How do the properties of Group 3 to Group 7 elements vary, and what are the trends of the halogens?
Periodic trends in the p-block, the chemistry of Group 7 (the halogens) including their reactions as oxidising agents, the reactions of halide ions, and qualitative tests for anions.
An Eduqas A-Level Chemistry PI2.1 answer on p-block trends, the halogens as oxidising agents, the reactions of halide ions, disproportionation of chlorine, and qualitative anion tests.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this topic is asking
Eduqas topic PI2.1 covers selected p-block chemistry, with the halogens (Group 7) as the central case study: their trends in physical properties and oxidising power, their reactions as oxidising agents (including displacement), the reactions and tests of halide ions, the disproportionation of chlorine, and qualitative tests for common anions. The reasoning links periodic trends to observable reactions.
Trends in the halogens
Halogens as oxidising agents and displacement
A halogen oxidises the halide ions of any halogen below it in the group, displacing the less reactive halogen. Chlorine displaces bromine and iodine; bromine displaces only iodine. The colour of the displaced halogen (orange bromine, brown iodine) signals the reaction, and the order confirms the trend in oxidising power.
Reactions of halide ions and silver nitrate test
Concentrated sulfuric acid also distinguishes halides: it oxidises bromide to bromine and iodide to iodine (and produces and ), but cannot oxidise chloride, illustrating the trend in reducing power of the halide ions (which increases down the group).
Disproportionation of chlorine
Chlorine disproportionates in alkali: in cold dilute sodium hydroxide it gives chloride and chlorate(I) (the basis of bleach), and in hot concentrated alkali it gives chloride and chlorate(V). In both, chlorine () is simultaneously reduced and oxidised.
Qualitative tests for anions
Eduqas expects the standard anion tests: carbonate (effervescence of with acid, turning limewater milky), sulfate (white precipitate with acidified barium chloride) and the halides (silver nitrate, as above). Acidifying first removes interfering ions.
Examples in context
Example 1. Water treatment. Chlorine is added to drinking water and pools because it is a strong enough oxidising agent to kill microorganisms; its disproportionation in water also forms chlorate(I), the active disinfectant.
Example 2. The silver-halide test in forensics. The distinct colours and ammonia solubilities of the silver halides let analysts identify which halide is present in a sample, a direct application of the qualitative tests.
Try this
Q1. Write the ionic equation for the reaction of chlorine with sodium iodide solution and state the colour change. [2 marks]
- Cue. ; the solution turns brown as iodine is displaced.
Q2. State the reagent and observation that confirm a carbonate ion. [2 marks]
- Cue. Add a dilute acid; effervescence of carbon dioxide is seen, which turns limewater milky.
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20194 marks(a) Describe and explain the trend in oxidising power of the halogens down Group 7. (b) Write an ionic equation for the reaction of chlorine with potassium bromide solution and state what is observed.Show worked answer →
(a) Oxidising power decreases down the group (1). Going down, the atomic radius increases and there is more shielding, so the atom less readily gains an electron to form the halide ion, making it a weaker oxidising agent (1).
(b) (1). The solution turns orange as bromine is displaced (chlorine, the stronger oxidising agent, oxidises bromide) (1).
Eduqas 20214 marksDescribe how you would use silver nitrate solution (followed by aqueous ammonia) to distinguish between separate solutions of sodium chloride, sodium bromide and sodium iodide.Show worked answer →
Acidify with dilute nitric acid, then add silver nitrate solution. Chloride gives a white precipitate, bromide a cream precipitate and iodide a yellow precipitate (2).
To confirm: the white (AgCl) dissolves in dilute aqueous ammonia; the cream (AgBr) dissolves only in concentrated ammonia; the yellow (AgI) is insoluble even in concentrated ammonia (2).
Related dot points
- Redox titrations with manganate(VII) and thiosulfate-iodine, constructing redox equations from half-equations, disproportionation, and using titration data to calculate amounts and concentrations.
An Eduqas A-Level Chemistry PI1.2 answer on redox titrations with manganate(VII) and iodine-thiosulfate, building redox equations from half-equations, disproportionation, and redox titration calculations.
- The characteristic properties of transition metals, variable oxidation states, complex-ion formation and shapes, coloured ions and the d-d transition, ligand substitution, and catalysis.
An Eduqas A-Level Chemistry PI2.2 answer on the characteristic properties of transition metals, variable oxidation states, complex ions and their shapes, the origin of colour, ligand substitution and catalysis.
- Periodicity of atomic radius, ionisation energy and melting temperature across Periods 2 and 3, the s, p and d blocks, and the trends explained by electronic structure and nuclear charge.
An Eduqas A-Level Chemistry C1.6 answer on periodicity across Periods 2 and 3: trends in atomic radius, ionisation energy and melting temperature, the s, p and d blocks, and their explanation from electronic structure.
- The four types of crystalline solid (ionic, simple molecular, giant covalent and metallic), their structures, and how structure and bonding explain physical properties.
An Eduqas A-Level Chemistry C1.5 answer on ionic, simple molecular, giant covalent and metallic crystalline solids and how their structure and bonding explain melting point, hardness, solubility and conductivity.
Sources & how we know this
- WJEC Eduqas GCE A Level Chemistry specification (from 2015) — WJEC Eduqas (2015)