Common Ion Effect

Category: 
Demonstration
Topics: 
Equilibrium
States of Matter

$\require{mhchem}$

 

Introduction

When concentrated hydrochloric acid is added to a large test tube containing saturated sodium chloride solution, white sodium chloride precipitates out due to the common ion effect.

Curriculum Notes 

This is a great demo to illustrate the common ion effect in a general chemistry course.  

One day of lead time is required for this project.

Discussion 

$\ce{HCl(g) + H2O(l) -> H3O+(aq) + Cl^{-}(aq)}$

$\ce{NaCl(s) <-> Na+(aq) + Cl^{-}(aq) }$

The increase in chloride ion concentration shift the dissolution equilibrium to the left, leading to the formation of solid sodium chloride.

Since the hydrochloric acid is ~12 M and the sodium chloride solution is only a little more than 5 M, the dilution effect is small enough that the I.P. > Ksp and a precipitate forms. 

Materials 

  • a large test tube filled about halfway with saturated sodium chloride solution.
  • a clamp or Erlenmeyer flask to hold the test tube so that the solution is visible.
  • a dropper bottle of concentrated hydrochloric acid.

Procedure 

A large test tube containing saturated sodium chloride is projected at the front of the classroom.  A couple of dropperfuls of concentrated hydrochloric acid is added to the solution.  A milky white sodium chloride precipitate forms immediately.  In time the solid will precipitate out.

Safety Precautions 

Care should be exercised when handling concentrated hydrochloric acid.  Wear goggles and gloves.  Dispose of waste appropriately.

Prep. Notes 

To prepare the saturated sodium chloride solution, slowly add 40 g of NaCl to 100 mL of water on a heated stirring plate and gently warm until the NaCl has stopped dissolving.  Allow to cool overnight.