carrying charge, allowing material to be conductive. At the electrodes, various oxidation and reduction processes occur, allowing electrons to move through the external circuit.
So of the five scenarios given, we’re looking for one which allows the fixed ions in the lattice of an ionic substance to become mobile. The first scenario is when the ionic substance is ground into a powder. Individual grains of an ionic substance are going to be no more conductive than a solid lump. The ions are still going to be fixed in position, unable to carry charge between the electrodes. Therefore, this is not a scenario that is going to make the ionic substance conductive and is not a correct answer.
What if the ions are in the gas state? On the surface, this looks quite promising. Individual ions should be mobile in the gas state and be able to carry charge between the electrodes. In actual fact, in the gas state, the ions are still more strongly attracted to each other than they are to the electric plates. And they form little molecules that are neutral overall. These neutral molecules are not attracted to the electrodes and so do not carry charge between them. Therefore, this is not a correct scenario.
The answer would’ve been very different if the scenario had been when in a plasma state. In a plasma, rather than gas, ions do exist separately and do respond to the electric field of the electrodes. However, this was not the scenario given. So we can move on to the next one.
What if we cool the ionic substance down with liquid nitrogen? Liquid nitrogen is extremely cold. The boiling point of nitrogen is minus 196 degrees Celsius. So liquid nitrogen is at least that cold. At room temperature, the lattice of an ionic substance will vibrate a little. But the ions will be fixed in place. At the supercold temperatures of liquid nitrogen, there will be even less vibration. Therefore, cooling with liquid nitrogen is definitely the wrong idea. It’s only going to reduce the mobility of the ions. Therefore, it’s not a correct answer.
What if we dissolved the ionic substance in water? When an ionic substance is dissolved in water, water molecules surround each ion. These hydrated ions are free to move through the rest of the water. This allows the individual ions of the ionic substance to migrate to the electrodes and undergo redox reactions. This means that when an ionic substance is dissolved in water, it makes the water conductive. The water allows the ions to be mobile. Therefore, this is the correct answer.
However, just to be safe, let’s look at the last scenario where an ionic substance is just below its melting point. The melting point of the ionic substance is the temperature at which it transitions from a solid into a liquid. Heating the ionic substance to just below its melting point means that it’s still a solid. Therefore, the ions are not mobile. So even though this is a step in the right direction, it’s not enough. Therefore, this scenario is not a correct answer.
So of the five scenarios given, the only one in which an ionic substance will be conductive is when it’s dissolved in water. If you’re interested, there are exotic materials called liquid crystals used in things like liquid crystal displays that do have melting points below room temperature. However, this question is focused on the most common ionic substances, which have high melting and boiling points, like sodium chloride.