Why ionic compounds melt high and conduct when molten

What carries electricity through melted salt? Push two wires from a battery into a pot of molten table salt, and a bulb in the circuit lights up. In a copper wire, the answer is electrons, so electrons feel like the obvious guess here too. They are the wrong guess. The clue is that a lump of solid salt, wired up the same way, lights nothing at all. Before the molten pot makes sense, the solid lump needs a close look.

Sodium chloride is made of ions. An ion is an atom that has lost or gained electrons, so it carries a charge. Each sodium ion is positive, and each chloride ion is negative. The ions are packed in a giant lattice. That is a regular pattern repeating in every direction, with each ion surrounded by ions of the opposite charge. Foundation and Higher students both need every step that follows. Opposite charges attract. That pull is called an electrostatic attraction, which means a pull caused by charge, and in the lattice it acts in all directions, from each ion to every neighbour. Now, which particles is solid sodium chloride built from: molecules, atoms or ions? Ions, and only ions. There is not a single molecule in it, because no small group of atoms is joined up and separate from the rest. That decides a word you must never use here. Intermolecular forces are forces between molecules. Sodium chloride has no molecules, so the word intermolecular has nothing to attach to, and it stays crossed out for the rest of this video.

Melting sodium chloride means freeing its ions from their fixed places, so they can slide past each other. To do that, the attraction between the ions has to be overcome. The explanation is a chain, and it has four links. Link one: sodium chloride is a giant lattice. Link two: so it has a huge number of strong electrostatic attractions between oppositely charged ions, acting in all directions. Link three: to melt it, a very large number of those attractions must be overcome. Link four: that takes a large amount of energy, so the melting point is high, at eight hundred and one degrees C. Your oven tops out at around two hundred and fifty degrees, and the salt in it stays a crisp solid. Eight hundred and one degrees is red-hot furnace territory. One wording trap sits inside link four. A high temperature is what has to be reached. The reason is the large amount of energy needed to overcome the attractions. Now a different compound. Potassium bromide melts at seven hundred and thirty-four degrees C, and it too is made of positive and negative ions. Pause and write the four-link chain for potassium bromide, numbered one to four. One, potassium bromide is a giant lattice. Two, it has many strong electrostatic attractions between oppositely charged ions, in all directions. Three, all of them must be overcome to melt it. Four, that needs a large amount of energy. Link two is the one an examiner's report singles out: "There were very few who stated that the electrostatic forces act in all directions in the giant structure, despite the statement in the specification; this was the mark that most students did not score." Strong bonds on its own is one link, not four. The fix is to count your links: if your answer has fewer than four, find the missing one before you move on. There is a second way to undo a good answer, and it comes back to that crossed-out word. A different examiner's report opens on a positive: "Most candidates scored at least one mark here for correctly identifying that bonds need a lot of energy to break". Then it adds: "Unfortunately a significant number of responses then went on to negate the bonding mark by mentioning intermolecular forces, molecules or atoms rather than electrostatic forces and ions." Both things are true at once: the energy idea was there, and the wrong particles cancelled it. The habit that keeps it is to name ions and electrostatic attraction, and never molecules, in any answer about an ionic compound.

The second property is conduction. Anything that conducts electricity needs charged particles that are free to move from place to place. Solid sodium chloride has plenty of charged particles. But its ions are held in fixed positions in the lattice. They can vibrate on the spot, and that is all, so the solid does not conduct. Now heat the lump until it is fully molten, past eight hundred and one degrees C. What has changed in the molten salt that lets it conduct? The lattice has broken up, so the ions are free to move. Moving ions carry the charge through the liquid, so molten sodium chloride conducts. Look at what stayed the same. The same ions were there all along, with the same charges. Melting only set them free. Nothing new appeared, and no electrons were let loose. Dissolving does the same job by another route. Stir salt into water and the lattice comes apart, with the ions spread through the water and free to move. So a solution of sodium chloride conducts too. A student writes this about a molten ionic compound: it conducts when molten because the electrons are free to move. One word in that sentence is doing all the damage. Which word in the student's sentence is wrong, and what should replace it? Electrons is wrong. Change it to ions: it conducts when molten because the ions are free to move. In a molten ionic compound, the particles carrying the charge are the ions. One more examiner's report shows how often that swap happens: "Only two fifths of students could give a correct answer in terms of the lack of mobility of the ions. Many students referred to the inability of electrons to move rather than ions." Electrons moving is the answer for a metal, and it has its own video in this chapter. For an ionic compound, the fix is one noun: ions, every time. Both properties come off one picture. The ions are held tightly in fixed places. Held tightly, so it takes a lot of energy to free them. Held in place, so nothing can move to carry a charge until the lattice breaks up. Together, they make a fingerprint. A high melting point, no conduction as a solid, and conduction when molten or dissolved, all point to an ionic compound.

A run of quick questions now, finishing at that pot of glowing salt. Why does solid sodium chloride melt at such a high temperature? Give the chain of four links. A giant lattice, many strong electrostatic attractions between oppositely charged ions in all directions, all of them overcome, and a large amount of energy. Now the vocabulary: why is intermolecular the wrong word for sodium chloride? It has no molecules, so there is nothing for an intermolecular force to act between. The right words are electrostatic attraction between ions. Here's a compound you have not met: potassium chloride, a solid made of ions. Does it conduct as a solid, and in water? Not as a solid, because its ions are locked in place. Yes once dissolved, because the lattice comes apart and the ions are free to move. One more: in the pot of molten table salt, which particles carried the charge to light the bulb? Ions, freed when the lattice melted. Electrons do the job in the copper wire, but inside the molten salt, the ions are the particles on the move.

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