What are the colours and physical states of chlorine, bromine and iodine at room temperature? Chlorine, Cl₂, is a greenish-yellow gas; bromine, Br₂, is a red-brown liquid; iodine, I₂, is a dark grey-black solid, often described as lustrous crystals. Their elemental particles are diatomic molecules, yet their physical states change down Group 17 because the intermolecular attractions between molecules generally become stronger as molecular size and electron-cloud polarizability increase. That physical trend is different from their familiar chemical reactivity trend, which decreases from chlorine to bromine to iodine in the standard school comparisons.
A child searching for Secondary 3 Chemistry tutor Hougang, chlorine bromine iodine colour and state, Group 17 halogens trend, why iodine is a solid but chlorine is a gas, difference between halogen and halide ions, or 2027 G3 Pure Chemistry revision near Kovan often remembers the names yet mixes up bromine water’s orange colour with liquid bromine’s red-brown appearance, or says iodine is purple at room temperature because its vapour can appear purple when heated. These are not identical situations. A useful Chemistry explanation names the substance, its physical state and the conditions of observation.
- Chlorine: Cl₂, greenish-yellow gas at ordinary room temperature.
- Bromine: Br₂, reddish-brown liquid at ordinary room temperature.
- Iodine: I₂, grey-black crystalline solid at ordinary room temperature; its vapour can appear violet or purple.
- Bonding: each is a diatomic non-metal molecule with covalent bonds inside a molecule.
- Physical trend: melting and boiling points increase down the familiar Cl₂ → Br₂ → I₂ sequence.
- Chemical trend: halogen oxidising/reactivity strength generally decreases down chlorine, bromine and iodine in the familiar school displacement reactions.
- Halogen ≠ halide: Cl₂ is elemental chlorine; Cl⁻ is chloride ion. They are not the same species or colour.
- SEC scope: 2027 G3 Chemistry K324 Section 8.2(b) explicitly names these three elements and their colours, states and displacement behaviour.
How Group 17 appears in the official syllabus
The 2027 Singapore–Cambridge SEC G3 Chemistry syllabus K324 describes chlorine, bromine and iodine as diatomic non-metals whose colour, state and displacement reactions show trends. Its Section 8.2(b) sets out this topic, while 8.1 links Periodic Table position, electron configuration and group properties.
This article places physical appearance first. A separate existing guide focuses specifically on Group 17 halogen displacement with chloride, bromide and iodide. Learning both subjects is important, but a parent should not treat the same word “trend” as proof that colour, boiling point and reactivity all increase in the same direction.
The G3 national course covers upper-secondary learning; schools sequence chapters differently between Secondary 3 and Secondary 4. Students studying G1, G2 or Combined Science should follow the appropriate syllabus rather than assume identical examination depth.
What is a halogen?
The word halogen refers to elements in Group 17 of the Periodic Table, which includes fluorine, chlorine, bromine, iodine and heavier members. The 2027 G3 Chemistry learning outcome specifically focuses on the chlorine–bromine–iodine comparisons.
These elements are non-metals with seven outer-shell electrons in the introductory electron-arrangement model. They often form compounds by gaining an electron to make a halide ion or by sharing electrons in covalent bonds.
In their familiar elemental forms, Cl₂, Br₂ and I₂ are molecules of two atoms of the same element. The symbols Cl, Br and I represent individual atoms; the formulas Cl₂, Br₂ and I₂ represent their normal diatomic elemental molecular forms.
A strong student should not automatically call a chlorine molecule a chloride ion. The chemical species, electron count and charge differ.
The Periodic Table tells you more than colour
Chlorine belongs to Group 17 and Period 3. In the familiar shell-arrangement model its electron configuration is 2,8,7. Bromine and iodine are farther down Group 17, with additional occupied electron shells.
Because they belong to the same group, their atoms share the characteristic seven outer electrons and show certain related chemical tendencies. Because they are different elements with different proton numbers and electron arrangements, their masses, sizes and physical properties differ.
The relationship is not “the colours change because the number of valence electrons changes.” Their outer-electron count remains seven. Other atomic and molecular characteristics change down the group.
This is useful for any question requiring students to predict a member’s behaviour from Periodic Table position rather than simply recite a chart.
Chlorine: the greenish-yellow gas
At ordinary room temperature and atmospheric pressure, elemental chlorine is a gas. In suitable concentrations it has a characteristic greenish-yellow appearance.
The elemental molecular formula is Cl₂. Two chlorine atoms are connected by a covalent bond within each chlorine molecule. Between separate Cl₂ molecules there are relatively weak intermolecular attractions compared with those operating between the larger Br₂ and I₂ molecules in the familiar trend.
Because its boiling point is well below ordinary room temperature, chlorine is gaseous under those conditions. A student’s observation may nevertheless describe a pale greenish-yellow gas, depending on concentration, optical path and lighting.
Do not try to identify suspected chlorine in real life by deliberately sniffing it. Chlorine gas is toxic and irritating. Observations and tests are for properly supervised teaching and professional detection.
Bromine: the red-brown liquid
At ordinary room temperature, elemental bromine is a liquid, typically described as reddish-brown. Its formula is Br₂.
Bromine atoms are larger and contain more electrons than chlorine atoms, so Br₂ molecules generally experience stronger attractive intermolecular interactions than Cl₂ molecules. The liquid state is therefore more stable at ordinary temperatures than a gaseous state under the same conditions.
This is a trend about attractions between whole molecules, not about bromine atoms having created a completely different bonding family. Within each Br₂ molecule, the bromine atoms still share a covalent bond.
Bromine is highly hazardous and its vapour is harmful. This description is scientific reference information, not an invitation to handle elemental bromine or compare colours in open containers.
Iodine: the grey-black solid
Elemental iodine, I₂, is commonly a dark grey-black crystalline solid at ordinary room temperature. Its crystals can have a metallic-looking lustre, but that appearance does not make iodine a metal.
Iodine atoms are larger and their electron clouds are more readily polarised compared with chlorine and bromine in the familiar group comparison. As a result, the intermolecular attractions between I₂ molecules can be stronger, supporting a solid state at room temperature.
Iodine vapour can appear purple or violet under suitable conditions. That is not a contradiction: it is iodine in a different physical state. The solid and gas still consist principally of I₂ molecules in the simple phase-change description.
A student who writes “iodine is a purple liquid at room temperature” has mixed up a vapour observation with the solid’s everyday state.
For a separate discussion see Why Iodine Sublimes: Purple Vapour, Crystals and Separation.
Why the melting and boiling points rise down the group
In a collection of simple molecules, melting and boiling involve overcoming intermolecular attractions sufficiently for the molecules to change their arrangement and mobility.
Cl₂ has smaller, less polarizable molecules than Br₂ and I₂ in the familiar series. As molecular electron clouds become larger and more polarizable, London dispersion attractions can become stronger. The attraction between separate molecules therefore tends to increase down the group.
Stronger intermolecular attractions generally require more energy to overcome, contributing to higher melting and boiling points. This explains the familiar sequence of gas, liquid and solid at a fixed ordinary room temperature.
The basic G3 idea is straightforward: bigger halogen molecules tend to have stronger forces between molecules. The more advanced name “London dispersion” can help connect the idea to later Chemistry, but a student should focus on the actual intermolecular nature of the forces.
The covalent bond inside Cl₂ is not the same as attraction between Cl₂ molecules
A common misconception is: “Iodine is solid because the I–I covalent bond is stronger than the Cl–Cl bond.” That is not the correct explanation of the boiling-point trend.
Boiling an ordinary molecular liquid into gas involves separating molecules from one another, not ordinarily splitting every I₂ into separate iodine atoms. The intramolecular covalent bond and the attractions between molecules are different interactions.
For example, liquid bromine becomes bromine vapour as molecules move apart; the familiar vaporisation process does not require every Br–Br bond to break into individual Br atoms.
The relevant energy for the ordinary phase-change explanation concerns intermolecular attractions, not automatically the single covalent bond inside each molecule.
I like to ask students to draw two Br₂ molecules and circle the bonds within each one separately from the space between molecules. That drawing makes the distinction visible.
Why a higher boiling point does not mean a more reactive halogen
The school chemical reactivity sequence for chlorine, bromine and iodine is generally Cl₂ more reactive than Br₂ more reactive than I₂ in the relevant halogen displacement comparisons.
The physical boiling-point sequence goes the opposite way: Cl₂ lower than Br₂ lower than I₂.
These two trends describe different chemical or physical processes. Boiling concerns overcoming attractions between molecules; displacement concerns electron-transfer tendencies between halogen and halide species.
A student who says “iodine must be most reactive because it is hardest to boil” has mixed an intermolecular physical measurement with a chemical tendency to gain electrons.
Use the correct mechanism for the specific question. The words melting point and displacement should lead to different explanations.
Chlorine, bromine and iodine are diatomic
A molecule of chlorine has formula Cl₂. Bromine is Br₂, and iodine is I₂. In each elemental molecule, two atoms of the same element share a covalent bond.
If a question asks for the reaction of chlorine with aqueous potassium bromide, the chlorine reagent is Cl₂, not Cl⁻. If it asks for bromide ions, those are Br⁻, not Br₂.
The formula difference matters because the elementary chemical and ionic equations depend on electron gain and loss. A halogen molecule can gain electrons to form halide ions; a halide ion can be oxidised to elemental halogen.
The group similarity allows a student to predict this pattern for unfamiliar question wording, but the exact molecules and ions still need the correct symbols.
Halogen versus halide: the most useful table
- Chlorine: elemental Cl₂; chloride: ion Cl⁻.
- Bromine: elemental Br₂; bromide: ion Br⁻.
- Iodine: elemental I₂; iodide: ion I⁻.
The elemental molecule and its corresponding halide ions are not the same chemical species. For instance, a colourless sodium chloride solution contains chloride ions in water, not necessarily greenish-yellow elemental chlorine gas.
That is why a student must not describe ordinary chloride solution as “green” simply because chlorine gas is greenish-yellow.
A substance’s formula identifies which atoms and charges are present. The name of the element may be part of both words, but the species differ.
Why the appearance of an aqueous solution can differ from the pure element
A pure liquid sample of bromine has a characteristic reddish-brown appearance. Bromine water, however, is bromine in an aqueous environment and is commonly described as orange or orange-brown in school tests.
The precise shade depends on concentration, solvent, lighting and other chemical conditions. A student who copies the pure elemental bromine description directly into every bromine-water question may be too rigid.
Iodine can appear dark in its solid form and violet in its vapour, while iodine in aqueous halide-containing mixtures can appear brown. In some organic solvents its colour may differ again.
The useful rule is to specify whether the question describes elemental solid, elemental liquid, vapour, or a solution before selecting a colour observation.
This is not a complication for its own sake. Chemistry tests often rely on knowing which chemical species and environment produced an observation.
Chlorine water and the bleaching observation
Chlorine gas and chlorine water are not necessarily described by the same intensity of colour. In moist conditions chlorine can also participate in chemistry that ultimately bleaches damp litmus paper after an initial acidic response.
That is different from a simple statement that elemental chlorine gas appears greenish-yellow. A gas test involving damp litmus is a chemical identification method with reactions occurring on the wet test material.
Students sometimes use “chlorine bleaches litmus” to answer a question asking for the room-temperature physical state of chlorine. The facts are related to the same element but answer different examination questions.
For a detailed gas-test explanation read Why Chlorine Bleaches Damp Litmus Paper.
How reactivity decreases down the three halogens
In the simple Group 17 displacement model, chlorine is generally more effective at oxidising appropriate bromide or iodide ions than bromine, while bromine can oxidise iodide ions under suitable conditions. Iodine is the least reactive of the three in this comparison.
The introductory electron-shell explanation connects this to the increasing number of occupied electron shells and changes in attraction for a gained electron farther down the group. The full thermodynamic picture includes more than one atomic property, but the familiar G3 trend is reliable for the specified school reactions.
Don’t explain decreasing chemical reactivity simply by saying “because the molecules are heavier.” Larger molecules and greater molar mass correlate with position down the group, but the electron-transfer chemistry is the relevant mechanism.
We can demonstrate the trend with a balanced displacement equation rather than merely assert a ranking.
A chlorine displacement equation
For aqueous bromide ions, a familiar reaction is:
Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂.
Chlorine gains electrons to form chloride ions. Bromide ions lose electrons and form bromine. The equation is a redox reaction that conserves atoms and charge.
If the original aqueous mixture contains potassium bromide, a molecular representation is:
Cl₂ + 2KBr → 2KCl + Br₂, with appropriate aqueous states.
The chemical product bromine may give an orange/brown appearance depending on the solvent and concentration. Do not insist that the reaction be observed as the dark colour of a pure bromine pool.
The more important lesson for this article is that a chemical reaction’s solution colour is not identical to the pure element’s room-temperature appearance.
A bromine displacement equation
Bromine can oxidise iodide under suitable aqueous conditions:
Br₂ + 2I⁻ → 2Br⁻ + I₂.
The resulting iodine may show brown appearance in an aqueous halide-containing environment, depending on conditions. This does not mean pure iodine at room temperature has changed from a grey-black solid into a permanently brown chemical compound.
The elemental iodine product is still represented by I₂. Its observed phase and appearance depend on the system.
A pupil who draws iodine as two separate I atoms has lost the familiar diatomic formula. A pupil who calls it iodide ions has confused reduction and oxidation direction.
For more full worked examples use the dedicated Group 17 Displacement Reactions article.
Why iodine normally cannot displace chloride from solution
In the standard school reactivity order, iodine is less reactive than chlorine. Therefore I₂ is not expected to oxidise Cl⁻ ions to Cl₂ under the usual displacement-test conditions.
A child may try to balance:
I₂ + 2Cl⁻ → 2I⁻ + Cl₂
and conclude the equation must happen because atoms and charge balance. That is an important misconception: an equation being balanced does not prove a proposed chemical change is favourable under the conditions.
The reactivity series supplies chemical evidence about direction. Atom conservation is necessary for a valid reaction equation but not sufficient for deciding whether a reaction will occur.
This principle also appears in metal displacement and reduction of metal oxides.
Worked example: identify a mystery Group 17 element from room-temperature data
An exam question presents three unknown elemental diatomic substances at room temperature: X is a greenish-yellow gas, Y a reddish-brown liquid, and Z a grey-black solid.
Within the stated list chlorine, bromine and iodine, X is chlorine, Y is bromine, and Z is iodine.
Now the question changes: which substance is expected to have the highest boiling point? Z, iodine, under the familiar trend.
Which is expected to displace iodide ions most readily under the usual conditions? X, chlorine.
Note that the answers to the latter two questions differ because one asks about physical phase attractions and the other about chemical reactivity.
This two-dimensional comparison is more valuable than reciting a three-row list.
Worked example: predict an unknown halogen from boiling data
Suppose a school question gives a hypothetical new member of a related simple-molecule series with a boiling point higher than bromine but lower than iodine, while its chemical reactivity is between theirs.
The data may support an intermediate position in a suitable trend, but students should not invent a new stable naturally occurring element between bromine and iodine in the actual Periodic Table. The hypothetical model would be a data interpretation task, not a real Periodic Table discovery.
A better question using actual named members might supply the room-temperature phase and ask which of Cl₂, Br₂, I₂ is most consistent. Use the observed state together with the given temperature range, rather than an invented number.
I teach students to respect the actual options. Predicting from trends is useful, but a model should not override known chemical identities.
A short question about melting point and molecular structure
A student writes: “Iodine has a higher melting point because the covalent I–I bond becomes stronger as we go down Group 17.”
The correct repair is to distinguish molecular bonding from intermolecular attractions. The familiar increase in melting and boiling points is explained by stronger attractions between I₂ molecules, associated with their size and electron-cloud polarizability.
The phase transition does not ordinarily require breaking the covalent bond inside every I₂ molecule. Molecules can move apart and remain I₂.
A useful answer says: “I₂ molecules are larger and more polarizable than Cl₂ molecules, so stronger intermolecular attractions require more energy to overcome.”
That is a causal explanation, not merely a trend statement.
What changes when iodine sublimes?
Solid iodine can enter the gas phase directly under suitable conditions, producing violet vapour. This is sublimation.
The chemical formula remains I₂. It is a physical change because the molecular identity stays the same. The appearance changes because the physical state and optical conditions differ.
This is a strong exam transfer task: the teacher gives a purple vapour observation after heating grey-black crystals. A learner must identify the original Group 17 element, the phase-change name and the unchanged molecule formula.
For this reason, the halogen colour/state chapter also reinforces the kinetic particle theory and physical versus chemical change lessons.
Why the halogens should not be treated as household demonstration chemicals
Chlorine and bromine are toxic and irritating; bromine is a corrosive liquid and volatile. Iodine vapour is irritating and can pose exposure hazards. These hazards make unsupervised colour comparisons or gas generation inappropriate.
Parents do not need to buy any halogens to help their child learn this topic. Paper cards, labelled diagrams and properly sourced images of actual elemental states can teach the chemistry without risk.
The student can also distinguish NaCl solution from Cl₂ gas using written species descriptions, rather than generating chlorine through electrolysis at home.
Responsible Chemistry education includes understanding that a scientifically interesting observation does not justify exposing children to hazardous substances.
Why a small-group tutorial should diagnose the first error
Imagine three learners tackling a Group 17 question.
Student A correctly names chlorine, bromine and iodine but writes Cl, Br, I as their ordinary elemental molecular formulas. The gap is diatomic elemental molecules.
Student B knows Cl₂, Br₂, I₂ but claims iodine’s high boiling point proves it is the most chemically reactive halogen. The gap is physical versus chemical trend.
Student C understands both but uses pure liquid bromine’s red-brown colour as a rigid description for dilute bromine water in a test. The gap is sample environment and observation conditions.
A well-run three-student tutorial should give each child a targeted correction, then an unfamiliar comparison question and a later independent retest. A class of three students by itself does not guarantee that individual misconceptions have been repaired.
Two-minute parent diagnostic
Ask three questions without showing a completed table:
“What are the colours and states of Cl₂, Br₂ and I₂ at room temperature?”
“Why does the boiling point rise down the group?”
“Does a higher boiling point mean greater ability to displace bromide?”
The expected progression is greenish-yellow gas, red-brown liquid and grey-black solid; larger molecules with stronger intermolecular attractions; and no, because displacement reactivity generally decreases from chlorine to iodine.
If the child can explain the different reasons for the two trends, the topic is much more secure than when they can only repeat colours.
Independent practice: eighteen questions
- 1. Name the Periodic Table group containing chlorine, bromine and iodine.
- 2. Give the ordinary elemental formula of chlorine gas.
- 3. Give the ordinary elemental formula of liquid bromine.
- 4. Give the ordinary elemental formula of solid iodine.
- 5. State the colour and physical state of chlorine at room temperature.
- 6. State the colour and state of bromine at room temperature.
- 7. State the colour and state of iodine at room temperature.
- 8. What colour can iodine vapour show under suitable conditions?
- 9. Does the usual boiling-point trend increase or decrease down these three elements?
- 10. What type of attraction between molecules contributes to that boiling-point trend?
- 11. Does boiling bromine normally turn every Br₂ molecule into two separate Br atoms?
- 12. Which is most reactive in the familiar displacement comparison: Cl₂, Br₂ or I₂?
- 13. Write a balanced ionic equation for chlorine displacing bromine from bromide ions.
- 14. Write an equation for bromine displacing iodine from iodide ions.
- 15. Is Cl⁻ the same chemical species as Cl₂?
- 16. Can aqueous bromine appear orange even though pure liquid bromine is red-brown?
- 17. Why isn’t the colour of iodine vapour a proof that iodine is a purple liquid at room temperature?
- 18. Which idea explains iodine’s higher boiling point: increased intermolecular attraction or stronger electrolysis at the cathode?
A good learner should give brief causal explanations for Questions 9, 10, 16 and 17, not just single-word answers.
Answers with reasoning
- 1. Group 17, the halogens.
- 2. Cl₂.
- 3. Br₂.
- 4. I₂.
- 5. Greenish-yellow gas.
- 6. Reddish-brown liquid.
- 7. Grey-black crystalline solid.
- 8. Violet or purple vapour.
- 9. Increase from chlorine to iodine.
- 10. Stronger intermolecular attractions, often described as London dispersion forces, between larger more polarizable molecules.
- 11. No. Vaporisation mainly separates molecules from one another; it does not ordinarily split their covalent bonds.
- 12. Cl₂.
- 13. Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂.
- 14. Br₂ + 2I⁻ → 2Br⁻ + I₂.
- 15. No. Cl⁻ is an anion; Cl₂ is a neutral diatomic molecule.
- 16. Yes, solution colour depends on solvent, concentration and conditions.
- 17. The observation is of iodine gas, while solid iodine is the ordinary room-temperature elemental form.
- 18. Intermolecular attraction.
After correction, change the question to “Which member has the lowest melting point?” or “What if the iodine is dissolved in water containing iodide?” The child should use the conditions, not a single memorised colour.
Seven-day learning plan
Day 1: learn the molecular formulas Cl₂, Br₂ and I₂ and distinguish them from halide ions. Day 2: learn the room-temperature physical states and representative colours. Day 3: explain how molecular size and intermolecular attraction affect melting and boiling.
Day 4: compare physical trends with chemical reactivity trends without merging them. Day 5: revisit the two halogen displacement equations. Day 6: describe iodine’s solid-to-vapour phase change and the difference between an element and its solution. Day 7: attempt mixed data questions without headings.
A week later, ask the child why iodine is a solid while chlorine is a gas, and why chlorine can displace iodide despite its lower boiling point. Two different correct mechanisms demonstrate the most important transfer.
Questions Hougang parents often ask
Is chlorine green or yellow?
The familiar elemental gas is described as greenish-yellow, with appearance depending on viewing conditions and concentration.
Is bromine a gas at room temperature?
Ordinary elemental bromine is a reddish-brown liquid, although it can produce vapour.
Is iodine always purple?
No. Ordinary solid iodine is grey-black; its vapour can appear purple or violet.
Why do boiling points rise down Group 17?
Larger, more polarizable diatomic molecules generally have stronger intermolecular attractions.
Does iodine’s higher boiling point make it the most reactive?
No. Chemical displacement reactivity generally decreases from chlorine to bromine to iodine.
Are chloride and chlorine the same?
No. Cl⁻ is chloride ion, while Cl₂ is elemental chlorine molecule.
Is this required in SEC 2027 G3 Chemistry?
Yes. K324 Section 8.2(b) explicitly names chlorine, bromine and iodine trends in colour, state and displacement.
Can children observe these pure halogens at home?
No. These chemicals and vapours can be hazardous. Use diagrams, school-supervised demonstrations and provided data.
The important distinction
A chemical can be described by what it looks like, what state it is in, how its molecules attract one another, and how it reacts. Those are different properties, and they need not follow identical trends.
The Group 17 halogens are a compact lesson in scientific precision. Chlorine, bromine and iodine differ in colour and room-temperature state, yet share diatomic elemental formulas and outer-electron patterns. Their physical attractions become stronger down the group while their familiar displacement reactivity decreases. Learning why both statements can be true is what prepares a Secondary 3 student for new data rather than a memorised table.
A data-based way to compare melting and boiling without memorising all temperatures
Imagine a school examination gives three unnamed halogen samples at ordinary atmospheric pressure and a table of approximate boiling temperatures:
Sample A boils well below room temperature. Sample B boils somewhat above room temperature. Sample C boils far above room temperature and is a solid at room temperature.
Within the stated chlorine, bromine and iodine choices, A is consistent with chlorine, B with bromine, and C with iodine.
The key reasoning is the relationship between boiling point and room temperature. If the boiling point is below room temperature, the ordinary sample can exist as a gas. If room temperature lies between the melting and boiling points, a liquid can be expected. If room temperature is below the melting point, the material can remain solid.
A student who merely remembers the colour names may not be able to infer state from unfamiliar temperature data. This exercise makes the physical basis explicit.
One fixed room temperature, three different phases
Chlorine is gas, bromine liquid and iodine solid at ordinary room conditions because that fixed temperature lies in different regions relative to each substance’s melting and boiling points.
There is no single rule that “all non-metals are gases” or “heavier elements are always solids”. Many non-metals have different molecular structures, bonding and intermolecular attractions.
The Group 17 sequence is useful because the same basic diatomic molecular form occurs across all three, while increasing molecular size makes the attractions between molecules stronger.
A parent can ask the student to imagine cooling chlorine or heating iodine. The molecules would undergo physical state changes under suitable conditions without ceasing to be the same chemical element.
That is the bridge between the Periodic Table and kinetic particle theory.
The importance of specifying concentration in a colour observation
Suppose a worksheet describes bromine water as pale orange and a different illustration describes liquid bromine as dark reddish-brown. Both can be consistent with bromine-containing systems.
The relevant differences include solvent, concentration, optical path and chemical environment. A diluted aqueous solution need not look like a pure liquid sample of the elemental substance.
Likewise, iodine may be dark grey as a solid, purple in vapour and brown in an aqueous iodide-containing mixture. A student should not force one colour onto every state or solvent.
The exam skill is to read the actual description and test medium. If a question gives the observation in a specific experiment, that supplied information should be used rather than overridden by a generic chart.
Why halide ion tests can reveal a different kind of evidence
The 2027 G3 qualitative-analysis syllabus lists chloride and iodide among the anions that can be tested using nitric acid followed by aqueous silver nitrate.
The familiar observations are a white precipitate for chloride and a yellow precipitate for iodide under the prescribed conditions. These colours are associated with silver halide precipitates, not with elemental chlorine gas or iodine vapour.
For chloride:
Ag⁺ + Cl⁻ → AgCl(s).
For iodide:
Ag⁺ + I⁻ → AgI(s).
The test therefore addresses a different question: which halide ion is present? It does not identify pure elemental chlorine or iodine merely by observing a coloured gas or solid.
This extension helps students avoid importing Group 17 elemental-colour facts into salt-analysis questions.
Two different iodine questions, two different answers
Question A: “State the colour and physical state of iodine at ordinary room temperature.” Answer: grey-black solid.
Question B: “Iodide ions are oxidised in a suitable aqueous reaction and a brown colour develops. What species can be present?” Answer: iodine-containing species formed by oxidation, with the observed colour in the specified aqueous environment.
A student who gives “purple gas” for Question A has confused the ordinary room state with a heated vapour observation. A student who says “the brown solution contains only iodide ions, not iodine” may have missed the oxidation.
These distinctions are not arbitrary examiner tricks. They follow from chemical species, oxidation state, physical state and solvent.
Which trend can an unfamiliar Group 17 table support?
Suppose a fictional table gives progressively higher boiling points from chlorine to iodine. It supports the physical property trend. It does not by itself prove the reactivity ranking.
To justify the chemical displacement ranking, you need evidence from reactions of halogens with halide solutions or a relevant supported electron-transfer explanation.
A good scientific answer does not derive chemical reactivity from a boiling-point table simply because both involve the same three element names.
Ask the learner to identify which data are physical measurements and which data concern chemical transformation. This helps them interpret unknown element data without forcing every property into one linear pattern.
Molecular shape and particle bonding in a phase change
Each ordinary Cl₂, Br₂ or I₂ particle is a molecule containing two atoms sharing a covalent bond.
When liquid Br₂ becomes bromine vapour, the usual physical change separates molecules from one another. It does not require the entire Br–Br molecular bond to be broken. The molecule remains Br₂.
When iodine sublimes, I₂ molecules leave the solid arrangement and travel in the gas phase. Again, the molecule’s identity can remain I₂ in the simple model.
This connection to kinetic particle theory reinforces the same rule from two directions: physical states concern particle spacing and interactions, while the chemical formula describes which atoms are bonded in each particle.
Five quick transfer questions after a week
- 1. A pure element is a red-brown liquid at room temperature and belongs to Group 17. Which element is it? Bromine.
- 2. A halogen sample gives violet vapour after suitable heating, but its ordinary room-temperature form is solid. Which element? Iodine.
- 3. What explains why iodine has a higher boiling point than chlorine? Stronger attractions between larger, more polarizable molecules.
- 4. Which is expected to be the stronger oxidising agent in the familiar Group 17 displacement comparison: chlorine or iodine? Chlorine.
- 5. Is a white AgCl precipitate the same material as greenish-yellow Cl₂ gas? No, they are chemically distinct species.
A learner who can explain why Questions 3 and 4 use different chemical ideas is showing mastery. Colour recall alone would not establish that.
Halogen Displacement: Chlorine, Bromine and Iodine · Why Iodine Sublimes: Purple Vapour · Periodic Table and Electron Configuration · 2027 SEC G3 Chemistry K324 syllabus · Hougang Chemistry Reference Library · Parent–student consultation.