Related topics: Acids and Bases
Where this sits
One row of the table, sodium to sulfur, and one property changing all the way across it: the oxides start basic, pass through amphoteric in the middle, and end acidic. Everything you need to explain that you already have: the periodicity of the elements themselves, and the structure and bonding that decides whether a solid is a giant lattice or a set of small molecules.
Reactions with oxygen & water
Every Period 3 element from sodium to sulfur burns in oxygen (chlorine and argon do not react with it directly). Two things are examined: the equation, and the observation, and the observation is usually a flame colour or the appearance of the solid, not a vague “it reacts”.
| Element | Reaction with oxygen | Observation |
|---|---|---|
| Sodium | 4Na + O2 → 2Na2O | yellow flame; white solid |
| Magnesium | 2Mg + O2 → 2MgO | brilliant white light; white solid |
| Aluminium | 4Al + 3O2 → 2Al2O3 | white solid |
| Silicon | Si + O2 → SiO2 | white solid |
| Phosphorus | P4 + 5O2 → P4O10 | white flame; clouds of white solid |
| Sulfur | S + O2 → SO2 | blue flame; colourless, choking gas |
The pattern behind the formulae
In its highest oxide each element uses all its outer electrons, so the oxidation state of the element equals its group number: Na(+1) in Na2O, Mg(+2) in MgO, Al(+3) in Al2O3, Si(+4) in SiO2, P(+5) in P4O10, S(+6) in SO3. That is why the formulae look irregular but are actually completely predictable.
Sulfur is the exception worth watching: burning sulfur in air gives SO2, and SO3 is made from it separately in the Contact process. Phosphorus in excess oxygen gives P4O10.
Worked example: balancing a burning equation
Write the equation for phosphorus burning in an excess of oxygen.
Step 1. Get the formulae right first: phosphorus exists as P4 molecules, and in excess oxygen the product is the highest oxide, P4O10, not P2O5, which AQA does not accept.
Step 2. Balance the phosphorus: one P4 gives one P4O10, so the phosphorus is already balanced.
Step 3. Balance the oxygen: the right-hand side needs 10 oxygen atoms, and O2 supplies them two at a time, so five molecules are needed.
P4 + 5O2 → P4O10
Writing 4P + 5O2 → P4O10 is also accepted. The same routine gives 4Na + O2 → 2Na2O and 4Al + 3O2 → 2Al2O3.
Sodium and magnesium with water
The two most reactive metals of the period also react with water, and they behave very differently, which is itself the point.
Sodium reacts vigorously with cold water. It floats, fizzes, melts into a ball and skates across the surface, giving off hydrogen and leaving a strongly alkaline solution:
2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)
Magnesium reacts only very slowly with cold water, forming the sparingly soluble Mg(OH)2 and a solution of about pH 9. With steam it is a different story. It burns with a brilliant white light to give the oxide directly:
Mg(s) + H2O(g) → MgO(s) + H2(g)
Note which product each reaction gives: sodium gives the hydroxide in solution, magnesium with steam gives the oxide as a solid.
Precision points
- Give the observation the question asks for. For magnesium burning that is a white solid or a bright white light; “white precipitate” is not accepted (nothing is precipitating), and neither is “effervescence”.
- Phosphorus is P4 and its highest oxide is P4O10. Equations built from P2O5 are rejected outright, including in later parts that depend on them.
- Burning sulfur gives SO2. Only write SO3 when the question is about the Contact process or tells you sulfur(VI).
- Sodium with water gives NaOH(aq); magnesium with steam gives MgO(s). Swapping the hydroxide and the oxide is the standard error.
- When state symbols are asked for, put them on every species.
Exam questions
Q1
[1 mark]
Give an equation for the reaction between phosphorus and an excess of oxygen.
Show answer
P4 + 5O2 → P4O10
1 mark
4P + 5O2 → P4O10 is also allowed.
Q2
[2 marks]
When a piece of sodium is added to 200 cm3 of water in a large beaker a vigorous reaction occurs. The temperature of the water increases by 25 °C. Give an equation, including state symbols, for the reaction of sodium with water. Suggest why it is dangerous to react a similar piece of sodium with 10 cm3 of water in a boiling tube.
Show answer
2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)
1 mark: ionic equations and multiples are allowed.
The temperature will rise much more (the same heat released into far less water), or the reactants can shoot out of the tube. 1 mark
“The mixture could explode”, “the glass could shatter” and “the hydrogen could ignite / is flammable” are all allowed. Simply saying the reaction is exothermic or vigorous is ignored. That is true in the beaker too, so it does not answer the question.
Q3
[2 marks]
Give an equation for the reaction of magnesium with steam. State one observation made.
Show answer
Mg + H2O → MgO + H2
1 mark
Observation: a white solid or white powder, or a bright white light / white flame. 1 mark
“White precipitate” is not accepted, and neither is “effervescence”.
Source: AQA A-Level Chemistry past papers.
Melting points & bonding
The melting points of the highest oxides climb to a peak and then fall off a cliff. Nothing about the elements themselves explains that: the explanation is entirely a change of structure partway across the period.
| Oxide | Structure & bonding | Melting point |
|---|---|---|
| Na2O | giant ionic lattice | high (~1548 K) |
| MgO | giant ionic lattice | highest (~3125 K) |
| Al2O3 | ionic with covalent character | high (~2345 K) |
| SiO2 | giant covalent (macromolecular) | high (~1883 K) |
| P4O10 | simple molecular | low (~573 K) |
| SO3 | simple molecular | low (~290 K) |
The giant oxides (ionic Na2O, MgO and Al2O3, and macromolecular SiO2) melt very high, because melting means breaking a huge number of strong bonds throughout the structure: electrostatic attractions between ions in the ionic lattices, covalent bonds in silicon dioxide. The simple molecular oxides P4O10, SO3 and SO2 melt low, because melting them only has to overcome the weak van der Waals and dipole–dipole forces between molecules. The strong covalent bonds inside each molecule stay intact and are irrelevant to the melting point.
Why MgO beats Na2O
Both are giant ionic, so the comparison is between lattices. Mg2+ carries twice the charge of Na+ and is smaller, so the attraction between the ions is much stronger and the lattice enthalpy much larger. That takes more energy to break, so MgO has the higher melting point, the highest of the whole set.
Pick a bar to see what actually has to be broken to melt that oxide.
Worked example: the four-mark comparison
Explain why aluminium oxide has a much higher melting point than sulfur trioxide.
Step 1. Name the structure of the first: Al2O3 is a giant ionic lattice.
Step 2. Name the force that has to be broken: there are strong electrostatic attractions between the Al3+ and O2− ions throughout the lattice, and a lot of energy is needed to overcome them.
Step 3. Name the structure of the second: SO3 is simple molecular.
Step 4. Name the force there: only weak van der Waals forces act between the molecules (SO3 is symmetrical, so it is non-polar) and little energy is needed to overcome them.
Four sentences, four marks: structure, force, structure, force. Answers lose marks by describing only one substance, or by talking about “breaking covalent bonds” when melting a molecular solid.
Precision points
- For SiO2 write macromolecular, giant covalent or giant molecular. AQA does not accept “giant” on its own, “giant atomic”, or “giant ionic”.
- Melting a molecular solid breaks intermolecular forces, never the covalent bonds inside the molecules. Saying you break covalent bonds in P4O10 contradicts the mark you are trying to earn.
- Name the force, don’t just say “stronger bonds”: electrostatic attraction between ions, covalent bonds, van der Waals forces, dipole–dipole forces.
- A comparison question needs both substances described. Half an answer scores half the marks even if it is perfectly correct.
Exam questions
Q4
[4 marks]
Explain, in terms of crystal structure and bonding, why silicon(IV) oxide has a higher melting point than phosphorus(V) oxide.
Show answer
SiO2 is macromolecular / giant covalent / a giant molecule. 1 mark
It has strong covalent bonds between the atoms, which need a lot of energy to break. 1 mark
P4O10 is molecular / a simple covalent molecule. 1 mark
Weak van der Waals forces act between the molecules, and they break easily. 1 mark
“Giant”, “giant atomic” and “giant ionic” are all rejected for SiO2.
Q5
[6 marks]
The table shows the melting points of three substances. Explain why the melting points of these substances are different. You should refer to the structure of and bonding in each substance.
| Substance | Melting point / K |
|---|---|
| sodium chloride | 1074 |
| chlorine | 172 |
| hydrogen chloride | 158 |
Show answer
This one is marked by levels of response, not by ticking points. Level 3 (5–6 marks) needs all three stages covered, essentially correct and complete, argued coherently from stage 1 through to stage 3. Level 2 (3–4) covers all three stages with gaps, or two stages well. Level 1 (1–2) is isolated but ordered statements.
Stage 1. Structure. NaCl is a giant ionic lattice; Cl2 and HCl are simple molecular.
Stage 2. The forces that have to be overcome. In NaCl, attractions between positive and negative ions; in Cl2, van der Waals forces; in HCl, dipole–dipole forces.
Stage 3. The comparison. Ionic bonding is far stronger than any intermolecular force, so NaCl melts highest. Between the two molecular substances, Cl2 is the bigger molecule with more electrons than HCl, so its van der Waals forces are stronger overall, which is why chlorine melts above hydrogen chloride despite HCl being the polar one.
That last point is the discriminator, and it is counter-intuitive: a permanent dipole does not automatically beat van der Waals forces. Contradictions cancel the statements they contradict.
Source: AQA A-Level Chemistry past papers.
Reactions with water & the pH trend
Whether an oxide dissolves, and whether the solution comes out alkaline or acidic, follows straight from the bonding. Ionic metal oxides give alkaline solutions. The giant covalent oxide in the middle does nothing at all. Molecular non-metal oxides give acidic solutions.
| Oxide | Reaction with water | Solution (universal indicator) |
|---|---|---|
| Na2O | Na2O + H2O → 2NaOH | pH ~13 |
| MgO | MgO + H2O → Mg(OH)2 (sparingly soluble) | pH ~9 |
| Al2O3 | insoluble, no reaction | pH 7 |
| SiO2 | insoluble, no reaction | pH 7 |
| P4O10 | P4O10 + 6H2O → 4H3PO4 | pH ~1 |
| SO2 | SO2 + H2O → H2SO3 | pH ~2–3 |
| SO3 | SO3 + H2O → H2SO4 | pH ~0–1 |
Why the metal oxides are alkaline
They contain the oxide ion, and the oxide ion is a powerful base. It takes a proton from water:
O2− + H2O → 2OH−
That is the reason for the high pH, and it is the sentence AQA wants. Na2O gives a higher pH than MgO not because it is a stronger base but because NaOH is much more soluble than Mg(OH)2: more hydroxide ions actually make it into solution.
The whole period in one row. Pick a tube for its equation and the reason behind it.
Al2O3 and SiO2 are the two that do not react. Al2O3 is ionic but its lattice is far too strong and its bonding has too much covalent character for water to break up; SiO2 is a giant covalent network. Neither dissolves, so both leave the water at pH 7. That is a positive fact, not a gap: questions ask for it.
The molecular non-metal oxides are the opposite: they react with water to make acids, and the further right you go the stronger the acid and the lower the pH.
Worked example: predicting the pH from the bonding
An oxide of a Period 3 element is a white solid that dissolves in water to give a solution of pH 1. Identify the bonding in the oxide and suggest which oxide it is.
Step 1. Read the pH: pH 1 is strongly acidic, so the oxide reacted with water to form an acid. That rules out every metal oxide.
Step 2. Acidic means non-metal means molecular: the oxide must be simple molecular with covalent bonding.
Step 3. Use the other clue: a white solid rules out SO2 (a colourless gas) and SO3 (a liquid or low-melting solid, and it would give a pH nearer 0). The white solid that dissolves to pH 1 is P4O10, forming phosphoric(V) acid:
P4O10 + 6H2O → 4H3PO4
Work in that order (pH first, then bonding, then the specific oxide) and the reasoning writes itself.
Precision points
- The reason the metal oxides are alkaline is the oxide ion reacting with water to give hydroxide ions. Non-ionic equations are ignored for that mark.
- Na2O beats MgO on pH because of solubility, not basicity. Say that more NaOH dissolves or dissociates, and do not write “molecules” or “atoms” of an ionic solid.
- SO2 + water gives H2SO3, sulfuric(IV) acid. Writing H2SO4 here is not accepted: that comes from SO3.
- Al2O3 and SiO2 are insoluble: pH 7, no reaction. Do not invent an equation for them with water.
- Equations built from P2O5 rather than P4O10 are not accepted anywhere.
Exam questions
Q6
[2 marks]
Sodium oxide forms a solution with a higher pH than magnesium oxide when equal amounts, in moles, of each oxide are added separately to equal volumes of water. State why both oxides form alkaline solutions. Suggest why sodium oxide forms a solution with a higher pH than the solution formed from magnesium oxide.
Show answer
The oxide ions react with water to form hydroxide ions. 1 mark
O2– + H2O → 2OH–
Sodium hydroxide is more soluble than magnesium hydroxide: more of it dissolves or dissociates. 1 mark
Saying sodium oxide is more soluble, or dissociates more, than magnesium oxide is also allowed. All non-ionic equations are ignored for the first mark, and “molecules” or “atoms” is not accepted.
Q7
[2 marks]
Give an equation for the reaction of phosphorus(V) oxide with water. Suggest a pH for the solution formed.
Show answer
P4O10 + 6H2O → 4H3PO4
1 mark: ionic equations are allowed.
pH: any value from −1 to +1. 1 mark
An equation written from P2O5 is not allowed.
Q8
[1 mark]
Give an equation for the reaction between sulfur dioxide and water.
Show answer
SO2 + H2O → H2SO3
1 mark
The dissociated forms are equally acceptable: SO2 + H2O → 2H+ + SO32−, or SO2 + H2O → H+ + HSO3−. H2SO4 is not accepted.
Source: AQA A-Level Chemistry past papers.
Acid–base character & reactions
The pH trend is really a trend in acid–base character, and that is the form the equations questions take. Basic oxides neutralise acids. Acidic oxides neutralise bases. And exactly one oxide in the middle does both.
| Character | Oxides | Reacts with… |
|---|---|---|
| Basic | Na2O, MgO | acids |
| Amphoteric | Al2O3 | both acids and bases |
| Acidic | SiO2, P4O10, SO2, SO3 | bases |
A basic oxide plus an acid gives a salt and water, like any neutralisation:
3MgO + 2H3PO4 → Mg3(PO4)2 + 3H2O
An acidic oxide plus a base does the same job from the other side:
P4O10 + 12NaOH → 4Na3PO4 + 6H2O
The amphoteric oxide
Al2O3 is amphoteric: it reacts with both acids and bases. That single fact is the reason aluminium sits where it does in this story, and “reacts with both an acid and a base” is the description AQA is looking for.
Al2O3 + 3H2SO4 → Al2(SO4)3 + 3H2O
Al2O3 + 2NaOH + 3H2O → 2NaAl(OH)4
Worked example: building a neutralisation equation
Write the equation for an excess of magnesium oxide reacting with phosphoric acid, H3PO4.
Step 1. Identify the salt: the metal is Mg2+, the acid gives PO43−. Balancing the charges needs three Mg2+ to two PO43−, so the salt is Mg3(PO4)2.
Step 2. The other product is always water for an oxide + acid.
Step 3. Balance from the salt outwards: Mg3(PO4)2 needs 3 MgO and 2 H3PO4; those bring 6 hydrogens, which make 3 waters.
3MgO + 2H3PO4 → Mg3(PO4)2 + 3H2O
Step 4. Check “excess”: an excess of the oxide means the acid is fully neutralised, so the normal salt is right. Had the acid been in excess you would get an acid salt instead: NaH2PO4 or Na2HPO4 in the sodium case, both of which AQA accepts when the wording allows.
Precision points
- Amphoteric means reacting with both acids and bases. Showing only one reaction does not demonstrate it.
- The aluminate product is written NaAl(OH)4. It only forms with aqueous sodium hydroxide, and the water in that equation is easy to drop. Check the hydrogens balance.
- Metal oxide + acid → salt + water, every time. Getting the salt formula right is the whole battle: balance the ion charges before you balance the equation.
- Watch which reagent is in excess: it decides between a normal salt and an acid salt.
- Mg(OH)2 is a weak, sparingly soluble base: a saturated solution is about pH 9, nowhere near 14. That is what makes it usable as an antacid.
Exam questions
Q9
[1 mark]
Which is not a correct statement about magnesium hydroxide?
Mg(OH)2 is only sparingly soluble, so a saturated solution reaches about pH 9, a long way from 14. That low solubility is exactly why it is safe to swallow as an antacid, so the first statement is correct. Mg(OH)2 written out as H2MgO2 is the same formula with the atoms in alphabetical order, so the third is correct too.
Q10
[1 mark]
Give an equation for the reaction of aluminium oxide with sulfuric acid.
Show answer
Al2O3 + 3H2SO4 → Al2(SO4)3 + 3H2O
1 mark: multiples allowed, state symbols ignored.
Q11
[2 marks]
An element in Period 3 forms an oxide that is insoluble in water. This oxide reacts with sulfuric acid and with aqueous potassium hydroxide. Give the formula for this oxide, and give an equation for the reaction of this oxide with sulfuric acid.
Show answer
Formula: Al2O3 1 mark
Al2O3 + 3H2SO4 → Al2(SO4)3 + 3H2O
1 mark: the ionic form Al2O3 + 6H+ → 2Al3+ + 3H2O is also allowed.
Three clues, one answer: insoluble rules out the sodium and phosphorus oxides, and reacting with both an acid and an alkali is the definition of amphoteric, which in Period 3 means aluminium oxide.
Q12
[1 mark]
Give an equation for the reaction of phosphorus(V) oxide with sodium hydroxide solution.
Show answer
P4O10 + 12NaOH → 4Na3PO4 + 6H2O
1 mark
The acid salts are equally acceptable: P4O10 + 4NaOH + 2H2O → 4NaH2PO4, or P4O10 + 8NaOH → 4Na2HPO4 + 2H2O.
Q13
[1 mark]
Give an equation to show how an excess of magnesium oxide reacts with phosphoric acid (H3PO4).
Show answer
3MgO + 2H3PO4 → Mg3(PO4)2 + 3H2O
1 mark
The ionic form 3MgO + 2H3PO4 → 3Mg2+ + 2PO43− + 3H2O is allowed.
Q14
[1 mark]
Give the equation for the reaction between H3PO4 and an excess of NaOH.
Show answer
H3PO4 + 3NaOH → Na3PO4 + 3H2O
1 mark
An excess of the alkali removes all three acidic protons, so the normal salt is the product here, not an acid salt.
Source: AQA A-Level Chemistry past papers.
The acids & anions formed
AQA asks specifically for the structures of the acids and the anions made when P4O10, SO2 and SO3 react with water. These come up as “draw the displayed formula” questions, where every bond has to be on the page.
How to build any of them
- Put the Period 3 atom in the centre (P or S) and surround it with oxygens.
- An –OH group is an acidic hydrogen: the number of them is the number of protons the acid can donate. H3PO4 has three, H2SO3 and H2SO4 have two.
- Any oxygen not carrying a hydrogen is double bonded to the central atom.
- To get the anion, take the hydrogens off the –OH groups. Each oxygen left behind carries a negative charge, and the total charge equals the number of hydrogens removed.
Telling SO2 and SO3 solutions apart
Both dissolve to give acids, so both turn indicators red, but sulfuric(VI) acid from SO3 is much the stronger, and that difference is testable.
Precision points
- “Displayed formula” means every bond drawn, including the O–H bonds. A condensed formula scores nothing.
- Put a negative charge on each oxygen atom that lost its hydrogen. Drawing the structure inside square brackets with the overall charge outside is also accepted, but never both at once, or the charge is counted twice.
- The anion from SO2 is sulfate(IV), SO32−; from SO3 it is sulfate(VI), SO42−. Mixing them up costs the mark.
- A test that distinguishes them has to give two different observations: state both, and make sure they actually differ.
Exam questions
Q15
[1 mark]
Draw the displayed formula of the molecule formed when phosphorus(V) oxide reacts with water.
Show answer
Phosphoric(V) acid, H3PO4: a central phosphorus with one P=O and three P–O–H groups. It is drawn in the figure above. 1 mark
The mark scheme requires that it must show all bonds, the three O–H bonds included.
Q16
[1 mark]
Give the displayed formula for the anion formed when sulfur trioxide reacts with water.
Show answer
The sulfate(VI) ion, SO42−: a central sulfur with two S=O bonds and two S–O− bonds, drawn in the figure above. 1 mark
The structure with the negative charges shown on two of the oxygens is what AQA prints; a version drawn with an expanded octet is also allowed.
Q17
[3 marks]
Identify a reagent or test that could be used to distinguish between aqueous solutions of sulfur dioxide and sulfur trioxide with the same concentrations. State the observation in each case.
Show answer
Reagent or test: universal indicator. 1 mark
With SO2(aq): orange-red. 1 mark
With SO3(aq): red. 1 mark
Several alternatives score equally. A pH meter: about pH 2–3 for SO2(aq), pH 0–1 for SO3(aq). A named metal carbonate, or Mg, Ca or Zn: slower effervescence with SO2(aq), faster with SO3(aq). Acidified barium chloride: no visible change with SO2(aq), a white precipitate with SO3(aq). An incomplete reagent loses the first mark but the observations are still marked.
Q18
[2 marks]
In the Contact process, sulfur(IV) oxide is converted into sulfur(VI) oxide using vanadium(V) oxide as a catalyst. Give two equations to show how the vanadium(V) oxide acts as a catalyst in this process.
Show answer
V2O5 + SO2 → V2O4 + SO3
1 mark
V2O4 + ½O2 → V2O5
1 mark: multiples allowed.
Only 1 mark in total if both equations are correct but written in the wrong order. The catalyst has to be used in the first step and regenerated in the second. That is what makes it a catalyst. This is the same variable-oxidation-state catalysis you meet with the transition metals.
Source: AQA A-Level Chemistry past papers.
The whole period in one tool
Every table on this page is a slice through the same story. Pick an element below and the tool assembles all of it: the oxide it forms, how it burns, what its structure is, where it melts, what it does in water and where that leaves the pH.
3.2.4 Period 3 elements & their oxides: Quick-reference summary
- Burning in oxygen: 4Na + O2 → 2Na2O · 2Mg + O2 → 2MgO · 4Al + 3O2 → 2Al2O3 · Si + O2 → SiO2 · P4 + 5O2 → P4O10 · S + O2 → SO2. In the highest oxide the oxidation state equals the group number.
- With water: Na gives NaOH(aq) vigorously; Mg is slow in cold water but with steam gives MgO(s) and a bright white light.
- Melting points: giant structures high (MgO highest: 2+ ions, small, strong lattice), molecular oxides low. The cliff between SiO2 and P4O10 is the switch from a giant structure to simple molecules.
- Oxides + water: Na2O → pH 13, MgO → pH 9 (both from O2− + H2O → 2OH−; NaOH is more soluble). Al2O3 and SiO2 are insoluble, pH 7. P4O10 → H3PO4 (pH 1), SO2 → H2SO3 (pH 2–3), SO3 → H2SO4 (pH 0–1).
- Character: Na2O and MgO basic · Al2O3 amphoteric (reacts with acids and bases) · SiO2, P4O10, SO2, SO3 acidic.
- Structures: H3PO4 is P with one P=O and three P–O–H; H2SO3 and H2SO4 are S with –OH groups and the rest double bonded. Remove each H to get PO43−, SO32− and SO42−, with the charge on the oxygens that lost them.