(I once asked a class, "If I gave you an unlabelled bottle of clear liquid, how would you find out what it is?" Most students shrugged. Then I said, "By the end of this chapter, you will know exactly how. You will know how to test for ions, how to separate mixtures, how to identify unknown substances, and how to measure how much of each substance is present. This is the chemistry of detection — and it is one of the most practical and powerful skills a chemist can have." In this chapter, we are going to learn how chemists analyse substances. How do we know what is in a sample? How do we measure how much? How do we separate complex mixtures into their components? These are the questions that chemical analysis answers. We will look at qualitative analysis — testing for the presence of specific ions and gases. We will look at quantitative analysis — measuring the amounts of substances. We will learn about chromatography, spectroscopy, and other powerful analytical techniques. By the end, you will have a toolkit of methods for investigating the chemical world. Let us begin.) Think about all the ways chemistry is used to analyse things. A doctor analyses your blood to check your health. An environmental scientist analyses water to check for pollution. A forensic scientist analyses evidence to solve a crime. A food scientist analyses products to ensure they are safe and authentic. A pharmaceutical company analyses drugs to ensure they are pure and effective. All of these rely on the techniques of chemical analysis. In this chapter, we are going to learn the fundamental techniques that make all of this possible. We will learn how to test for common ions and gases. We will learn how to separate mixtures using chromatography. We will learn how to use spectroscopy to identify substances. And we will learn how to perform accurate quantitative analysis. These are skills that you will use throughout your studies and, if you pursue a career in science, throughout your working life. Let us begin.
Chemical analysis is the process of determining the composition of a substance. There are two main types:
| Type | Question It Answers | Examples |
|---|---|---|
| Qualitative analysis | What is present? | Testing for the presence of specific ions or gases. |
| Quantitative analysis | How much is present? | Titration, gravimetric analysis, spectroscopy. |
Cations are positive ions. They can be identified by their reactions with sodium hydroxide solution and ammonia solution, and by flame tests.
| Cation | Observation with NaOH | Observation with Excess NaOH |
|---|---|---|
| Cu²⁺ | Blue precipitate | Precipitate insoluble in excess |
| Fe²⁺ | Green precipitate | Precipitate insoluble in excess |
| Fe³⁺ | Red-brown precipitate | Precipitate insoluble in excess |
| Zn²⁺ | White precipitate | Precipitate soluble in excess |
| Al³⁺ | White precipitate | Precipitate soluble in excess |
| Ca²⁺ | White precipitate | Precipitate insoluble in excess |
| Mg²⁺ | White precipitate | Precipitate insoluble in excess |
| NH₄⁺ | No precipitate | Ammonia gas released on warming |
| Cation | Flame Colour |
|---|---|
| Na⁺ | Golden yellow |
| K⁺ | Lilac |
| Ca²⁺ | Brick red |
| Cu²⁺ | Blue-green |
| Li⁺ | Crimson red |
| Ba²⁺ | Apple green |
Anions are negative ions. They can be identified by specific chemical tests.
| Anion | Test | Observation |
|---|---|---|
| Cl⁻ (chloride) | Add dilute HNO₃, then AgNO₃ | White precipitate (AgCl), soluble in ammonia |
| Br⁻ (bromide) | Add dilute HNO₃, then AgNO₃ | Cream precipitate (AgBr), partially soluble in ammonia |
| I⁻ (iodide) | Add dilute HNO₃, then AgNO₃ | Yellow precipitate (AgI), insoluble in ammonia |
| SO₄²⁻ (sulphate) | Add dilute HCl, then BaCl₂ | White precipitate (BaSO₄) |
| CO₃²⁻ (carbonate) | Add dilute HCl | Fizzing; CO₂ gas turns limewater milky |
| NO₃⁻ (nitrate) | Brown ring test: add FeSO₄, then conc. H₂SO₄ | Brown ring at junction of liquids |
| SO₃²⁻ (sulphite) | Add dilute HCl, then acidified K₂Cr₂O₇ | Orange dichromate turns green |
| Gas | Test | Observation |
|---|---|---|
| O₂ | Insert glowing splint | Splint relights |
| H₂ | Hold lit splint at mouth of tube | "Pop" sound |
| CO₂ | Bubble through limewater | Limewater turns milky |
| NH₃ | Hold damp red litmus near gas | Litmus turns blue |
| Cl₂ | Hold damp litmus near gas | Litmus bleached (turns white) |
| SO₂ | Hold damp acidified K₂Cr₂O₇ paper | Orange turns green |
| HCl | Hold glass rod dipped in NH₃ solution | White fumes (NH₄Cl) |
(These tests are the detective tools of chemistry. With a few simple reagents, you can identify almost any common ion or gas. It is like solving a puzzle — each test narrows down the possibilities until only one answer remains.)
Chromatography is a technique for separating mixtures based on the different rates at which their components move through a stationary phase.
Principle: The mixture is carried by a mobile phase (a liquid or gas) through a stationary phase (a solid or a liquid on a solid). Components that interact more strongly with the stationary phase move more slowly; those that interact more with the mobile phase move faster. This separates the components.
| Type | Stationary Phase | Mobile Phase | Applications |
|---|---|---|---|
| Paper chromatography | Water in paper fibres | Solvent | Separating dyes and pigments |
| Thin-layer chromatography (TLC) | Silica or alumina on a glass plate | Solvent | Analysing drugs, pesticides, and organic compounds |
| Gas-liquid chromatography (GLC) | Liquid on a solid support in a column | Carrier gas (e.g., nitrogen) | Analysing volatile mixtures, drug testing, environmental monitoring |
The retention factor (Rf) is a measure of how far a component travels relative to the solvent front:
Rf = distance travelled by component / distance travelled by solvent front
The Rf value is characteristic of a particular substance under particular conditions. It can be used to identify unknown components by comparison with known standards.
Spectroscopy is the study of the interaction between matter and electromagnetic radiation. It is one of the most powerful tools in chemical analysis.
When atoms are excited (e.g., by heat in a flame), their electrons jump to higher energy levels. When the electrons fall back, they emit light of specific wavelengths. The emitted light is separated into a spectrum, and the wavelengths are characteristic of the element. This allows identification and quantification of elements.
Applications: Analysing metal ions in water, identifying elements in stars, quality control in industry.
In AAS, a sample is vaporised and atomised. A light source emits light of a specific wavelength (characteristic of the element being analysed). The atoms in the sample absorb some of this light, and the amount absorbed is proportional to the concentration of the element. AAS is highly sensitive and can detect very low concentrations.
Applications: Detecting heavy metals in water and soil, analysing blood for lead, quality control in food and pharmaceuticals.
IR spectroscopy measures the absorption of infrared radiation by molecules. Different bonds in a molecule absorb different frequencies of IR radiation, giving a unique "fingerprint" spectrum. This can be used to identify functional groups and entire molecules.
Applications: Identifying organic compounds, analysing polymers, forensic analysis, quality control.
Mass spectrometry involves ionising a sample and separating the ions based on their mass-to-charge ratio. It gives the molecular mass and structural information about a compound.
Applications: Identifying unknown compounds, drug testing, environmental analysis, proteomics.
Titration is a quantitative technique used to determine the concentration of a solution by reacting it with a solution of known concentration.
Key terms:
Procedure:
Calculation:
Moles of titrant = concentration × volume
Moles of analyte = moles of titrant × (stoichiometric ratio)
Concentration of analyte = moles of analyte / volume of analyte
Example: 25.0 cm³ of NaOH solution is titrated with 0.100 mol/dm³ HCl. The average volume of HCl used is 20.0 cm³. Calculate the concentration of the NaOH solution.
Solution:
Moles of HCl = 0.100 × 0.0200 = 0.00200 mol.
NaOH + HCl → NaCl + H₂O, so 1 mole NaOH reacts with 1 mole HCl.
Moles of NaOH = 0.00200 mol.
Concentration of NaOH = 0.00200 / 0.0250 = 0.0800 mol/dm³.
Gravimetric analysis is a quantitative technique in which the mass of a product is measured to determine the amount of a substance in a sample.
Steps:
Example: A sample of impure sodium chloride is dissolved in water, and excess silver nitrate is added. The silver chloride precipitate is filtered, dried, and weighed. If the mass of AgCl is 2.87 g, calculate the mass of NaCl in the original sample. (Molar masses: AgCl = 143.5 g/mol, NaCl = 58.5 g/mol.)
Solution:
Moles of AgCl = 2.87 / 143.5 = 0.0200 mol.
NaCl + AgNO₃ → AgCl + NaNO₃, so 1 mole NaCl produces 1 mole AgCl.
Moles of NaCl = 0.0200 mol.
Mass of NaCl = 0.0200 × 58.5 = 1.17 g.
Good laboratory technique is essential for accurate and safe chemical analysis. Key practices include:
| Field | Application |
|---|---|
| Medicine | Blood tests, drug monitoring, diagnosis of disease. |
| Environmental science | Monitoring water and air quality, detecting pollutants. |
| Forensic science | Analysing evidence from crime scenes (DNA, drugs, poisons, explosives). |
| Food science | Checking food safety, authenticity, and nutritional content. |
| Pharmaceutical industry | Quality control, purity testing, drug development. |
| Geology | Analysing rocks and minerals, dating geological samples. |
| Archaeology | Analysing artefacts, dating ancient materials. |
(Chemical analysis is the eyes and ears of chemistry. Without it, we would be blind to the composition of the world around us. Every time you get a blood test, drink a glass of safe water, or take a medicine, you are benefiting from the techniques described in this chapter. They are the quiet heroes of modern science.)
When choosing an analytical technique, consider:
Question: A solution gives a white precipitate with sodium hydroxide solution that dissolves in excess. It also gives a white precipitate with barium chloride solution after acidification with hydrochloric acid. Identify the cation and anion.
Solution: The white precipitate with NaOH that dissolves in excess suggests Zn²⁺ or Al³⁺. The white precipitate with BaCl₂ after acidification suggests SO₄²⁻ (BaSO₄). So the solution could contain ZnSO₄ or Al₂(SO₄)₃.
Question: 25.0 cm³ of HCl solution is titrated with 0.100 mol/dm³ NaOH. The average volume of NaOH used is 22.5 cm³. Calculate the concentration of the HCl solution.
Solution:
Moles of NaOH = 0.100 × 0.0225 = 0.00225 mol.
HCl + NaOH → NaCl + H₂O, so 1 mole HCl reacts with 1 mole NaOH.
Moles of HCl = 0.00225 mol.
Concentration of HCl = 0.00225 / 0.0250 = 0.0900 mol/dm³.
Question: A sample of impure barium chloride is dissolved in water, and excess sodium sulphate is added. The barium sulphate precipitate is filtered, dried, and weighed. If the mass of BaSO₄ is 2.33 g, calculate the mass of BaCl₂ in the original sample. (Molar masses: BaSO₄ = 233 g/mol, BaCl₂ = 208 g/mol.)
Solution:
Moles of BaSO₄ = 2.33 / 233 = 0.0100 mol.
BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl, so 1 mole BaCl₂ produces 1 mole BaSO₄.
Moles of BaCl₂ = 0.0100 mol.
Mass of BaCl₂ = 0.0100 × 208 = 2.08 g.
Question: In a paper chromatography experiment, a dye spot travels 4.0 cm from the baseline, and the solvent front travels 10.0 cm. Calculate the Rf value of the dye.
Solution: Rf = distance travelled by dye / distance travelled by solvent = 4.0 / 10.0 = 0.40.
1. Distinguish between qualitative and quantitative analysis.
Qualitative analysis determines what substances are present. Quantitative analysis determines how much of each substance is present.
2. Describe the test for chloride ions in solution.
Add dilute nitric acid, then silver nitrate solution. A white precipitate of silver chloride forms, which is soluble in ammonia.
3. What is the flame colour for potassium ions?
Lilac.
4. Describe the test for carbon dioxide gas.
Bubble the gas through limewater. If the limewater turns milky, the gas is carbon dioxide.
5. What is chromatography used for?
Chromatography is used to separate mixtures into their components based on their different rates of movement through a stationary phase.
6. Calculate the Rf value of a substance that travels 3.5 cm when the solvent front travels 7.0 cm.
Rf = 3.5 / 7.0 = 0.50.
7. 25.0 cm³ of NaOH solution is neutralised by 20.0 cm³ of 0.100 mol/dm³ HCl. Calculate the concentration of the NaOH solution.
Moles of HCl = 0.100 × 0.0200 = 0.00200 mol. Moles of NaOH = 0.00200 mol. Concentration of NaOH = 0.00200 / 0.0250 = 0.0800 mol/dm³.
Question 1 [10 marks]
(a) Distinguish between qualitative and quantitative analysis. [2]
(b) Describe the tests for: (i) Cu²⁺, (ii) Fe²⁺, (iii) Fe³⁺ using sodium hydroxide solution. [6]
(c) Describe the test for sulphate ions in solution. [2]
(a) Qualitative analysis determines what substances are present; quantitative analysis determines how much. (2 marks: 1 for each.)
(b) (i) Cu²⁺: blue precipitate with NaOH, insoluble in excess. (ii) Fe²⁺: green precipitate with NaOH, insoluble in excess. (iii) Fe³⁺: red-brown precipitate with NaOH, insoluble in excess. (6 marks: 2 for each ion + colour.)
(c) Add dilute hydrochloric acid, then barium chloride solution. A white precipitate of barium sulphate confirms the presence of sulphate ions. (2 marks: 1 for method, 1 for result.)
Question 2 [10 marks]
(a) Explain the principle of chromatography. [3]
(b) In a paper chromatography experiment, a dye travels 4.5 cm and the solvent front travels 9.0 cm. Calculate the Rf value. [2]
(c) Describe how chromatography can be used to identify an unknown substance. [3]
(d) State one application of chromatography. [2]
(a) Chromatography separates mixtures based on the different rates at which their components move through a stationary phase. The mobile phase carries the components, and those that interact more strongly with the stationary phase move more slowly. (3 marks: 1 for separation, 1 for mobile phase, 1 for stationary phase.)
(b) Rf = 4.5 / 9.0 = 0.50. (2 marks: 1 for formula, 1 for answer.)
(c) The Rf value of the unknown substance is measured and compared with the Rf values of known standards run under the same conditions. If the Rf values match, the substance is identified. (3 marks: 1 for Rf measurement, 1 for comparison with standards, 1 for same conditions.)
(d) Any one of: separating dyes and pigments, analysing drugs, forensic analysis, environmental monitoring. (2 marks: 1 for use, 1 for clarity.)
Question 3 [10 marks]
(a) Describe the procedure for a titration. [4]
(b) 25.0 cm³ of HCl solution is titrated with 0.100 mol/dm³ NaOH. The average volume of NaOH used is 22.5 cm³. Calculate the concentration of the HCl solution. [4]
(c) State two precautions to ensure accurate results in a titration. [2]
(a) Rinse the burette with titrant and fill it. Pipette a known volume of analyte into a conical flask. Add indicator. Slowly add titrant, swirling, until the indicator changes colour. Record the volume. Repeat to obtain concordant results. (4 marks: 1 for rinsing/filling burette, 1 for pipetting analyte, 1 for adding indicator, 1 for repeating.)
(b) Moles of NaOH = 0.100 × 0.0225 = 0.00225 mol. Moles of HCl = 0.00225 mol. Concentration of HCl = 0.00225 / 0.0250 = 0.0900 mol/dm³. (4 marks: 1 for moles of NaOH, 1 for mole ratio, 1 for moles of HCl, 1 for concentration.)
(c) Any two of: read meniscus at eye level; use a white tile; rinse burette and pipette with the appropriate solutions; repeat to get concordant results; add titrant dropwise near the end point. (2 marks: 1 for each.)
Challenge 1: Explain why it is important to acidify the solution with dilute nitric acid before testing for halides with silver nitrate.
It is important to acidify with dilute nitric acid before testing for halides with silver nitrate because other ions in the solution could interfere with the test. For example, carbonate ions (CO₃²⁻) would react with silver nitrate to form a white precipitate of silver carbonate, which could be mistaken for silver chloride. Sulphate ions (SO₄²⁻) could also form a white precipitate with silver nitrate. Adding dilute nitric acid removes these interfering ions by reacting with them — carbonates fizz and release CO₂, and sulphates remain in solution as nitric acid does not form a precipitate with them. The nitric acid also ensures that the solution is acidic, which is necessary for the silver halide precipitates to form correctly. Without acidification, the results would be unreliable.
Challenge 2: Explain why atomic absorption spectroscopy is more sensitive than flame emission spectroscopy for detecting trace metals.
Atomic absorption spectroscopy (AAS) is more sensitive than flame emission spectroscopy for detecting trace metals because it measures the absorption of light by ground-state atoms, which are the vast majority of atoms in a sample. In flame emission spectroscopy, only the small fraction of atoms that are thermally excited and emit light are detected. Since most atoms remain in the ground state, the emission signal is much weaker than the absorption signal. AAS also uses a specific light source (a hollow cathode lamp) that emits the exact wavelength needed, which reduces background interference and increases sensitivity. AAS can detect concentrations in the parts per billion range, making it ideal for trace metal analysis. Flame emission is useful for elements that are easily excited (like sodium and potassium), but AAS is superior for most trace metal analysis.
Challenge 3: A student performs a titration and obtains the following results: 22.5 cm³, 22.4 cm³, and 22.6 cm³. They calculate the average as 22.5 cm³. Is this correct? Explain your reasoning and suggest how the student could improve their results.
The student's calculation of the average is correct. The three values (22.5, 22.4, and 22.6 cm³) are all within 0.2 cm³ of each other, which is considered concordant. The average of the three values is (22.5 + 22.4 + 22.6) / 3 = 67.5 / 3 = 22.5 cm³. However, to improve the reliability of the results, the student could: (1) repeat the titration more times to get more data points; (2) check that the burette and pipette are clean and rinsed with the appropriate solutions; (3) read the meniscus at eye level to avoid parallax error; (4) add the titrant dropwise near the end point to avoid overshooting; (5) use a white tile to make the colour change easier to see; (6) ensure the solution is well mixed by swirling the flask constantly. These improvements would increase the accuracy and reliability of the results.
"Chemical analysis is the eyes and ears of chemistry — the tools that reveal the invisible composition of the world. Now you know how to use them."
