Topic 4.4: Physical and Chemical Changes

Physical and chemical changes: follow the bonds

Here’s an airbag frozen at full bloom, and in that blink it did two different things. It turned solid sodium azide into nitrogen gas, and then that gas filled the bag. Let’s light up the bonds in the pellet. The sodium and azide ions are held together by ionic bonds, and inside each azide ion, the three nitrogens are held together by covalent bonds.

When it goes off, those bonds break, and brand-new bonds form, a triple bond between the two nitrogens in every N₂. Bonds broke and new bonds formed, so the pellet made new substances, and that’s a chemical change.

Now look inside the bag. Every triple bond stays exactly where it is, and the only attractions changing are the weak ones between neighboring molecules, which fade as they spread apart. No bonds broke, so that’s a physical change.

Last lesson you sorted changes by the clues you could see, and now we can explain them with the bonds. A process where chemical bonds break or form is a chemical process, and a process where only the attractions between particles change, like in melting or boiling, is a physical process, and those attractions are the intermolecular forces you met in the last unit.

Chemists typically classify processes this way, and I’m circling typically, because later we’ll meet a case where it gets fuzzy. And you can see the difference from the outside, too. A toxic white solid turned into colorless nitrogen gas, which barely reacts with anything, and silvery sodium metal, so the products have completely new properties, while the gas spreading through the bag is still the same nitrogen.

Getting hot doesn’t prove a change is chemical, and neither does happening fast, because the test is always what happened to the bonds. Some airbag inflators skip the pellets. They store compressed argon gas and just let it out into the bag. So what kind of process is filling a bag that way, and why?

The flame that jumps

The airbag does all of this in a few hundredths of a second, so to watch both kinds of change slowly, let’s go somewhere a lot calmer. At Christmas in 1860, the scientist Michael Faraday stood at a bench like this one at the Royal Institution in London, and gave a series of lectures for young people about the chemistry of a single candle.

He blew a candle out very gently, and a thin white stream rose up from the wick. Then he held a lit taper a couple of inches above the wick, and in his own words, you will observe a train of fire going through the air till it reaches the candle. So what was in that stream that could carry fire all the way back down to the wick?

Let’s open the candle up. Wax is made of long molecules of carbon and hydrogen, and in the solid they’re packed side by side, held together by the attractions between neighbors. Let’s follow one of them, a molecule with twenty carbons. Near the flame the wax melts and the molecules start sliding past each other, then the liquid climbs up the wick, and right at the top it turns into a vapor, with the molecules flying free.

It’s the same molecule the whole way up. Melting and vaporizing only changed its neighbors, so those are physical changes. Then it reaches the edge of the flame, where oxygen from the air is waiting, and now its bonds break, and its atoms leave in brand-new molecules, carbon dioxide and water. That part is the chemical change.

So one candle does three things. It melts and it vaporizes, which only change neighbors, and it burns, which changes partners. Real wax is a mix of big molecules, so to see exactly which bonds change, we use our methane, which burns the same way, with carbon–hydrogen and oxygen–oxygen bonds breaking and carbon–oxygen and oxygen–hydrogen bonds forming. And that’s what carried the fire, because that stream was unburned wax, as vapor and tiny droplets, so the flame could burn its way right back down to the wick.

Now you try it. Blow the candle out, then bring the lit taper into the vapor above the wick before it cools. Faraday also warned that you have to be quick, because if you wait, the wick cools, and the vapor turns back into tiny droplets of liquid and solid wax, which drift away as white smoke, and then the relight fails. It’s still wax, so that’s one more physical change. When liquid wax on the wick turns into vapor, what breaks?

Salt: the change that won’t pick a side

For molecules like wax and nitrogen, partners and neighbors are two different things, so the line between physical and chemical is sharp. In a salt crystal, though, they’re the same thing. Inside a salt crystal every sodium ion is bonded to the six chloride ions around it, and every chloride to six sodiums, so an ion’s neighbors are its bonds.

Now watch what water does to it. The slightly negative oxygen ends of the water molecules turn toward a sodium ion, and it lifts right out of the crystal, wrapped in water. Chloride ions get pulled out too, with the slightly positive hydrogen ends of the water facing them. And these are the same sodium and chloride ions the whole time. They were already ions inside the crystal, and their charges never change.

So here’s what everybody agrees on. Before, there are sodium and chloride ions held together by ionic bonds, and after, the same ions sit apart, each one held by attractions to water molecules, which we call ion–dipole attractions. The name says what they are, an ion pulling on the oppositely charged end of a polar water molecule.

If you call it physical, your argument is that the ions keep their identity and nothing new forms, so if you boil the water away, the very same salt comes back. If you call it chemical, your argument is that ionic bonds, which are real chemical bonds, broke, and new ion–dipole interactions formed.

Both are good arguments, and that’s why we said typically back there. On the AP exam either label can work, as long as you back it up by naming what broke and what formed. Compare that with sugar. When sugar dissolves, whole sugar molecules leave the crystal and nothing inside them breaks, so that one’s physical, no argument.

Here’s a sodium ion and a chloride ion that just left the crystal. Turn each water molecule so it faces its ion the right way. Which of these arguments about dissolving salt is not a valid one?

Earning the point

Let’s finish with the kind of question the AP exam could ask about our airbag. Sodium azide dissolves easily in water, and in an inflator it decomposes, and the question wants you to compare what happens to the bonds in each. Which statement compares the two correctly?

For part b, this is the sentence that earns the point, that the covalent bonds inside the azide ion break and triple bonds between nitrogen atoms form. Saying it made a gas, or that it got hot, doesn’t earn it, because neither one says what happened to the bonds. For part a, you can call dissolving physical or chemical, as long as you name the ionic bonds that break and the ion–dipole attractions that form, and say that each azide ion stays in one piece.

The blink, explained

So let’s go back to the crash hall one last time, and this time, you’re running the test. Fire it, and every time it freezes, call it: chemical or physical? When the pellets went off, bonds broke and new ones formed, so that part was chemical, and when the gas filled the bag, the same molecules just spread out, so that part was physical.

And every nitrogen atom that caught that dummy’s head was sitting inside a pellet a moment before. You can now turn a reaction into a complete picture of its particles and tell a chemical change from a physical one by what its bonds do, and next time we’ll put real masses on those pictures, which chemists call stoichiometry.