Precipitation Reactions

If you've ever poured sugar into a glass of water, you'll notice that the water pretty much looks the same it did before you poured in the sugar. This property of the sugar to just seamlessly dissolve into the water is known as solubility. Sugar has a high solubility, allowing it to dissociate very well in water due to its slight polarity. This is expanded on the section dedicated entirely to Solubility.

Substances with high solubilities are known as soluble, and usually mix seamlessly with whatever they're dissolved in(known as a solvent). Substances with low solubilities are known as insoluble, and they tend to precipitate during reactions.

The image above shows lead(II) nitrate mixing with potassium iodide. This mixture forms a precipitate of lead(II) iodide, which is collected as a solid at the bottom of the beaker.

The equation for this reaction is this:

2KI(aq) + Pb(NO₃)₂(aq) → PbI₂(s) + 2KNO₃(aq)

Note that for precipitation reactions, it's a good idea to label the phase of each species so you can easily tell what the precipitate(the solid species) is. In this case, it's PbI₂.

While the format above is good for precipitation reactions, we can express this in terms of the net ionic equation, where we represent the ions that coming together to form a precipitate.

Pb²⁺(aq) + 2I^-(aq) → PbI₂(s)

The net ionic equation for this precipitation would be the one above. It purely represents the ions that came together to form the precipitate, not worrying about the ions that smoothly transitioned and dissolved into the solvent.

This is the more easily approachable way to write out a precipitation reaction. Note that the charge is also conserved for the reaction.