Chapter 1 · Question 9

What is a double displacement reaction? Explain with the example of the reaction between sodium sulphate and barium chloride solutions. What is a precipitation reaction and how is it related to the above example?

Answer Revealed

Direct Answer:

A double displacement reaction involves an exchange of ions between two reactant compounds, forming two new compounds. When sodium sulphate solution reacts with barium chloride solution, a white insoluble solid — barium sulphate — precipitates out:

\text{Na}_2\text{SO}_4\text{(aq)} + \text{BaCl}_2\text{(aq)} \rightarrow \text{BaSO}_4\text{(s)} + 2\text{NaCl(aq)}

. A precipitation reaction is any reaction in which an insoluble solid (precipitate) is formed. The above reaction is also a precipitation reaction because

\text{BaSO}_4

is insoluble in water and appears as a white precipitate.

Simple Explanation

In a double displacement, the two compounds swap partners — like two couples exchanging dance partners. Sodium sulphate and barium chloride swap ions: the sodium goes with chlorine, and the barium goes with sulphate. The barium sulphate formed is a white solid that settles at the bottom because it does not dissolve — this is called a precipitate. So this reaction is both a double displacement and a precipitation reaction.

Exam-Ready Structure

Double displacement reactions and precipitation reactions are closely related concepts in the NCERT chapter: 1. Definition (Double Displacement): A reaction in which two different atoms or groups of atoms (ions) are exchanged between two reacting compounds. General form:

AB + CD \rightarrow AD + CB

. In such reactions, ions are exchanged between the reactants to form new products. 2. Example — reaction of sodium sulphate with barium chloride:

\text{Na}_2\text{SO}_4\text{(aq)} + \text{BaCl}_2\text{(aq)} \rightarrow \text{BaSO}_4\text{(s)} + 2\text{NaCl(aq)}

. Here, the

\text{Na}^+

ions from sodium sulphate combine with

\text{Cl}^-

ions from barium chloride to form sodium chloride (

\text{NaCl}

), while

\text{Ba}^{2+}

ions combine with

\text{SO}_4^{2-}

ions to form barium sulphate (

\text{BaSO}_4

). 3. Observation: A white, insoluble substance (

\text{BaSO}_4

) is formed that settles at the bottom of the test tube. 4. Definition (Precipitation Reaction): Any reaction in which an insoluble solid (called a precipitate) is formed when two solutions are mixed. The solid that separates out is called the precipitate. 5. Relation: The reaction between sodium sulphate and barium chloride IS a precipitation reaction because it produces the insoluble white precipitate of barium sulphate. Every precipitation reaction is a type of double displacement reaction, but not every double displacement reaction necessarily involves a precipitate — some may produce neutralisation (acid-base reaction forming salt and water). 6. Another NCERT example: Lead nitrate and potassium iodide react to form a yellow precipitate of lead iodide:

\text{Pb(NO}_3)_2\text{(aq)} + 2\text{KI(aq)} \rightarrow \text{PbI}_2\text{(s)} + 2\text{KNO}_3\text{(aq)}

. This is also both a double displacement and a precipitation reaction.

Key Points

Double displacement: ions are exchanged between two compounds ( $AB + CD \rightarrow AD + CB$ )
Example: $\text{Na}_2\text{SO}_4\text{(aq)} + \text{BaCl}_2\text{(aq)} \rightarrow \text{BaSO}_4\text{(s)} + 2\text{NaCl(aq)}$
White precipitate of $\text{BaSO}_4$ is formed — it is insoluble in water
Precipitation reaction: any reaction producing an insoluble solid (precipitate)
Another example: $\text{Pb(NO}_3)_2 + 2\text{KI} \rightarrow \text{PbI}_2\text{(s)} + 2\text{KNO}_3$ (yellow precipitate)

Common Mistakes

Confusing double displacement with a single displacement — in double displacement, ions are exchanged; in single displacement, one element pushes out another
Writing the precipitate with (aq) instead of (s) — the precipitate is insoluble, so it must have (s) as the state symbol

What is a double displacement reaction? Explain with the example of the reaction between sodium sulphate and barium chloride solutions. What is a precipitation reaction and how is it related to the above example?