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Qualitative Analysis: Dry Test, Borax Bead Test, Microcosmic Salt Bead Test – Procedure, Reactions, Practice Problems and FAQ

Qualitative Analysis: Dry Test, Borax Bead Test, Microcosmic Salt Bead Test – Procedure, Reactions, Practice Problems and FAQ

How can alkali metal ions be identified in a lab setting?

On the provided samples, we may do a flame test, and based on the distinctive colour of the flame, we can determine which specific alkali metal or group 1 metal is present. Lithium, for instance, gives the flame a crimson red hue; sodium, a yellow hue; potassium, a violet or lilac hue; rubidium, a reddish-violet hue; and caesium, a blue hue. Similar to this, there are other ways to determine what basic and acid radicals are present in the given salt. The dry test is one of the methods that help us in identifying the cation and anion present in the given salt.

Let's learn more about the dry tests without further ado!

TABLE OF CONTENTS

  • Qualitative Analysis
  • Borax Bead Test
  • Borax Bead Test – Chemical Changes
  • Microcosmic Salt Bead Test
  • Microcosmic Salt Bead Test – Chemical Changes
  • Sodium Carbonate Bead Test
  • Sodium Carbonate Bead Test – Observations
  • Practice Problems
  • Frequently Asked Questions – FAQ

Qualitative Analysis

  • Qualitative analysis is the process of determining a substance's nature and the identities of its constituents.
  • Qualitative analysis of inorganic salts means the identification of cations and anions present in the salts.
  • Qualitative analysis is carried out through reactions that are easily detectable to human senses such as sight and smell.
  • A dry test involves noting the appearance and physical properties, such as colour, odour, solubility, etc. of the salt.
  • Examples: Heating of dry salt, flame test, borax bead test, microcosmic salt bead test, charcoal cavity test, etc.

Borax Bead Test

Borax: It is one of the most important compounds of boron.

Formula: Na2B4O7.10H2O.. Practically, borax is present as the tetranuclear unit [B4O5(OH)4]2-. So, the formula of borax is correctly represented as Na2[B4O5(OH)4].8H2O

Structure:

Appearance: White crystalline solid

This test is used for coloured salts because borax reacts with the corresponding metal salts to form metal borates and show their characteristic colours.

Procedure:

  • A loop is made at the one end of the platinum wire and borax powder is taken in it. The platinum wire containing the borax powder is heated in the hottest part of the flame until it becomes red hot.
  • The borax powder swells up due to the loss of water of crystallisation and shrinks on forming a transparent, colourless bead that consists of boric anhydride and sodium metaborate.

  • The hot bead is dipped in the salt sample, heated in the flame and the colour of the bead is observed for both the non-luminous flame as well as in the luminous flame while it is hot and when it is cold.

Borax Bead Test – Chemical Changes

  1. On heating borax, it loses its water of crystallisation and forms sodium metaborate and boric anhydride.

Na2B4O7.10H2O (s)Na2B4O7(l)+10H2O (l)

Na2B4O7(s)2NaBO2(s)+B2O3(s)

  1. Treatment with metal salt:

CoSO4(s)+B2O3(s)Co(BO2)2+SO3

Blue colour bead Co(BO2)2 (cobalt metaborate) formation takes place.

The colour of the borax bead for different metals is given below.

Metal

Oxidising Flame

Reducing Flame

Hot

Cold

Hot

Cold

Cr

Yellow

Green

Green

Green

Mn

Violet

Violet

Colourless

Colourless

Fe

Yellowish brown

Yellow

Green

Green

Co

Blue

Blue

Blue

Blue

Ni

Violet

Reddish brown

Grey

Grey

Cu

Green

Blue

Colourless

Opaque

Microcosmic Salt Bead Test

Microcosmic salt:  {Na(NH_4)HPO_4}_  (Ammonium Sodium Phosphate). This salt is termed as microcosmic salt.

Microcosmic Salt Bead Test is used for coloured salts because microcosmic salts react with corresponding transition metal salts to form metal orthophosphate and show their characteristic colours. The procedure is the same as the borax bead tets.

Microcosmic Salt Bead Test – Chemical Changes

  1. On heating, the microcosmic salt loses its water of crystallisation and forms metal metaphosphate which is transparent and liberates ammonia gas.

Na(NH4)HPO4.4H2O(s)NaPO3(s)+NH3(g)+5H2O(aq)

  1. Treatment with metal oxide.

CuO(s)+NaPO3(s)NaCuPO4(s)

Blue coloured bead of NaCuPO4 is formed.

  1. To identify halides (X-, X can be Cl, Br & I)

NaCuPO4+2NaXNa3PO4+2NaX

Cl-→ blue colour across the bead

Br- → blue colour across the bead and green on top of the flame

I- → green colour on entire flame

Metal

Oxidising Flame

Reducing Flame

Hot

Cold

Hot

Cold

Copper

Green

Blue

Colourless

Red

Iron

Brown

Yellow

Green

Green

Cobalt

Blue

Blue

Blue

Blue

Nickel

Violet

Brown

Grey

Grey

Sodium Carbonate Bead Test

A small amount of sodium carbonate (Na2CO3) is heated in a platinum wire loop in the bunsen flame. This results in the formation of a white, opaque bead. The bead is dipped in a small amount of KNO3 and then in the salt sample and heated. The colour change of the bead is noted.

Sodium Carbonate Bead Test – Observations

  1. The formation of a green bead confirms the presence of a manganese compound.

MnO+Na2CO3+O2 Na2MnO4+CO2

  1. The formation of a yellow bead confirms the presence of a chromium compound.

2Cr2O3+4Na2CO3+3O2 4Na2CrO4+4CO2

Practice Problems

  1. Find the total number of sp2hybridised boron atoms in borax.
  1. 1
  2. 2
  3. 3
  4. 4

Answer: B

Solution: Borax contains 2 sp2and 2 sp3hybridised boron atom per molecule.

So, option B is the correct answer.

  1. The colour shown by cobalt in the borax bead test is:
  1. Green
  2. Purple
  3. Blue
  4. Olive

Answer: C

CoSO4(s)+B2O3(s)Co(BO2)2+SO3

When a cobalt salt is subjected to borax bead test, a blue coloured bead Co(BO2)2 (cobalt metaborate) is formed.

So, option C is the correct answer.

  1. Glassy bead appearance in microcosmic salt bead test is due to
  1. Metal metaphosphate
  2. Boric anhydride
  3. Sodium metaborate
  4. None of these

Answer: A

On heating, the microcosmic salt loses its water of crystallisation and forms metal metaphosphate which is transparent and liberates ammonia gas.

Na(NH4)HPO4.4H2O(s)NaPO3(s)+NH3(g)+5H2O(aq)

So, option A is the correct answer.

  1. The formation of a yellow bead in the sodium carbonate bead test confirms the presence of
  1. Iron
  2. Copper
  3. Manganese
  4. Chromium

Answer: D

The formation of a yellow bead confirms the presence of a chromium compound.

2Cr2O3+4Na2CO3+3O2 4Na2CrO4+4CO2

So, option D is the correct answer.

Frequently Asked Questions – FAQ

1. What is the difference between oxidising and reducing flame?
Answer:
A non-luminous flame is an oxidising flame and a luminous flame is a reducing flame. Oxidising flame is a flame produced with an excess amount of oxygen. With a good amount of oxygen, the flame height shortens, and the colour darkens and burns with sound.

2. What are meta acids?
Answer: When one water molecule is removed from 1 molecule of acid, meta acids are formed

H3BO3(Boric acid)HBO2 (metaboric acid)

BO2-- metaborate ion

So, the salt formed from metaboric acid is metaborate. NaBO2 - sodium metaborate

H3PO4(Phosphoric acid)HPO3 (metaphosphoric acid)

PO3-- meta phosphate ion

NaPO3-sodium metaphosphate

Note: When the name of acid ends with -ic acid, its anion name ends with - ate and the name of acid ends with -us acid its anion name ends with - ite.

H2SO4 (sulphuric acid)-SO42- (sulphate ion)

H2SO3 (sulphurous acid)-SO32- (sulfite ion)

What is the use of borax?

Borax is used to prevent fungal foot diseases in animals, used to treat wood products that are infected with woodworm. It is also used as a buffering agent.

3. What is the difference between hot and cold beads?
Answer: 
"Hot" describes a bead that has just been removed from the Bunsen burner's flame, while "cold" describes a bead that has reached room temperature.

4. Are the colours observed for cations the same in the borax bead test and microcosmic bead test?
Answer: Yes, generally colours produced by cations are the same in both the borax bead test and the microcosmic bead test.

Related Topics

Salt Analysis - Classification, Definition

Percentage Composition

Analysis of Group I Acidic Radicals

Analysis of Group II Acidic Radicals

Analysis of Group III Acidic Radicals

Brown Ring Test

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