Instrumental methods, forensic, environmental, clinical applications

Chemistry · Senior Secondary (HKDSE) · 6 min read · Updated: 21 Aug 2026

Chemistry Study Notes: Instrumental Methods & Their Applications

Hey everyone! Welcome to one of the most exciting topics in chemistry – Analytical Chemistry. Think of it as the 'detective work' of science. In this chapter, we'll learn how chemists figure out what is in a sample and how much of it there is. These skills are super important for everything from solving crimes and checking our food for safety to protecting the environment. Let's get our detective hats on and dive in!

Section 1: The Chemist's Toolkit - Identifying Unknowns

Before we use fancy machines, let's look at some classic, hands-on tests that chemists use to identify substances. These are the fundamental skills every chemist needs!

A. The Flame Test: Seeing an Element's True Colours

Have you ever wondered how fireworks get their brilliant colours? It's all about chemistry! When we heat certain metal ions, they give off a characteristic colour of light. We can use this to identify them.

How it works: A clean platinum or nichrome wire loop is dipped into concentrated hydrochloric acid and the sample, then held in a non-luminous Bunsen flame.

Sodium (\(\text{Na}^+\)): Intense Golden Yellow
Potassium (\(\text{K}^+\)): Lilac (a pale purple)
Calcium (\(\text{Ca}^{2+}\)): Brick Red
Copper (\(\text{Cu}^{2+}\)): Bluish Green

Quick Review: Flame Test Colours

Memory trick: Think of everyday things! Salty crisps (Sodium) are golden. A copper statue turns green.

B. Testing for Common Gases

Here’s how we can identify some common, invisible gases in the lab.

Hydrogen (\(\text{H}_2\)): Use a lit splint. If hydrogen is present, you'll hear a 'squeaky pop'.
Oxygen (\(\text{O}_2\)): Use a glowing splint. If oxygen is present, the splint will relight.
Carbon Dioxide (\(\text{CO}_2\)): Bubble the gas through limewater (calcium hydroxide solution). If \(\text{CO}_2\) is present, a white precipitate forms, turning the limewater milky/cloudy.
Chlorine (\(\text{Cl}_2\)): Use damp blue litmus paper. It will first turn red (acidic nature) and then quickly bleach white.
Ammonia (\(\text{NH}_3\)): Use damp red litmus paper. It will turn blue because ammonia is an alkaline gas.
Sulfur Dioxide (\(\text{SO}_2\)): Turns acidified potassium dichromate solution from orange to green.

C. Testing for Ions in Solution

Many substances are dissolved in water, so we need systematic ways to test for aqueous ions by observing precipitate formation and solubility in excess reagents.

Testing for Cations (Positive Ions)

Add aqueous sodium hydroxide, \(\text{NaOH(aq)}\), or aqueous ammonia, \(\text{NH}_3\text{(aq)}\), drop by drop until in excess:

Copper(II) (\(\text{Cu}^{2+}\)): Forms a blue precipitate of \(\text{Cu(OH)}_2\). Dissolves in excess \(\text{NH}_3\text{(aq)}\) to form a deep blue solution, but remains insoluble in excess \(\text{NaOH(aq)}\).
Iron(II) (\(\text{Fe}^{2+}\)): Forms a dirty green precipitate of \(\text{Fe(OH)}_2\), insoluble in excess alkali.
Iron(III) (\(\text{Fe}^{3+}\)): Forms a reddish-brown precipitate of \(\text{Fe(OH)}_3\), insoluble in excess alkali.
Aluminium (\(\text{Al}^{3+}\)): Forms a white precipitate of \(\text{Al(OH)}_3\). It dissolves in excess \(\text{NaOH(aq)}\) to give a colourless solution, but is insoluble in excess \(\text{NH}_3\text{(aq)}\).
Zinc (\(\text{Zn}^{2+}\)): Forms a white precipitate of \(\text{Zn(OH)}_2\). It dissolves in both excess \(\text{NaOH(aq)}\) and excess \(\text{NH}_3\text{(aq)}\) to give colourless solutions.
Calcium (\(\text{Ca}^{2+}\)): Forms a white precipitate of \(\text{Ca(OH)}_2\) with \(\text{NaOH(aq)}\) (insoluble in excess \(\text{NaOH(aq)}\)), but gives no precipitate with \(\text{NH}_3\text{(aq)}\).
Magnesium (\(\text{Mg}^{2+}\)): Forms a white precipitate of \(\text{Mg(OH)}_2\), which is insoluble in both excess \(\text{NaOH(aq)}\) and excess \(\text{NH}_3\text{(aq)}\).
Ammonium (\(\text{NH}_4^+\)): Add \(\text{NaOH(aq)}\) and warm gently. Ammonia gas is evolved, turning damp red litmus paper blue.

Testing for Anions (Negative Ions)

Carbonate (\(\text{CO}_3^{2-}\)): Add dilute \(\text{HCl(aq)}\). Effervescence occurs; the colourless gas evolved turns limewater milky.
Sulfate (\(\text{SO}_4^{2-}\)): Add dilute \(\text{HCl(aq)}\) followed by \(\text{BaCl}_2\text{(aq)}\). A white precipitate of \(\text{BaSO}_4\) forms.
Halide Ions (\(\text{Cl}^-\), \(\text{Br}^-\), \(\text{I}^-\)): Acidify with dilute \(\text{HNO}_3\text{(aq)}\), then add \(\text{AgNO}_3\text{(aq)}\):
- \(\text{Cl}^-\): White precipitate (\(\text{AgCl}\)), soluble in dilute \(\text{NH}_3\text{(aq)}\).
- \(\text{Br}^-\): Cream precipitate (\(\text{AgBr}\)), soluble only in concentrated \(\text{NH}_3\text{(aq)}\).
- \(\text{I}^-\): Yellow precipitate (\(\text{AgI}\)), insoluble in both dilute and concentrated \(\text{NH}_3\text{(aq)}\).

D. Finding Functional Groups in Organic Compounds

\(\text{C=C}\) (Alkenes): Add bromine in organic solvent or bromine water. Rapid decolourisation from orange-brown to colourless occurs.
\(\text{-COOH}\) (Carboxylic Acids): Add \(\text{Na}_2\text{CO}_3\text{(aq)}\) or \(\text{NaHCO}_3\text{(aq)}\). Effervescence of \(\text{CO}_2\) gas is observed.
Carbonyl Compounds (Aldehydes & Ketones): Add 2,4-dinitrophenylhydrazine (2,4-DNPH). A bright orange/yellow precipitate forms.
Aldehydes (\(\text{-CHO}\)) vs Ketones (\(\text{>C=O}\)):
1. Tollens' Reagent: Warm with ammoniacal silver nitrate solution. Aldehydes form a silver mirror on the tube wall; ketones give no reaction.
2. Fehling's Solution: Warm with Fehling's solution. Aldehydes reduce the deep blue solution to form a brick-red precipitate of \(\text{Cu}_2\text{O}\); ketones do not react.
3. Acidified \(\text{K}_2\text{Cr}_2\text{O}_7\): Aldehydes are oxidised, turning the solution from orange to green.

Section 2: Separation & Purification Methods

A. Common Separation Techniques

Crystallisation: Used to separate and purify soluble solids from solutions by gradual cooling and solvent evaporation.
Simple Distillation: Separates a volatile liquid from non-volatile solids or liquids with significantly different boiling points.
Fractional Distillation: Separates miscible liquids with close boiling points using a fractionating column.
Liquid-Liquid Extraction: Uses a separating funnel to partition a solute between two immiscible solvent layers based on relative solubilities.

B. Chromatography

Separates components based on their differential distribution between a stationary phase and a mobile phase.

In paper or thin-layer chromatography (TLC), retention is quantified by the retardation factor (\(R_f\)):

\( R_f = \frac{\text{distance travelled by the component}}{\text{distance travelled by the solvent front}} \)

C. Purity Determination

Pure substances: Exhibit sharp, distinct melting and boiling points.
Impure substances: Melt and boil over a wider temperature range; impurities lower the melting point and elevate the boiling point.

Section 3: Modern Instrumental Analysis

A. Colorimetry

Measures the light absorbance of coloured solutions. Concentration is determined by constructing a calibration curve (Absorbance vs. Concentration) using standard solutions and measuring the unknown under the same wavelength filter.

B. Infrared (IR) Spectroscopy

Infrared radiation excites molecular vibrations. Characteristic absorption bands identify specific functional groups:

Alcohol \(\text{O–H}\) (hydrogen-bonded): Broad absorption band at \(3230\text{–}3670\text{ cm}^{-1}\).
Carboxylic acid \(\text{O–H}\) (hydrogen-bonded): Very broad, jagged absorption band spanning \(2500\text{–}3300\text{ cm}^{-1}\) (often overlapping the \(\text{C–H}\) stretching region).
Carbonyl \(\text{C=O}\): Strong, sharp absorption at \(1680\text{–}1750\text{ cm}^{-1}\) (found in aldehydes, ketones, carboxylic acids, and esters).
Carbon-Carbon double bond \(\text{C=C}\): Moderate absorption at \(1610\text{–}1680\text{ cm}^{-1}\).

C. Mass Spectrometry (MS)

Samples are ionised and separated according to their mass-to-charge ratio (\(m/z\)).

Molecular Ion Peak (\(M^+\)): Corresponds to the unfragmented radical cation, providing the relative molecular mass (\(M_r\)) of the compound.
Isotope Peaks: Chlorine-containing compounds show an \(M : M+2\) peak ratio of approximately \(3 : 1\) (due to \(^{35}\text{Cl}\) and \(^{37}\text{Cl}\)). Bromine-containing compounds show an \(M : M+2\) ratio of approximately \(1 : 1\) (due to \(^{79}\text{Br}\) and \(^{81}\text{Br}\)).
Fragmentation Patterns: Characteristic fragment ions (e.g. \(m/z = 15\) for \(\text{CH}_3^+\), \(m/z = 29\) for \(\text{C}_2\text{H}_5^+\) or \(\text{CHO}^+\), \(m/z = 43\) for \(\text{CH}_3\text{CO}^+\)) allow deduction of structural features.

Section 4: Real-World Applications

Environmental Monitoring: Quantitative detection of trace air pollutants (e.g. \(\text{CO}\), \(\text{NO}_x\), volatile organic compounds) and water quality assessment (e.g. heavy metal ions, dissolved oxygen, permanganate index).
Forensic Science: Identification of illicit substances, accelerants from arson scenes, toxicological analysis, and fiber/paint comparisons using GC-MS and IR spectroscopy.
Clinical Diagnostics: Colorimetric and spectroscopic determination of blood glucose, cholesterol, and serum electrolytes to guide medical interventions.

Quick check

Can you answer these now?

Open each question to check the key ideas from this chapter.

What characteristic flame colours are observed for \(\text{Na}^+\), \(\text{K}^+\), \(\text{Ca}^{2+}\), and \(\text{Cu}^{2+}\) ions in a flame test?

\(\text{Na}^+\) gives an intense golden yellow flame, \(\text{K}^+\) gives a lilac flame, \(\text{Ca}^{2+}\) gives a brick-red flame, and \(\text{Cu}^{2+}\) gives a bluish-green flame.

How can common laboratory gases such as \(\text{H}_2\), \(\text{O}_2\), \(\text{CO}_2\), \(\text{Cl}_2\), \(\text{NH}_3\), and \(\text{SO}_2\) be identified?

\(\text{H}_2\) gives a squeaky pop with a lit splint; \(\text{O}_2\) relights a glowing splint; \(\text{CO}_2\) turns limewater milky; \(\text{Cl}_2\) turns damp blue litmus red then bleaches it; \(\text{NH}_3\) turns damp red litmus blue; \(\text{SO}_2\) turns acidified \(\text{K}_2\text{Cr}_2\text{O}_7\) from orange to green.

How can cations in aqueous solution be differentiated using \(\text{NaOH(aq)}\) and \(\text{NH}_3\text{(aq)}\)?

Different cations form characteristic hydroxide precipitates that vary in colour and solubility in excess \(\text{NaOH(aq)}\) or excess \(\text{NH}_3\text{(aq)}\).

How are carbonate, sulfate, and halide ions (\(\text{Cl}^-\), \(\text{Br}^-\), \(\text{I}^-\)) identified in aqueous solution?

\(\text{CO}_3^{2-}\) produces \(\text{CO}_2\) gas with dilute acid; \(\text{SO}_4^{2-}\) forms a white \(\text{BaSO}_4\) precipitate with \(\text{BaCl}_2\text{(aq)}\) in dilute acid; halides form silver halide precipitates with acidified \(\text{AgNO}_3\text{(aq)}\) that have distinct colours and solubilities in ammonia.

What chemical tests are used to identify alkenes, carboxylic acids, aldehydes, and ketones?

Alkenes decolourise bromine water; carboxylic acids release \(\text{CO}_2\) gas with carbonates; aldehydes and ketones give an orange precipitate with 2,4-DNPH, with aldehydes further forming a silver mirror with Tollens' reagent or a red precipitate with Fehling's solution.

How is the retardation factor (\(R_f\)) calculated in chromatography?

\(R_f = \frac{\text{distance travelled by the component}}{\text{distance travelled by the solvent front}}\)

How do impurities affect the melting and boiling points of a substance?

Impurities lower and broaden the melting point, whereas they elevate and broaden the boiling point range of a substance.

How is colorimetry used to determine the concentration of a coloured substance in solution?

Colorimetry measures light absorbance at a selected wavelength and determines concentration by comparing the sample's absorbance against a calibration curve of standard solutions.

What characteristic infrared absorption wavenumbers identify \(\text{O–H}\) (alcohol), \(\text{O–H}\) (carboxylic acid), and \(\text{C=O}\) functional groups?

Alcohol \(\text{O–H}\) absorbs broadly at \(3230\text{–}3670\text{ cm}^{-1}\), carboxylic acid \(\text{O–H}\) absorbs very broadly at \(2500\text{–}3300\text{ cm}^{-1}\), and carbonyl \(\text{C=O}\) absorbs strongly and sharply at \(1680\text{–}1750\text{ cm}^{-1}\).

What information does the molecular ion peak (\(M^+\)) and the isotope peak ratio provide in a mass spectrum?

The \(M^+\) peak gives the relative molecular mass (\(M_r\)) of the compound, while \(M : M+2\) ratios of \(3:1\) and \(1:1\) indicate the presence of chlorine and bromine atoms, respectively.

What are the key real-world analytical applications in environmental monitoring, forensic science, and clinical diagnostics?

They include detecting trace pollutants (\(\text{CO}\), \(\text{NO}_x\), heavy metals) in environmental monitoring, identifying illicit drugs and accelerants in forensics, and determining blood glucose/electrolytes in clinical diagnostics.

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