Answered

You may have noticed an (s) next to one of the products in some double replacement reactions. An (s) next to a compound signifies that it is an insoluble solid, also known as a precipitate. Precipitates can be either cloudy or chunky substances of various colors in an ionic solution. To predict if a product in a double replacement reaction forms a precipitate, you need to use the solubility rules.



For each reaction below, select which compound will form a precipitate. Note: It is possible that no precipitates form.


  1. $Na _2 S+ Cd \left( NO _3\right)_2 \rightarrow CdS +2 NaNO _3$
  2. $FeCl _3+3 KOH \rightarrow 3 KCl + Fe ( OH )_3$
  3. $Na _2 CO _3+ Ba \left( NO _3\right)_2 \rightarrow BaCO _3+2 NaNO _3$

Verified Answer :

  • Precipitation occurs when an insoluble solid forms from aqueous reactants, following standard solubility rules.
  • In reaction \(Na_2S + Cd(NO_3)_2 \rightarrow CdS + 2NaNO_3\), the precipitate is \(\boxed{CdS}\).
  • In reaction \(FeCl_3 + 3KOH \rightarrow 3KCl + Fe(OH)_3\), the precipitate is \(\boxed{Fe(OH)_3}\).
  • In reaction \(Na_2CO_3 + Ba(NO_3)_2 \rightarrow BaCO_3 + 2NaNO_3\), the precipitate is \(\boxed{BaCO_3}\).

Explanation

  1. Solubility Rules Overview

To determine whether a product in a double replacement reaction forms a precipitate, we rely on established solubility rules. A precipitate, denoted as \((s)\), is an insoluble solid that forms when two aqueous solutions are mixed. Conversely, compounds that remain dissolved in the solution are denoted as \((aq)\) for aqueous. Generally, most alkali metal salts (like those containing \(Na^+\)) and nitrates (like \(NO_3^-\)) are soluble in water. On the other hand, many sulfides, hydroxides, and carbonates are insoluble unless paired with specific cations.

  1. Analyzing Reaction 1: \(Na_2S + Cd(NO_3)_2 \rightarrow CdS + 2NaNO_3\)

For the first reaction, \(Na_2S + Cd(NO_3)_2 \rightarrow CdS + 2NaNO_3\), we evaluate the solubility of the products \(CdS\) and \(NaNO_3\). Sodium nitrate (\(NaNO_3\)) contains both an alkali metal ion (\(Na^+\)) and a nitrate ion (\(NO_3^-\)), both of which are highly soluble in water, so it remains \((aq)\). Cadmium sulfide (\(CdS\)), however, does not follow the solubility rules that allow for dissolution; sulfides are typically insoluble except when paired with Group 1 or Group 2 metals or ammonium. Therefore, \(CdS\) is the precipitate.

  1. Analyzing Reaction 2: \(FeCl_3 + 3KOH \rightarrow 3KCl + Fe(OH)_3\)

For the second reaction, \(FeCl_3 + 3KOH \rightarrow 3KCl + Fe(OH)_3\), the products are \(KCl\) and \(Fe(OH)_3\). Potassium chloride (\(KCl\)) is a salt of a Group 1 metal and a halide, which is soluble, resulting in an \((aq)\) state. Iron(III) hydroxide (\(Fe(OH)_3\)), on the other hand, is a hydroxide that does not fall under the common exceptions (which are Group 1 metals, ammonium, barium, strontium, and calcium). Consequently, iron(III) hydroxide is insoluble and forms a precipitate.

  1. Analyzing Reaction 3: \(Na_2CO_3 + Ba(NO_3)_2 \rightarrow BaCO_3 + 2NaNO_3\)

For the third reaction, \(Na_2CO_3 + Ba(NO_3)_2 \rightarrow BaCO_3 + 2NaNO_3\), the products are \(BaCO_3\) and \(NaNO_3\). As established, sodium nitrate (\(NaNO_3\)) is soluble \((aq)\). Barium carbonate (\(BaCO_3\)) is a carbonate compound. Most carbonates are insoluble in water, and while there are exceptions, barium carbonate is not among them; it is a well-known insoluble solid. Therefore, \(BaCO_3\) is the precipitate.

Examples

In daily life, the concept of precipitates is vital in water treatment plants. Water often contains dissolved minerals like calcium and magnesium, which cause hardness. By adding specific chemicals, these minerals are forced to form insoluble precipitates that can then be filtered out, leaving behind softened, cleaner water. Understanding whether a substance remains dissolved or becomes a solid is essential for processes ranging from environmental cleanup to culinary chemistry!