Home
JEE Chemistry
Qualitative Analysis

Qualitative Analysis

Qualitative analysis in Chemistry can help determine which elements or compounds are present in a sample through tests that observe color changes, precipitate formation, or other reactions.

1.0Qualitative Analysis of Inorganic Salts

Qualitative analysis identifies the nature and constituents of a substance, particularly the cations and anions present in inorganic salts. These salts are often products of the neutralization of acids by bases. In such reactions, the part contributed by the acid is termed the anion, and the part contributed by the base is termed the cation. For instance, in CuSO4 and NaCl, Cu²⁺ and Na⁺ are the cations, while SO₄²⁻ and Cl⁻ are the anions.

Scales of Analysis:

Type of Analysis

Requirements

Macro 

Uses 0.1 to 0.5 grams of the substance and about 20 mL of solution.

Semi-micro Analysis

Requires about 0.05 grams of the substance and 1 mL of solution.


Micro Analysis

Involves very small quantities of the substance.


  •  Detection Methods: Qualitative analysis employs reactions that produce visible changes detectable by sight or smell:
  • Formation of a Precipitate: Solid formation in a solution.
  • Color Change: Observable change in the color of the substance.
  • Gas Evolution: Release of gas bubbles during a reaction.

2.0Systematic Steps

Preliminary Examination

Preliminary examination in qualitative analysis involves a series of initial tests and observations to gather basic information about a substance before performing more detailed chemical tests. These steps help in narrowing down the possible identities of the components in a sample. The preliminary examination typically includes:

 

Physical State and Appearance

Observing the sample's color, texture, and form (solid, liquid, gas).

Odor

any characteristic smells which can give clues about certain compounds.

Solubility

Testing the solubility of the substance in water and other solvents.

Flame Test

Heating a small amount of the sample in a flame to observe characteristic colors that indicate the presence of certain metal ions.

Heating Effects

Heating the sample to see if it melts, decomposes, or gives off gases, which can provide insights into its composition.

pH Test

Measuring the pH of an aqueous solution of the sample to determine its acidic or basic nature.

Magnetism

Checking if the sample is attracted to a magnet, indicating the presence of ferromagnetic materials.

Physical State and Appearance

Observing the sample's color, texture, and form (solid, liquid, gas).

Odor

any characteristic smells which can give clues about certain compounds.

Solubility

Testing the solubility of the substance in water and other solvents.

Flame Test

Heating a small amount of the sample in a flame to observe characteristic colors that indicate the presence of certain metal ions.

Heating Effects

Heating the sample to see if it melts, decomposes, or gives off gases, which can provide insights into its composition.

pH Test

Measuring the pH of an aqueous solution of the sample to determine its acidic or basic nature.

Magnetism

Checking if the sample is attracted to a magnet, indicating the presence of ferromagnetic materials.

3.0Anion Determination 

Anion determination in qualitative analysis involves identifying the specific anions (negatively charged ions) present in a given sample. The process usually follows a systematic approach with several stages to accurately identify the anions.

Systematic Testing for Common Anions:

Anions

Test

Observation

          Confirmation

Carbonate (CO3-2) and Bicarbonate (HCO3) Ions:

Add dilute hydrochloric acid (HCl) to the sample.


Effervescence (bubbling) due to the release of carbon dioxide (CO₂) gas.

Pass the gas through lime water (calcium hydroxide solution). A white precipitate of calcium carbonate confirms the presence of carbonate or bicarbonate ions.

Sulphate (SO₄²⁻) Ion

Add barium chloride (BaCl₂) solution to the sample acidified with dilute hydrochloric acid.


A white precipitate of barium sulphate indicates the presence of sulphate ions.



Sulphite (SO₃²⁻) Ion:


Add dilute hydrochloric acid.


Smell of sulfur dioxide (SO₂) gas and no precipitate with barium chloride in acidic medium.

The gas turns potassium dichromate paper green.


Chloride (Cl⁻) Ion:

Add silver nitrate (AgNO₃) solution to the sample acidified with dilute nitric acid.


A white precipitate of silver chloride that is soluble in ammonia solution indicates the presence of chloride ions.



Bromide (Br⁻) Ion:


Add silver nitrate (AgNO₃) solution to the sample acidified with dilute nitric acid.


A pale yellow precipitate of silver bromide that is partially soluble in ammonia indicates bromide ions.



Iodide (I⁻) Ion:

Add silver nitrate (AgNO₃) solution to the sample acidified with dilute nitric acid.


A yellow precipitate of silver iodide that is insoluble in ammonia indicates iodide ions.



Nitrate (NO₃⁻) Ion:


Add a few drops of concentrated sulfuric acid (H₂SO₄) followed by a few drops of freshly prepared ferrous sulfate (FeSO₄) solution.

A brown ring at the junction of the two liquids indicates the presence of nitrate ions (Brown Ring Test).


Phosphate (PO₄³⁻) Ion

Add ammonium molybdate solution in the presence of nitric acid and heat gently.

A yellow precipitate of ammonium phosphomolybdate indicates the presence of phosphate ions.



Acetate (CH₃COO⁻) Ion

Add a few drops of concentrated sulfuric acid (H₂SO₄) and warm the solution.


The characteristic smell of vinegar (acetic acid) indicates the presence of acetate ions.



Note - Some anions may require additional confirmatory tests using different reagents to ensure accurate identification.

4.0Cation Determination

Similar to anion tests, cation tests involve wet tests with specific reagents that precipitate or change the color of particular cations.

Confirmatory tests validate the presence of the suspected cations.

1. Dry Tests

Heating of Dry Salt: Observing changes when the salt is heated.

Color Change

Example

Interpretation

Blue copper(II) sulfate turns white when heated, indicating loss of water.

Presence of hydrated salts.

Gas Evolution

Example

Interpretation

Carbonates release CO₂ gas, making lime water milky.

Presence of carbonates or ammonium salts.

Residue Formation

Example

Interpretation

Sodium bicarbonate forms sodium carbonate and releases CO₂ and water.

Presence of bicarbonates or hydrates.

Melting or Sublimation

Example

Interpretation

Ammonium chloride sublimes to form white fumes.

Presence of salts that melt or sublime.

Characteristic Odor

Example

Interpretation

Acetates release a vinegar-like smell.

Presence of acetates or sulfides.

2. Flame Test

Exposing the salt to a flame and noting the color.

Colour of the flame observed by naked eye

Inference

Green flame with blue centre

Cu+2

Crimson red

Sr+2

Apple green

Ba+2

Brick red

Ca+2

Lilac

K+

Yellow

Na+

Colour variations for different elements under Flame Test


3. Borax Bead Test

Heating the salt with borax and observing the color change.

Metal Ion

Oxidizing Flame Observation

Reducing Flame Observation

Interpretation

Copper (Cu²⁺)

Blue-green bead

Red bead

Presence of copper ions

Chromium (Cr³⁺)

Green bead

Green bead (color may vary slightly)

Presence of chromium ions

Iron (Fe³⁺/Fe²⁺)

Yellow to brown bead

Green bead

Presence of iron ions

Cobalt (Co²⁺)

Deep blue bead

Deep blue bead

Presence of cobalt ions

Nickel (Ni²⁺)

Brown bead

Grey bead

Presence of nickel ions

Manganese (Mn²⁺)

Violet bead

Colorless or slightly pink bead

Presence of manganese ions

Borax Bead

4. Charcoal Cavity Test 

Heating the salt in a charcoal cavity and noting changes.

Procedure

Preparation:

  • Materials: Charcoal block, Bunsen burner, blowpipe, salt sample.
  • Safety: Wear goggles and gloves, ensure good ventilation.

Creating the Cavity: Carve a small cavity on the charcoal block.

Placing the Sample: Place a small amount of the salt sample in the cavity.

Heating the Sample:

  • Use a blowpipe to direct the flame onto the sample.
  • Observe changes such as color, fumes, or residue formation.

Observations and Interpretations:

          Cations

    Observation

          Interpretation

Lead (Pb²⁺)

Yellow oxide coating, metallic globules.

Presence of lead ions.

Copper (Cu²⁺)

Red/black metallic bead, green flame.

Presence of lead ions.

Zinc (Zn²⁺):

White oxide turns yellow when hot.

Presence of zinc ions.

Tin (Sn²⁺)

White oxide coating, metallic tin globules.

Presence of tin ions.

Antimony (Sb+3)

White fumes, white oxide coating.

Presence of antimony ions

Bismuth (Bi+3)

Yellow oxide coating, metallic globules.

Presence of bismuth ions.

5.0Qualitative Analysis of Organic Compounds

Qualitative analysis of organic compounds involves identifying specific functional groups, elements, and molecular structures in a sample. Key techniques include:

  1. Preliminary Tests: Observing physical properties and solubility.
  2. Elemental Analysis: Detecting elements like carbon, hydrogen, nitrogen, halogens, and sulfur using methods such as combustion analysis and Lassaigne’s test.
  3. Functional Group Analysis: Identifying groups like alcohols, phenols, aldehydes, ketones, carboxylic acids, and amines through specific chemical tests (e.g., Tollen’s test, Fehling’s test).
  4. Spectroscopic Methods: Using IR spectroscopy for functional groups, NMR spectroscopy for molecular structure, and mass spectrometry for molecular weight and fragmentation patterns.
  5. Chromatography: Separating compounds with thin-layer chromatography (TLC) and gas chromatography (GC).

Frequently Asked Questions

Qualitative analysis in organic chemistry is a method used to identify the chemical components of a substance without determining their quantity. It focuses on detecting the presence of specific ions or compounds in a sample.

Common techniques include: Flame tests: Identifying metal ions based on the color they produce in a flame. Precipitation reactions: Using specific reagents to form insoluble precipitates with certain ions. Blowpipe tests: Heating substances with a blowpipe to observe characteristic reactions. Charcoal cavity tests: Heating samples on a charcoal block to observe changes. Borax bead tests: Forming beads with borax to identify metal ions by the color of the bead in a flame.

The flame test is used to identify metal ions based on the characteristic color they emit when heated in a flame. Each metal ion produces a unique color, allowing for easy identification.

Qualitative analysis focuses on identifying the presence or absence of certain chemical compounds or elements within a sample. This method provides information about what is in the sample without quantifying the amounts. It is particularly useful in organic chemistry for identifying unknown substances. Qualitative analysis examples involve Flame test, Borax bead test etc. Quantitative analysis determines the amount or concentration of a particular compound or element within a sample. This method provides precise measurements that are critical for understanding the composition and purity of substances. For example Titration, Gravimetric Analysis.

Join ALLEN!

(Session 2024 - 25)


Choose class
Choose your goal
Preferred Mode
Choose State