Topic 4.2: Net Ionic Equations

Net ionic equations: writing what changed

Back at our fountain, dissolved carbon dioxide became carbon dioxide gas, and that one short line is the equation for the entire show. The water, sweetener, caffeine and color got thrown into the air too, but they didn’t change, so they stay out of the equation, and leaving out what didn’t change is exactly what a net ionic equation does.

Every change has a balanced equation, physical ones too. Ice melting is H₂O solid becoming H₂O liquid, and sugar dissolving is the same sugar molecule going from solid to aqueous, which means dissolved in water. Salt is made of ions, so when it dissolves it comes apart into separate sodium ions and chloride ions.

Balanced means two things. Every element has the same number of atoms on both sides, so no mass appears or disappears, and the total charge matches too, zero on the left, and plus one and minus one on the right, which add up to zero. And a coefficient and a subscript mean different things, because two C L minus means two separate chloride ions, while C L two is chlorine gas, a different substance. Now try one yourself, with calcium chloride. Which equation shows solid calcium chloride dissolving in water?

Three ways to write one reaction

Let’s zoom into that white cloud. The silver nitrate gave us silver and nitrate ions, the salt water gave us sodium and chloride ions, all floating separately. When the liquids mix, the silver and chloride ions grab each other and lock into the solid, while the sodium and nitrate ions keep drifting around like nothing happened.

The molecular equation names the compounds you poured, silver nitrate plus sodium chloride makes solid silver chloride plus sodium nitrate. But dissolved ionic compounds really float around as separate ions, so let’s split them up, keeping the silver chloride together because it’s a solid. That’s the complete ionic equation, everything that’s actually in the water.

Now there’s a sodium ion on the left and the same sodium ion on the right, and the same for nitrate, so they’re spectator ions, and we cancel them. What’s left is the net ionic equation, silver ion plus chloride ion makes solid silver chloride, the part that actually changed. One silver and one chlorine on each side, and plus one and minus one add up to zero, matching the neutral solid.

So each way of writing it has its own job. The molecular equation tells you what to pour, the complete ionic equation shows what’s really in the water, and the net ionic equation shows only what changed.

Now look back in the water at the ions we just canceled. The sodium and nitrate ions are still right here, so canceling them changed the equation, not the beaker. We canceled sodium and nitrate out of the equation, so go find where they actually ended up, and tap them.

Golden rain

Let’s do the next one together, with two more clear liquids, lead nitrate and potassium iodide. And look at that, a storm of bright yellow. The yellow solid is lead iodide, P b I two, and it takes two potassium iodides for each lead nitrate, because the solid needs two iodides. Now you try. Split it in your head before I write.

One lead ion and two nitrates, and since each nitrate is minus one, that lead ion must be two plus, then two potassium ions and two iodides, and the lead iodide stays whole as a solid. Your turn again. Which ones are the spectators this time? The two potassium ions and the two nitrate ions show up unchanged on both sides, so they cancel. That leaves one lead ion plus two iodide ions making solid lead iodide, and plus two with two minus ones makes zero, just like the neutral solid.

Attach iodide ions to this lead ion until the charges balance, and see how many it takes. Here’s why it’s called golden rain. Heat the mixture and the yellow solid dissolves, and as it slowly cools over an hour or so, it grows back as these glittering plates.

It looks like gold, but it’s still lead iodide. This time, pour in extra iodide. When the solid forms, two iodide ions are left over in the water. They float in the water with the potassium and the nitrate.

But iodide reacted. Four iodides went into the solid. A spectator is an ion that never joins the solid. Look at what’s in the water. Which ions are the spectators?

What splits, what stays

So how do you decide what to split? Dissolved ionic compounds split into ions, and so do strong acids like hydrochloric acid, which come apart completely in water, but solids, liquids, gases and dissolved molecules like sugar stay whole.

Here’s sugar water next to salt water. The sugar dissolves but stays in one piece, while the salt comes apart into ions, so both are aqueous and only one of them splits. So aqueous doesn’t automatically mean split. Weak acids, like the acetic acid in vinegar or hydrogen sulfide, mostly stay together as molecules too, and for now I’ll always tell you when an acid is weak.

Now mix salt water with potassium nitrate solution. Split everything and every ion shows up unchanged on both sides, so everything cancels, and that means no reaction, just ions mixing, which is a physical change. Sodium, potassium and nitrate keep showing up as spectators because their compounds always dissolve, and so do ammonium compounds, and for anything else an AP question will tell you which solid forms.

Black smoke

Now it’s your turn, somewhere very far from a lab. In April of 1979, the research submarine Alvin was exploring the Pacific sea floor off Mexico when its lights found these. Chimneys of rock pouring out black smoke, more than a mile down, in water that’s otherwise close to freezing. When the crew brought their temperature probe back up, its plastic tip had melted, and a later probe measured the water shooting out at about three hundred fifty degrees Celsius, still liquid because of the crushing pressure down there.

But that black stuff only looks like smoke, because it’s really billions of tiny grains of solid, our precipitate clue again, telling the truth. The hot water carries dissolved iron ions with a two plus charge and hydrogen sulfide, a weak acid that stays as molecules, and in the cold seawater they form grains of solid iron sulfide while hydrogen ions drift away.

Using what the vent water carries and what the grains are made of, which is the right net ionic equation, with atoms and charge balanced? The real smoke is a mix of sulfides of iron, zinc and copper, so our equation is a simplified model of one of them.

On the AP exam

This one comes from a real AP free-response question from 2023. Solid calcium carbonate, the stuff in chalk and limestone, reacts with hydrochloric acid to make carbon dioxide, water and dissolved calcium chloride, and they want the net ionic equation. What’s the net ionic equation for this reaction?

On the real exam, that balanced net ionic equation is the line that earns the point. The states weren’t required on that question, but writing them is how you know what to split. Notice where this carbon dioxide came from. Its carbon started out locked in the calcium carbonate, so a reaction made this CO₂, while our fountain’s was already dissolved in the soda, or so we said, and one experiment can check that.

The test

Back in the field, here are two bottles of the same soda. This one’s fresh, and this one sat open overnight, so a lot of its dissolved carbon dioxide has escaped while everything else is still there.

Suppose the gas really was already dissolved in the soda. What do you predict the flat bottle will do? Go ahead and run the experiment yourself, and drop a Mentos into each bottle.

One test can’t rule out every other idea, but lab measurements agree that the candy barely makes bubbles once the dissolved gas runs low, so I’m calling the fountain a physical change. So let’s give the whole field one more go.

All hundred and one of these fountains are the same physical change, dissolved carbon dioxide becoming gas, and everything else in those bottles stays out of the equation because it didn’t change. So whenever something fizzes or changes color, ask whether anything new formed, and when you write the equation, write what changed, and only what changed, without pretending the rest disappeared.