Period 3 Elements and Reactions with Oxygen and Water Revision Notes | A Level AQA Chemistry

1. The Big Picture: A Periodic Trend Worth Knowing

Across Period 3, there is a clean, examinable shift: elements on the left form basic oxides, elements on the right form acidic oxides, and aluminium sits awkwardly in the middle as an amphoteric oxide. Once you see this pattern, every reaction in this topic slots into place.

This topic also connects directly to your understanding of bonding. Ionic metal oxides tend to be basic; covalent non-metal oxides tend to be acidic. The periodic trend in oxide character is really a trend in bonding character — and that is the kind of reasoning AQA examiners reward.

The chart above gives you the headline story: pH falls as you move from left to right across Period 3. Notice that Al2O3 and SiO2 are shown near neutral — Al2O3 because it barely reacts with water, and SiO2 because it does not react with water at all (so the pH of water in contact with it remains approximately 7). Note that SiO2 is still classified as a weakly acidic oxide because it reacts with hot concentrated NaOH, even though this is not reflected in the water pH shown here. The real acidic action comes from the non-metal oxides on the right.

2. Reactions of Na and Mg with Water

Sodium reacts vigorously with cold water. You will have seen this in a demonstration: the sodium skims across the surface, melts into a ball, fizzes, and may ignite with a yellow flame. The products are sodium hydroxide and hydrogen gas.

The balanced equation, with state symbols, is: $2Na(s) + 2H_2O(l) \rightarrow 2NaOH(aq) + H_2(g)$. The sodium hydroxide fully dissociates in solution, giving a strongly alkaline solution with a pH of approximately 14.

Magnesium behaves very differently. With cold water, the reaction is extremely slow — you might see a few tiny bubbles over several minutes, and the solution becomes only weakly alkaline. However, magnesium reacts much more readily with steam, producing magnesium oxide and hydrogen: $Mg(s) + H_2O(g) \rightarrow MgO(s) + H_2(g)$.

This contrast between sodium and magnesium is a classic AQA comparison question. Sodium reacts rapidly with cold water; magnesium reacts very slowly with cold water but vigorously with steam. The reason lies in the reactivity series — sodium is higher than magnesium, so it has a greater tendency to lose electrons and react.

One more point on magnesium and water: when magnesium does react slowly with cold water, the equation is $Mg(s) + 2H_2O(l) \rightarrow Mg(OH)_2(aq) + H_2(g)$, giving a slightly alkaline solution (pH around 9–10) because magnesium hydroxide is only sparingly soluble. This is a different product from the steam reaction, so the conditions matter.

3. Reactions of Period 3 Elements with Oxygen

Moving across Period 3, each element reacts with oxygen to form a characteristic oxide. The key oxides you need for AQA are: $Na_2O$, $MgO$, $Al_2O_3$, $SiO_2$, $P_4O_{10}$, $SO_2$, and $SO_3$. Notice that sulphur forms two oxides — both are on the specification.

A few things to notice in that table. First, sodium forms $Na_2O$ rather than $Na_2O_2$ under normal combustion conditions — AQA specifies $Na_2O$ as the product here. Second, phosphorus forms $P_4O_{10}$ because phosphorus exists as $P_4$ molecules; the equation must reflect this. Third, sulphur burns in oxygen to form $SO_2$ directly. $SO_3$ is not produced by simple combustion of sulphur; it requires the catalytic oxidation of $SO_2$ using a vanadium(V) oxide ($V_2O_5$) catalyst under the conditions of the Contact Process.

The trend in bonding across these oxides mirrors the trend in the elements themselves. Metal oxides on the left are ionic; non-metal oxides on the right are covalent. Silicon dioxide is a special case — it is covalent but forms a giant covalent lattice rather than simple molecules, which is why it has a very high melting point and does not react readily with water.

4. Reactions of Period 3 Oxides with Water and the pH of Solutions Formed

This is where the periodic trend becomes most visible. Each oxide has a characteristic reaction (or non-reaction) with water, and the pH of the resulting solution tells you directly whether the oxide is basic, acidic, or neither.

Start with the metal oxides on the left. Sodium oxide reacts vigorously with water to form sodium hydroxide: $Na_2O(s) + H_2O(l) \rightarrow 2NaOH(aq)$. Because NaOH is a strong alkali that fully dissociates, the solution has a pH of approximately 14. This is a strongly basic oxide.

Magnesium oxide reacts slowly with water to form magnesium hydroxide: $MgO(s) + H_2O(l) \rightarrow Mg(OH)_2(aq)$. Magnesium hydroxide is only sparingly soluble, so the solution is weakly alkaline with a pH of around 9. The reaction is much less vigorous than sodium oxide, consistent with magnesium being less reactive than sodium.

Aluminium oxide does not react with water under normal laboratory conditions, so no pH change is observed when it is added to water. However, it is amphoteric: it reacts with acids (behaving as a base) and with alkalis (behaving as an acid). With hydrochloric acid: $Al_2O_3(s) + 6HCl(aq) \rightarrow 2AlCl_3(aq) + 3H_2O(l)$. With sodium hydroxide: $Al_2O_3(s) + 2NaOH(aq) + 3H_2O(l) \rightarrow 2NaAl(OH)_4(aq)$.

Silicon dioxide does not react with water under normal conditions, so the pH of water in contact with SiO2 remains approximately 7. However, SiO2 is classified as a weakly acidic oxide because it does react with hot concentrated sodium hydroxide solution: $SiO_2(s) + 2NaOH(aq) \rightarrow Na_2SiO_3(aq) + H_2O(l)$. The giant covalent lattice with strong Si–O bonds throughout means that water alone cannot break these bonds at room temperature.

Now the non-metal oxides, where the chemistry becomes more acidic. Phosphorus(V) oxide reacts vigorously and exothermically with water to form phosphoric(V) acid: $P_4O_{10}(s) + 6H_2O(l) \rightarrow 4H_3PO_4(aq)$. Phosphoric acid is a weak acid, so the solution has a pH of around 1–2 depending on concentration.

Sulphur dioxide dissolves in water to form sulphurous acid (also called sulfurous acid): $SO_2(g) + H_2O(l) \rightarrow H_2SO_3(aq)$. This is a weak acid, giving a pH of around 2–3. Sulphur trioxide reacts much more vigorously with water to form sulphuric acid: $SO_3(g) + H_2O(l) \rightarrow H_2SO_4(aq)$. Sulphuric acid is a strong acid that fully dissociates, giving a pH close to 1 or below.

Worked example: Predicting the pH and Equation for an Oxide Reacting with Water

Predict the product and approximate pH when SO3(g) is dissolved in water. Write a balanced equation with state symbols.

1. Sulphur is a non-metal in Period 3 (Group 6). Non-metal oxides form acidic solutions. — Position in the period tells you the acid-base character of the oxide before you write any equation.
2. $SO_3$ is sulphur trioxide — sulphur in oxidation state +6, the highest common oxide of sulphur. — Distinguishing SO2 from SO3 matters: SO3 gives a strong acid; SO2 gives a weak acid.
3. $SO_3(g) + H_2O(l) \rightarrow H_2SO_4(aq)$ — Sulphur trioxide reacts with water to form sulphuric acid. Check atom balance: S: 1=1, O: 3+1=4=4, H: 2=2. Balanced.
4. $H_2SO_4$ is a strong diprotic acid. It fully dissociates in dilute solution: $H_2SO_4(aq) \rightarrow 2H^+(aq) + SO_4^{2-}(aq)$ — Full dissociation means a high concentration of H+ ions and therefore a very low pH.
5. pH ≈ 1 (or below 1 for concentrated solutions) — This is the lowest pH of any Period 3 oxide solution — SO3 forms the strongest acid in this series.

Answer: $SO_3(g) + H_2O(l) \rightarrow H_2SO_4(aq)$. The solution is strongly acidic with a pH of approximately 1.

5. The Full Period 3 Oxide Pattern — Bringing It Together

You now have all the individual reactions. The power of this topic is seeing them as one coherent trend. As you move from Na to S across Period 3, the oxides shift from strongly basic through amphoteric to strongly acidic. This is directly linked to the change from metallic to non-metallic character.

Notice the pattern in the table: the two oxides that do not react with water ($Al_2O_3$ and $SiO_2$) sit at the transition between metallic and non-metallic character. This is not a coincidence — it reflects the intermediate nature of their bonding. $Al_2O_3$ has significant ionic character but also covalent character; $SiO_2$ is fully covalent with a robust giant lattice.

6. Why Does the Trend Exist? Bonding and Electronegativity

Understanding the trend at a deeper level will help you answer 'explain' questions confidently. The key is the nature of the E–O bond in each oxide, where E is the Period 3 element.

In ionic metal oxides such as $Na_2O$ and $MgO$, the oxide ion $O^{2-}$ is present. When these dissolve in water, the oxide ion acts as a base and accepts a proton from water: $O^{2-}(aq) + H_2O(l) \rightarrow 2OH^-(aq)$. This is why metal oxides produce alkaline solutions — the oxide ion is a strong base.

In covalent non-metal oxides such as $SO_3$ and $P_4O_{10}$, the E–O bond is polar covalent. When these react with water, the water molecule attacks the electrophilic central atom (S or P), and the O–H bond in water breaks to release $H^+$ ions into solution. This is why non-metal oxides produce acidic solutions.

Aluminium sits at the boundary. Its oxide has enough ionic character to react with acids (the $O^{2-}$ accepts $H^+$) but enough covalent character to react with bases (the Al–O bond can be attacked by $OH^-$). This dual behaviour is the definition of amphoteric character.

7. Exam Technique and Common Mistakes

This topic rewards precision. Examiners distinguish between students who have a vague sense of the trend and those who can write accurate equations, state correct pH values, and explain the bonding reasons. The following guidance targets the most common mark losses.

The long-answer question above models exactly what a 6-mark 'explain' response looks like. Notice the structure: each point links an observation (the oxide type or pH) to a reason (the bonding or ion present). Listing facts without reasons will not reach the top mark band — AQA rewards the 'because' chain throughout.

Worked example: How to Explain Period 3 Oxides in an Exam Answer

1. Identify the oxide and state whether it is ionic, giant covalent, or simple molecular.
2. Link the bonding to oxide character: ionic oxides are basic, aluminium oxide is amphoteric, and the covalent non-metal oxides are acidic.
3. State what happens with water, including the product where relevant and whether the resulting solution is alkaline, acidic, or shows no reaction.

A quick way to check these equations is to count atoms on both sides before using them as evidence for oxide trends. In Period 3, the chemistry is not just about memorising products; it is about linking bonding, reaction with water, and the resulting acid-base behaviour.