Experiment 58

Objective

To identify the presence of mercury ions in a sample by observing the formation of white and yellow precipitates with sodium hydroxide.

Introduction

Mercury salts react with sodium hydroxide (NaOH) to form distinct precipitates. Mercurous salts yield a white precipitate, while mercuric salts form a yellow precipitate of mercuric oxide (HgO). This reaction serves as a confirmatory test for mercury salts.

Reactions:

1.    Reaction with mercurous chloride:

Hg2Cl2 + 2NaOH → 2NaCl + Hg2O ↓ + H2O

2.    Reaction with mercuric chloride:

HgCl2 + 2NaOH → 2NaCl + HgO ↓ + H2 O

Materials

Chemicals

·       Mercury salt sample

·       Sodium hydroxide (NaOH)

·       Distilled water

Apparatus

·       Test tube

·       Test tube holder

·       Bunsen burner

Reagents

·       Dissolve 3–4 ml of the sample in a test tube.

·       Prepare a 5% NaOH solution in a 250 ml beaker.

Procedure

1.    Place 3–4 ml of the sample solution in a test tube.

2.    Add 6–7 ml of NaOH solution to the sample.

3.    Wait a few minutes for the reaction to complete.

4.    Observe and record the color and type of precipitate formed.

Observation

The addition of NaOH to a mercurous salt results in a white precipitate, while a mercuric salt forms a yellow precipitate, indicating the presence of mercury.

Questions

1.    What is a mercury salt?

2.    What happens when NaOH is added to mercury salt?

3.    What color forms with mercurous salts?

4.    What color forms with mercuric salts?