Analía Bellizzi – Chemistry Classes

Ronald Reagan Senior High School

Precipitation Reactions – Testing for Ions

Qualitative Tests for Ions and Gases

Introduction

When we need to find out if an ion is present in a solution, we need to know which are the substances containing those ions that are insoluble, so we can produce the precipitate. 

In the case of group 1 ions, Na+, K+, etc., form all soluble compounds, so there is not possibility of producing any precipitation. 

For these elements and some more, we use a FLAME TEST. 

1- FLAME TESTS

flame test
There are different ways of performing a flame test. One of the most common ways is the following:
  1. A cleaned, moistened flame test wire is dipped into a solid sample of the compound.
  2. It is then put into the edge of a blue Bunsen flame.
  3. The flame color produced indicates which  metal ion is present in the compound.

Flame colors and the metal ion they represent for each ion present

GROUP 1 METAL ION FLAME COLOR
LITHIUM
Li+

RED (HOT PINK)

SODIUM
Na+

YELLOW-ORANGE

POTASSIUM
K+
LILAC
GROUP 2 METAL ION FLAME COLOR
CALCIUM
Ca2+

BRICK RED (ORANGE RED)

STRONTIUM
Sr2+

RED (EMERGENCY FLARE RED)

BARIUM
Ba2+

LIME GREEN

OTHER ELEMENTS ION FLAME COLOR
BORIC ACID
H3BO3

GREEN

IRON (II)
Fe2+

RED BROWN

COPPER
Cu2+

GREEN (TEAL)

2- PRECIPITATION REACTIONS

When we cover solubility rules, we said that most of the metallic hydroxides were insoluble. 
We use this property to identify the presence of some positive ions in solution. 

Reactions with NaOH (aq)

To an aqueous solution of the sample, add aqueous sodium hydroxide a little at a time (until a change is observed)
Observe the color of precipitate 

add some more and observe the solubility of the precipitate in excess.
If there is no precipitate, warm the mixture and test for ammonia gas.

Reactions with NH3 (aq)

To an aqueous solution of the sample, add aqueous ammonia a little at a time (until a change is observed)

Observe the color of precipitate if present, and the color and solubility of the precipitate in excess.
A diagram to illustrate the test for cations using sodium hydroxide and ammonia solution (Ammonium Hydroxide) is as follows

TEST FOR CATIONS USING SODIUM HYDROXIDE AND AMMONIA

ELEMENT ION REACTION WITH NAOH REACTION WITH NH3
COPPER
Cu2+

light blue precipitate, insoluble in excess

light blue precipitate, soluble in excess, giving a dark blue solution

IRON (II)
Fe2+

green ppt., insoluble in excess

green ppt., insoluble in excess