Experiment 37

Objective

To identify the presence of calcium ions in a sample by observing precipitation with ammonium carbonate and subsequent reaction with hydrochloric acid.

Introduction

Calcium salts react with ammonium carbonate to form a white precipitate of calcium carbonate (CaCO₃). When hydrochloric acid (HCl) is added to this precipitate, carbon dioxide gas is released as bubbles, confirming the presence of calcium ions.

Reaction:

1.    Formation of calcium carbonate:

CaCl2 ​+ (NH4​)2​CO3 ​→ CaCO3 ​↓ + 2NH4​Cl

2.    Reaction with hydrochloric acid:

CaCO3 ​+ 2HCl → CaCl2 ​+ CO2 ​↑ + H2​O

Materials

Chemicals

·       Calcium chloride (CaCl₂)

·       Ammonium carbonate ((NH₄)₂CO₃)

·       Hydrochloric acid (HCl)

·       Distilled water

Apparatus

·       Test tube

·       Test tube holder

·       Bunsen burner

Reagents

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

·       5% HCl solution prepared in a 250 ml beaker

Procedure

1.    Place 5–6 ml of the sample solution in a test tube.

2.    Add a small amount of ammonium carbonate to the sample.

3.    Observe any changes in the reaction, noting the formation of a white precipitate.

4.    Add hydrochloric acid to the solution and observe the dissolution of the precipitate, along with the release of gas bubbles.

Observation

A white precipitate forms upon the addition of ammonium carbonate. When HCl is added, the precipitate dissolves, releasing carbon dioxide gas as bubbles, indicating the presence of calcium ions.

Questions

1.    What is a calcium salt?

2.    How is a calcium salt identified?

3.    What happens when ammonium carbonate is added to a calcium salt?

4.    What reagents are used in the calcium test?