Inorganic Chemistry

Going down Group 2 from magnesium to barium, how do the solubilities of the hydroxides and the sulfates change?

• ABoth hydroxides and sulfates become more soluble
• BHydroxides become more soluble; sulfates become less soluble✓
• CHydroxides become less soluble; sulfates become more soluble
• DBoth hydroxides and sulfates become less soluble

Answer: Group 2 hydroxide solubility increases down the group (Mg(OH)₂ is sparingly soluble, Ba(OH)₂ is much more soluble), whereas sulfate solubility decreases down the group (MgSO₄ is soluble, BaSO₄ is insoluble — the basis of the sulfate test).

Why does barium sulfate, rather than barium chloride or nitrate, make the use of acidified barium chloride a reliable test for sulfate ions?

• ABaSO₄ is a soluble white solid (like BaCl₂)
• BBaSO₄ dissolves to give a bright yellow solution
• CBaSO₄ decomposes on contact with acid
• DBaSO₄ is an insoluble white precipitate✓

Answer: Adding acidified BaCl₂ to a solution containing SO₄²⁻ gives a white precipitate of insoluble BaSO₄. The acid (e.g. dilute HCl or HNO₃) removes carbonate ions that would otherwise also precipitate, making the test specific for sulfate.

When chlorine reacts with cold dilute sodium hydroxide solution, the products include NaCl and NaClO. What does this tell you about the behaviour of chlorine in this reaction?

• AChlorine is only oxidised
• BChlorine is only reduced
• CChlorine undergoes disproportionation✓
• DChlorine is not oxidised or reduced

Answer: Cl₂ + 2NaOH → NaCl + NaClO + H₂O. Chlorine goes from 0 to −1 (in NaCl, reduced) and from 0 to +1 (in NaClO, oxidised) simultaneously. Being both oxidised and reduced in the same reaction is disproportionation.

A few drops of chlorine water are added to a colourless solution of potassium iodide. What is observed, and why?

• ANo change, because Cl₂ is a weaker oxidising agent than I₂
• BA white precipitate forms, because AgI is produced
• CThe solution turns pale green, because I⁻ reduces Cl₂ to Cl⁻ only
• DA brown colour forms, because Cl₂ oxidises I⁻ to I₂✓

Answer: Chlorine is a stronger oxidising agent than iodine, so it displaces iodine: Cl₂ + 2I⁻ → 2Cl⁻ + I₂. The liberated iodine gives a brown (yellow-brown) colour in solution.

Concentrated sulfuric acid is added to solid sodium halides. With which halide is the reducing power of the halide ion great enough to reduce sulfur from +6 in H₂SO₄ all the way to H₂S?

• ANaF
• BNaCl
• CNaBr
• DNaI✓

Answer: Reducing power increases down Group 7. Iodide is the strongest reducing agent of the common halides and can reduce S from +6 (H₂SO₄) to −2 (H₂S). Bromide only reduces it to SO₂ (+4); chloride and fluoride do not reduce sulfur at all (they give HCl/HF).

A solution gives a brick-red colour in a flame test and forms a white precipitate with dilute sulfuric acid that is insoluble. Which cation is present?

• ABa²⁺
• BSr²⁺
• CCa²⁺✓
• DMg²⁺

Answer: A brick-red flame colour is characteristic of calcium, Ca²⁺. Calcium also forms a white, sparingly soluble precipitate of CaSO₄ with sulfuric acid. (Ba²⁺ gives an apple-green flame; Sr²⁺ gives red/crimson; Mg²⁺ gives no characteristic colour.)

Across Period 3, the oxides change in character. Which sequence correctly describes the acid–base nature of Na₂O, Al₂O₃ and SO₃?

• AAcidic, amphoteric, basic
• BBasic, acidic, amphoteric
• CAmphoteric, basic, acidic
• DBasic, amphoteric, acidic✓

Answer: Period 3 oxides change from basic on the left (Na₂O, a metal oxide) through amphoteric (Al₂O₃) to acidic on the right (SO₃, a non-metal oxide). So the order is basic, amphoteric, acidic.

Which equation correctly represents the reaction of phosphorus(V) oxide with water?

• AP₄O₁₀ + 2H₂O → 4HPO₃
• BP₄O₁₀ + 6H₂O → 4H₃PO₄✓
• CP₄O₆ + 6H₂O → 4H₃PO₃
• DP₄O₁₀ + 6H₂O → 4H₃PO₃

Answer: Phosphorus(V) oxide reacts vigorously with water to give phosphoric(V) acid: P₄O₁₀ + 6H₂O → 4H₃PO₄. The oxidation state of phosphorus (+5) is unchanged, consistent with an acid–base (not redox) reaction.

Transition metals and their compounds are characteristically coloured. The colour of a transition-metal complex ion arises because:

• Aelectrons are promoted between split 3d energy levels, absorbing certain visible wavelengths✓
• Bthe complete removal of the 4s electrons releases energy as visible light
• Cthe ligands themselves emit characteristic phosphorescence when illuminated
• Dthe d sub-shell is completely full in all complexes

Answer: Ligands split the 3d orbitals into two energy levels. Electrons absorb visible-light photons of the right energy (ΔE) to be promoted to the higher level; the colour seen is the complementary colour of the light absorbed. A full or empty d sub-shell shows no such d–d transition (e.g. Zn²⁺ is colourless).

Which statement explains why scandium is often not regarded as a typical transition metal, whereas iron is?

• AScandium has no 3d electrons whatsoever in its atoms, so there is no d sub-shell for it to use in its bonding
• BScandium cannot form stable ions of its own, so it never appears as a hydrated complex in solution (like Fe³⁺)
• CSc³⁺, its only common ion, has an empty 3d sub-shell, so it forms no coloured/partly-filled d-orbital ions✓
• DIron has a completely full 3d sub-shell in each of its ions, and that is what gives it its transition properties

Answer: A transition metal forms at least one stable ion with a partially filled d sub-shell. Scandium's only common ion is Sc³⁺ (3d⁰, empty), so it shows none of the characteristic transition-metal properties (variable oxidation states, coloured ions). Iron forms Fe²⁺ (3d⁶) and Fe³⁺ (3d⁵), both partly filled.

When excess concentrated hydrochloric acid is added to an aqueous solution of copper(II) sulfate, a colour change occurs. Which change and product are correct?

• APale blue [Cu(H₂O)₆]²⁺ changes to yellow/green [CuCl₄]²⁻✓
• BPale blue [Cu(H₂O)₆]²⁺ changes to deep blue [Cu(NH₃)₄(H₂O)₂]²⁺
• CColourless [CuCl₄]²⁻ changes to pale blue [Cu(H₂O)₆]²⁺
• DGreen [CuCl₄]²⁻ changes to a white precipitate of CuCl

Answer: Chloride ligands replace water in a ligand substitution: [Cu(H₂O)₆]²⁺ + 4Cl⁻ ⇌ [CuCl₄]²⁻ + 6H₂O. This changes the geometry (octahedral to tetrahedral) and the colour from pale blue to yellow/green. (The deep blue ammine forms with ammonia, not HCl.)

Why does reactivity increase down Group 2?

• AIonisation energies rise as atomic radius increases
• BNuclear charge falls steadily down the group
• CThe elements gain electrons more readily down the group
• DIonisation energies fall as atomic radius increases✓

Answer: Group 2 metals react by losing two electrons, and those electrons are further out and better shielded further down, so less energy is needed. Reactivity with water increases markedly from magnesium to barium.

How does the solubility of Group 2 sulfates change down the group?

• AIt increases down the group, with barium sulfate freely soluble
• BIt stays essentially constant throughout the whole of the group
• CIt decreases, with magnesium sulfate insoluble
• DIt decreases, with barium sulfate essentially insoluble✓

Answer: Sulfate solubility falls down the group while hydroxide solubility rises. The insolubility of barium sulfate is what makes acidified barium chloride the standard test for sulfate ions.

Why does reactivity decrease down Group 7?

• ALarger atoms attract an extra electron less strongly✓
• BLarger atoms attract an extra electron more strongly
• CElectronegativity increases steadily down the group
• DThe halogens become better oxidising agents down the group

Answer: Halogens react by gaining an electron, and with greater radius and more shielding the nucleus holds an incoming electron less tightly. So oxidising power falls from fluorine to iodine.

What is observed when chlorine water is added to potassium bromide solution?

• AThe solution turns purple as iodine is displaced
• BThe solution stays colourless as no reaction occurs
• CA white precipitate forms immediately in the solution
• DThe solution turns orange as bromine is displaced✓

Answer: Chlorine is the stronger oxidising agent, so it takes electrons from bromide ions and bromine is released, colouring the solution orange. Chlorine cannot displace fluorine, which sits above it.

How are silver halide precipitates distinguished?

• ABy colour and their solubility in dilute nitric acid
• BBy colour and their solubility in ammonia solution✓
• CBy their melting points measured after filtration
• DBy the pH of the solution remaining after filtering

Answer: Silver chloride is white and dissolves in dilute ammonia, silver bromide cream and dissolves only in concentrated ammonia, silver iodide yellow and dissolves in neither. Nitric acid is added first to remove interfering ions.

Why is nitric acid added before the silver nitrate in the halide test?

• ATo remove carbonate ions which would also precipitate✓
• BTo remove sulfate ions which would also precipitate
• CTo increase the solubility of the silver halide formed
• DTo convert the halide ions into halogen molecules

Answer: Carbonate ions would give a silver carbonate precipitate and a false positive, so dilute nitric acid is added to destroy them first. Sulfate is dealt with in a separate test using barium chloride.

Why does the melting point rise from sodium to aluminium in Period 3?

• AFewer delocalised electrons and lower ionic charge strengthen bonding
• BThe structure changes from giant to simple molecular
• CThe atomic radius increases steadily across the period
• DMore delocalised electrons and higher ionic charge strengthen bonding✓

Answer: Sodium gives one electron to the sea, magnesium two and aluminium three, and the cation charge rises correspondingly, so metallic bonding strengthens. Silicon then peaks as a giant covalent structure before the melting point collapses at phosphorus.

What is produced when a Group 2 oxide reacts with water?

• AAn acidic solution of the oxide
• BA neutral salt solution and hydrogen
• CAn alkaline hydroxide solution✓
• DA solution of the metal and oxygen gas

Answer: Group 2 oxides are basic and react to give hydroxides, with alkalinity increasing down the group as the hydroxides become more soluble. Magnesium hydroxide is only sparingly soluble, which is why milk of magnesia is a mild antacid.

Why is chlorine added to drinking water despite being toxic?

• AIt removes dissolved metal ions from the water supply
• BIt kills pathogens, and the benefit outweighs the risk✓
• CIt raises the pH of the water to a safer level
• DIt improves the taste and appearance of the water

Answer: Chlorination has all but eliminated waterborne disease such as cholera and typhoid. The quantities used are small, though the possible formation of chlorinated organic compounds is a recognised concern.