Given melting point, conductivity and solubility for an unknown substance, decide whether it is ionic, simple molecular, giant covalent or metallic, and defend the choice.
Four models in four weeks: an ionic lattice, a set of simple molecules, a giant covalent network, and a metal. Every solid in the club's crate of unlabelled fragments is one of those four, which is what makes the next trick work. What changes now is the direction of the argument. Until this week you were handed a structure and worked out the properties. From here you get a card of measurements and have to name the structure, which is harder and is how a real bench works.
Three measurements do nearly all of it. The melting point separates simple molecular from the rest, because it takes very little heat to pull whole molecules apart, though a few metals melt low too. Does it conduct as a solid? Only a metal does, and graphite. Does it conduct once melted or dissolved, having failed as a solid? Only an ionic compound switches on that way. Solubility in water is a useful extra: giant covalent substances never dissolve, ionic ones often do, and plenty of small molecules dissolve as well, so solubility supports a case rather than settling one.
Sample A melts at 801 degrees. It will not conduct as a solid, it does conduct once molten, and it dissolves in water. Run the questions in order. 801 is far too high for separate molecules, so molecular is out. It fails the solid test, so it is not a metal. It passes the molten test, and only an ionic compound switches on at that point. A network other than graphite would have failed both conduction tests, and would not have dissolved either. Sample A is ionic, and all four measurements agree.
Ionic and metallic are the pair people mix up, because both of them conduct. The test that separates them is when. A metal's electrons are already loose, so it conducts cold, solid and straight out of the drawer. An ionic compound's ions are pinned until the lattice comes apart, so it conducts only after melting or dissolving. Graphite conducts cold too, so the meter alone cannot tell it from a metal. A hammer can: a metal flattens and stays whole, because the electron sea keeps holding the sliding layers, while only a weak pull holds one graphite sheet to the next, so the piece crumbles.
Sometimes one measurement pulls the other way. Sample B melts at 232 degrees, low enough to suggest simple molecular, but it conducts as a solid, and no molecular substance ever does. When measurements disagree, evidence that rules something out completely beats evidence that only suggests. Sample B is a metal, a soft low-melting one: tin. The argument also runs backwards. Told a sample is giant covalent, you can predict its card before reading it: a melting point in the thousands, no conduction unless it is graphite, and insoluble in water.
Every solid at this level is ionic, simple molecular, giant covalent or metallic, so ruling three out names the fourth. A low melting point points to molecules unless the sample conducts. Conducting as a solid means a metal, or graphite. Conducting only once molten means ionic. None of those means a giant covalent network. When two lines disagree, the one that rules a type out completely beats the one that only suggests.
Adapted from OpenStax Chemistry 2e (CC-BY 4.0), openstax.org · license
The crate of unlabelled fragments comes out. Four models are on the board from the last four weeks, and every piece in the crate is one of them. Sample A is a white solid that melts at 801 degrees.
Which structure type does that number rule out straight away?
Learn first
Jobs that lean on this skill. Follow one to see everything it is built on.
24 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.