Question

Why is this reaction still not balanced? `"CaCl"_2cdot2"H"_2"O"(aq) + "K"_2"CO"_3(aq) -> "CaCO"_3(s) + 2"KCl"(aq)`

Also, what does the dot mean in front of the 2 x water?

Answer 1

Well, you can compare the left and right sides and see that there is no hydrogen atom at all on the products side as it is written right now.

So, you must have water on the products side when you dissolve a hydrate into an aqueous solution (even though it is implied for `"K"_2"CO"_3` that it is in water).

`color(blue)("CaCl"_2cdot2"H"_2"O"(aq) + "K"_2"CO"_3(aq) -> "CaCO"_3(s) + 2"KCl"(aq) + 2"H"_2"O"(l))`

(Also, dihydrate means there are two water molecules, not one, and calcium chloride must have a calcium cation of a `+2` charge. `"CaCl"_color(blue)(2)cdotcolor(blue)(2)"H"_2"O"`, not `"CaCl"cdot"H"_2"O"`)

In terms of the phases:

The `(s)` and `(aq)` are actually supposed to mean something specific. By saying something is a solid, you are saying that it did NOT dissolve successfully.

You are saying that you physically SEE a solid in solution, and not just a transparent solution. You physically SEE white `"CaCO"_3(s)` in solution in real life, because it's not very soluble in water, but you DON'T physically see any `"KCl"(aq)` in solution because it IS quite soluble in water.

In terms of what the reaction actually describes:

The balanced chemical reaction you are meant to write describes the process after everything is already placed into water and mixed.

Even though you know that it's already all in water, you do still have to balance the equation.

You have to place the liquid water molecules on the products side to account for the two water molecules that came from the calcium chloride dihydrate.