Experiment 39

Objective

To identify the presence of carbonate ions in a sample by observing gas evolution and precipitation reactions.

Introduction

Most carbonate salts are insoluble in water except for sodium and potassium carbonates. When an acid, such as HCl, is added to a soluble carbonate salt, carbon dioxide (CO₂) gas is released as bubbles. This CO₂ gas, when passed through calcium hydroxide (lime water), forms a white precipitate of calcium carbonate, which dissolves upon excess CO₂ flow, confirming the presence of carbonate ions.

Reaction:

1.    Reaction with hydrochloric acid:

Na2​CO3 ​+ HCl → NaCl + CO2 ​↑ + H2​O

Materials

Chemicals

·       Sodium carbonate (Na₂CO₃)

·       Hydrochloric acid (HCl)

·       Calcium hydroxide solution (lime water)

·       Distilled water

Apparatus

·       Test tube

·       Test tube holder

Reagents

·       15–20 ml of sample in a test tube

·       5% HCl solution prepared in a 250 ml beaker

Procedure

1.    Place 15–20 ml of the sample solution in a test tube.

2.    Add hydrochloric acid to the sample.

3.    Observe the reaction, noting the release of CO₂ gas bubbles.

4.    Pass the gas through calcium hydroxide solution and observe the formation of a white precipitate.

5.    Continue flowing the gas to observe the dissolution of the precipitate.

Observation

Upon adding HCl to the sample, carbon dioxide gas is released as bubbles. When passed through calcium hydroxide, a white precipitate forms initially, which then dissolves upon continued CO₂ flow.

Questions

1.    What is a carbonate salt?

2.    Why are most carbonate salts insoluble in water?

3.    Which carbonate salts are soluble?

4.    What reagent is used in the carbonate ion test?