UNIT 2 CHEMISTRY · PEARSON AAQ · C1.3.2
Period 3 Reactions with Water
FREE CHEMISTRY REVISION · TOPIC 24
Distinguish the reaction equation from what you actually observe.
Compare sodium, magnesium and aluminium with liquid water, explain protective surface layers, and retain Pearson’s specified hydroxide products.
Pearson BTEC Level 3 AAQ Applied Science
Unit 2: Principles and Applications of Chemistry · C1.3.2
What you will learn
Describe the trends and observations for Period 3 elements reacting with water, focusing on the products NaOH, Mg(OH)₂ and Al(OH)₃ specified in C1.3.2. Link observations to reaction rate, distinguish an alkaline solution from an insoluble white solid, and write balanced hydroxide-forming equations with the correct conditions.
This page concerns the elements with water. The reactions of Period 3 chlorides with water belong to a later specification point. Steam reactions can produce different products; do not substitute those products for the hydroxides required here.
1. A metal–water reaction: hydroxide and hydrogen
In the hydroxide-forming model, a metal reacts with liquid water to form a metal hydroxide and hydrogen. The metal is oxidised: its oxidation number increases from zero. Hydrogen in water is reduced from +1 to 0 in H₂. Oxygen remains −2.
The formula of the hydroxide depends on the metal’s charge. Each OH⁻ group has charge −1, so Na⁺ requires one OH⁻, Mg²⁺ requires two and Al³⁺ requires three. Write NaOH, Mg(OH)₂ and Al(OH)₃. Brackets show that the subscript applies to the entire hydroxide group.
Observation is different from explanation
“Bubbles form” is an observation; “hydrogen is produced” identifies the gas. “The reaction slows” is an observation; “a surface layer restricts water reaching fresh metal” is an explanation. Give both when the question asks you to describe and explain.
The comparisons below describe controlled demonstrations. They are revision explanations rather than instructions for carrying out these reactions.
2. Sodium with cold water
Sodium reacts rapidly with cold water. A small piece typically floats, fizzes and moves over the surface. Heat released can melt the metal into a silvery ball; the ball becomes smaller as sodium is consumed. The hydrogen can sometimes ignite with a yellow/orange flame, but ignition is not needed for the reaction to occur.
Sodium hydroxide dissolves, giving a colourless alkaline solution. A suitable indicator shows the alkalinity, but the solution is not intrinsically purple or pink: the colour depends on the indicator. Do not describe NaOH as a white precipitate in this comparison.
2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)
Each Na changes 0 → +1. Two sodium atoms lose two electrons in total, matching the formation of one H₂ molecule. The presence of OH⁻ in the solution explains its alkalinity. The rapid fizzing is evidence of gas evolution, not evidence that water is merely boiling away.
3. Magnesium with liquid water
Magnesium reacts much more slowly with cold liquid water. A few small hydrogen bubbles may form on its surface; over a short observation time the reaction can appear slight. Warmer liquid water increases the rate, although the metal’s surface condition also matters.
The specified hydroxide is magnesium hydroxide, Mg(OH)₂, a white, sparingly soluble solid. A white coating or cloudiness can appear as the reaction proceeds. Only a small amount dissolves, so the liquid becomes alkaline without all the hydroxide disappearing into solution.
Mg(s) + 2H₂O(l) → Mg(OH)₂(s) + H₂(g)
Magnesium changes 0 → +2. Its existing oxide film and the accumulating hydroxide layer restrict further contact with water. This helps explain why a simple prediction from metal reactivity alone does not give the observed rate.
Liquid water is not steam
Hot steam gives the familiar magnesium oxide reaction, rather than the hydroxide-forming liquid-water equation above. For C1.3.2, keep Mg(OH)₂ as the specified product and name the condition. Do not label a steam equation as formation of magnesium hydroxide.
4. Aluminium: the importance of passivation
Ordinary aluminium in cold water generally shows no obvious visible reaction. The exposed metal quickly acquires a thin, adherent oxide film. This film is a barrier between the water and the metal underneath: this protection is called passivation.
Aluminium can react with water when the protective barrier is effectively disrupted under suitable conditions. The hydroxide-forming overall equation, required to connect with the specified Al(OH)₃ product, is:
2Al(s) + 6H₂O(l) → 2Al(OH)₃(s) + 3H₂(g)
Conditional model: water must reach reactive aluminium; not the usual observation for intact aluminium in cold water.
Aluminium hydroxide is a white, poorly soluble solid. Aluminium changes 0 → +3; two Al atoms supply six electrons in the formal redox accounting, corresponding to three H₂ molecules. The balanced equation describes what can form, but it does not demonstrate that the reaction is fast.
Do not assume that briefly scratching the metal guarantees a sustained reaction: the protective film can reform. Reactions in strongly alkaline solution may produce dissolved aluminate species instead, so they must not be presented as an ordinary water-only route to a white Al(OH)₃ product.
5. Compare the observations and products
| Element | Typical cold-water observation | Specified hydroxide | Essential qualification |
|---|---|---|---|
| Na | Rapid fizzing and movement; metal consumed | NaOH(aq), colourless alkaline solution | Hydrogen may ignite; indicator colour is not the colour of NaOH |
| Mg | Slow/slight bubbling; white coating may develop | Mg(OH)₂(s), sparingly soluble white solid | Surface films slow reaction; warmer liquid water reacts faster |
| Al | Usually no visible reaction with an intact film | Al(OH)₃(s) in the conditional water-reaction model | Oxide film must be effectively disrupted; equation alone does not establish rate |
For the ordinary cold-water comparison, the observed reaction becomes much less vigorous from Na to Mg to passivated Al. Across these metals, forming a positive ion involves removal of increasing numbers of electrons; however, surface films, activation barriers, temperature and surface area are crucial to the observed rate. Do not infer a smooth numerical rate trend from oxidation numbers alone.
Silicon, phosphorus and sulfur do not undergo an analogous obvious reaction with cold water under ordinary comparison conditions. Chlorine can react with water and argon is inert, but chlorine’s water products are not additional products to learn for this hydroxide-focused specification point. A lack of visible reaction is not proof that an element has no possible chemistry with water under any conditions.
6. Original particle model: dissolved and solid hydroxides
7. Four worked examples
Example 1 — balance the sodium equation
Question: Complete Na + H₂O → NaOH + H₂.
Step 1: One H₂ molecule contains two H atoms. Use 2H₂O so that two H atoms can enter H₂ while two remain in hydroxide groups.
Step 2: The two O atoms require 2NaOH, and hence 2Na. Answer: 2Na + 2H₂O → 2NaOH + H₂. Atom totals on each side are Na = 2, O = 2 and H = 4.
Example 2 — identify magnesium’s product
Question: A metal reacts slowly with liquid water and develops a white, poorly soluble coating. Explain the observation for magnesium.
Step 1: Mg forms Mg²⁺, so two OH⁻ groups are needed: Mg(OH)₂.
Step 2: Write Mg + 2H₂O → Mg(OH)₂ + H₂. The gas explains bubbling, while the white solid explains coating/cloudiness. Its surface layer slows access of water to fresh metal; the hydroxide is not entirely dissolved.
Example 3 — reconcile aluminium’s equation and observation
Question: Aluminium can form Al(OH)₃ with water. Why does an ordinary aluminium piece show no obvious reaction in cold water?
Step 1: The equation 2Al + 6H₂O → 2Al(OH)₃ + 3H₂ is balanced for hydroxide formation.
Step 2: That equation does not give the reaction rate. An adherent oxide film restricts contact between water and the metal. Answer: passivation suppresses the visible reaction; Al(OH)₃ formation requires conditions that overcome that barrier.
Example 4 — compare hydrogen using coefficients
Question: In their hydroxide-forming equations, how many H₂ molecules correspond to two Na atoms, two Mg atoms and two Al atoms?
Step 1: 2Na gives 1H₂. Doubling Mg + 2H₂O → Mg(OH)₂ + H₂ gives 2Mg → 2H₂.
Step 2: 2Al → 3H₂ in the conditional equation. Answer: 1, 2 and 3 H₂ molecules respectively. This is a stoichiometric comparison, not a claim that aluminium reacts most quickly.
Common misconceptions
- “A balanced aluminium equation proves rapid cold-water reaction.” Equations describe stoichiometry; the oxide film controls access and rate.
- “All metal hydroxides dissolve.” NaOH dissolves readily; Mg(OH)₂ and Al(OH)₃ are poorly soluble.
- “MgOH₂ is the same as Mg(OH)₂.” The correct formula needs brackets to show two OH groups.
- “Fizzing means oxygen is produced.” The gas in these hydroxide-forming metal–water reactions is hydrogen.
- “A white coating means all reaction has instantly stopped.” A layer can slow further reaction; the observed rate depends on its effectiveness and the conditions.
Exam tips
- Write the condition: cold liquid water, warmer liquid water or a conditional activated aluminium surface.
- Give visible observations before naming products and explanations.
- Use (aq) for sodium hydroxide solution and (s) for the poorly soluble hydroxides in this model.
- Check both O and H totals after balancing. Keep H₂ as a diatomic gas.
- For trends, distinguish reaction rate from the amount of hydrogen predicted by coefficients.
Check your understanding
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