Question

Two aqueous solutions react to form a precipitate. What kind of reaction is this?

Answer 1

When two aqueous solutions react to form a precipitate, a chemical reaction called a double replacement (displacement) reaction has occurred. The general equation for a double replacement reaction is AX + BY `rarr` AY + BX, where A and B are cations, and X and Y are anions. In a double replacement reaction, the cations switch places with the anions. One product of a double replacement must be water, an insoluble gas, or an insoluble solid called a precipitate.

The reason that a precipitate forms is that one of the products is insoluble in water. We can determine what the precipitate is by using solubility rules.

For example,

Silver nitrate `("AgNO"_3")` and sodium chloride `("NaCl")` react to produce silver chloride `("AgCl")` and sodium nitrate `("NaNO")`. One of the products is a precipitate.

`"AgNO"_3("aq")+"NaCl(aq)"` `rarr` `"AgCl"+"NaNO"_3"`

To determine which product is the precipitate, you need to check the solubility rules to determine which product is insoluble.

As you can see, all nitrates are soluble, and silver chloride is insoluble. Therefore, silver chloride is the precipitate.

So, now you can finish writing the equation.

`"AgNO"_3("aq")+"NaCl(aq)"` `rarr` `"AgCl(s)"+"NaNO"_3("aq")"`