How do properties vary across and down the p-block?
Trends in Group 3 to Group 7, the inert pair effect, oxides and chlorides of Period 3, oxidising and reducing behaviour, and tests for ions.
A focused answer to WJEC A-Level Chemistry Unit 3, covering trends across and down the p-block, the inert pair effect, the oxides and chlorides of Period 3, oxidising and reducing behaviour, and qualitative tests for anions.
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What this dot point is asking
WJEC wants you to describe trends across and down the p-block, explain the inert pair effect, classify the oxides and chlorides of Period 3, discuss oxidising and reducing behaviour, and carry out qualitative tests for ions.
The answer
Trends across and down the p-block
The inert pair effect
Period 3 oxides and chlorides
The oxides go from basic (, ) through amphoteric () to acidic (, ). The chlorides change from ionic (dissolving to neutral solutions) to covalent (hydrolysing to acidic solutions, releasing fumes).
Period 3 chlorides
The chlorides change character across the period in step with the bonding. Sodium and magnesium chlorides are ionic solids that dissolve to give roughly neutral solutions. Aluminium chloride is covalent (it sublimes and exists as dimers) and hydrolyses in water to give an acidic solution. Silicon tetrachloride and phosphorus chlorides are simple covalent molecules that hydrolyse vigorously, fuming in moist air as they release hydrogen chloride: . The trend from ionic to covalent mirrors the change from metal to non-metal across the period.
Tests for cations and gases
Beyond the halide tests, you should know that adding sodium hydroxide to a metal-ion solution gives characteristic precipitates: pale green for iron(II), orange-brown for iron(III), and a white precipitate that redissolves in excess for aluminium and zinc (amphoteric hydroxides). Ammonium ions release ammonia gas (turning damp red litmus blue) when warmed with sodium hydroxide. These qualitative tests, combined with the anion tests, let you identify an unknown salt by a short sequence of observations.
Examples in context
Lead in pigments and stability. The inert pair effect explains why lead's stable ion is , underlying lead(II) compounds used historically in paints and glazes. Halide tests in analysis. The silver nitrate then ammonia sequence is the standard qualitative test for chloride, bromide and iodide in water and unknown salts.
Try this
Q1. Classify aluminium oxide as acidic, basic or amphoteric. [1 mark]
- Cue. Amphoteric.
Q2. State the colour of the precipitate when silver nitrate is added to aqueous sodium iodide. [1 mark]
- Cue. Yellow.
Q3. State why the silver(I) ion test for halides is carried out in acidic conditions. [1 mark]
- Cue. Nitric acid removes carbonate and hydroxide ions that would give interfering precipitates.
Q4. Write the equation for the hydrolysis of silicon tetrachloride. [1 mark]
- Cue. .
Q5. State the test result for ammonium ions with warm sodium hydroxide. [1 mark]
- Cue. Ammonia gas is released, turning damp red litmus blue.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC 20194 marksDescribe and explain the trend in the acid-base nature of the Period 3 oxides from sodium oxide to sulfur trioxide when added to water.Show worked answer β
Metal oxides (sodium oxide, magnesium oxide) are basic; they react with water to give alkaline hydroxides, for example .
Across the period the oxides become more acidic. Aluminium oxide is amphoteric (reacts with both acids and bases), while the non-metal oxides such as and are acidic, giving acidic solutions, for example .
The trend from basic to amphoteric to acidic follows the change from metal to non-metal across the period.
Markers reward basic metal oxides, amphoteric aluminium oxide, acidic non-metal oxides, and linking the trend to metallic to non-metallic character.
WJEC 20213 marksDescribe a chemical test, with the expected observation, to distinguish between separate aqueous solutions of sodium chloride and sodium iodide.Show worked answer β
Add dilute nitric acid then aqueous silver nitrate to each solution.
Sodium chloride gives a white precipitate of silver chloride, which dissolves in dilute ammonia.
Sodium iodide gives a yellow precipitate of silver iodide, which is insoluble in ammonia.
Markers reward acidified silver nitrate, the white versus yellow precipitates, and the ammonia solubility difference confirming the identification.
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Sources & how we know this
- WJEC A-level Chemistry specification β WJEC (2015)