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Drop a small piece of potassium into water and it skitters across the surface in a burst of lilac flame; leave a gold ring in that same water for a hundred years and it stays untouched. Metals differ enormously in how readily they react, and chemists capture that whole spread of behaviour in one ordered list called the reactivity series. This list ranks metals from the most reactive at the top down to the least reactive at the bottom, and once you know the order you can start to predict how any metal will behave rather than having to remember each case separately. This lesson opens Topic C4 (Predicting and identifying reactions and products) of OCR Gateway Combined Science A by building that series from things you can actually watch happen on the bench — the reactions of metals with water, with dilute acid and with oxygen.
By the end of this lesson you should be able to recall the order of the reactivity series, describe the reactions of metals with water, dilute acid and oxygen, write word and balanced symbol equations for those reactions, and use how vigorous and how fast a reaction is to place an unknown metal in the series.
This lesson builds AO1 recall of the reactivity series and the reactions of metals, AO2 application when you write balanced symbol equations for those reactions, and AO3 analysis when you place an unknown metal in the series from how vigorously and how fast it reacts.
The reactivity series is simply a list of metals put in order of how readily they take part in reactions — the most reactive at the top and the least reactive at the bottom. The version you need to know includes two non-metals, carbon and hydrogen, which act as useful marker points within the list:
The reason the two non-metals earn a place is that each marks a boundary you will keep coming back to. A metal's position relative to hydrogen tells you whether it reacts with dilute acid, and its position relative to carbon is used elsewhere in the course when deciding how a metal can be obtained from its ore. In this lesson the one we lean on is the hydrogen line.
Exam Tip: A handy mnemonic for the order is "Please Stop Calling Me A Careless Zebra, Instead Call Someone Good" — Potassium, Sodium, Calcium, Magnesium, Aluminium, (Carbon), Zinc, Iron, (Hydrogen), Copper, Silver, Gold. Learn it until it is automatic, because almost every prediction in C4 rests on it.
How a metal behaves with water — and how violently — is one direct measure of its reactivity, and the pattern maps neatly onto the series.
metal+water→metal hydroxide+hydrogen
For sodium, the balanced equation is:
2Na+2H2O→2NaOH+H2
metal+steam→metal oxide+hydrogen
For magnesium, this is Mg+H2O→MgO+H2.
What you actually see grades smoothly as you move down the series. Potassium reacts so fiercely with cold water that the hydrogen released ignites with a lilac flame while the metal shoots across the surface; sodium fizzes quickly and melts into a ball that darts about; lithium fizzes steadily but more calmly; calcium sinks and gives off a steady stream of bubbles. The lower down the series, the gentler the reaction.
| Metal | Reaction with cold water | What you see |
|---|---|---|
| Potassium | Violent | Ignites with a lilac flame, moves rapidly, may spark |
| Sodium | Vigorous | Fizzes fast, melts into a ball, darts across the surface |
| Lithium | Steady | Fizzes gently, floats and moves slowly |
| Calcium | Moderate | Sinks, steady stream of bubbles, cloudy mixture |
| Magnesium | Very slow in cold water (reacts with steam) | Few bubbles in cold water |
| Copper, silver, gold | No reaction | Nothing happens |
Exam Tip: A frequent misconception is that any metal reacting with water gives a metal oxide. Be precise: with cold water the product is a hydroxide, and only with steam is it the oxide. Writing an oxide for the cold-water reaction of sodium is an easy mark to throw away.
Most metals above hydrogen in the series react with dilute acid (such as hydrochloric or sulfuric acid) to give a salt and hydrogen gas:
metal+acid→salt+hydrogen
For magnesium with hydrochloric acid:
Mg+2HCl→MgCl2+H2
The rate of fizzing is a direct guide to reactivity: the more reactive the metal, the faster bubbles of hydrogen stream off and the more quickly the metal disappears into solution. Because the acid supplies hydrogen ions and hydrogen sits partway down the series, the acid effectively asks each metal, "are you reactive enough to push hydrogen out?"
| Metal | Reaction with dilute acid | Rate of fizzing |
|---|---|---|
| Magnesium | Vigorous | Very fast — rapid stream of bubbles |
| Zinc | Moderate | Steady fizzing |
| Iron | Slow | Slow bubbles, especially when cold |
| Copper, silver, gold | No reaction | None |
The very reactive metals such as potassium and sodium react with dilute acid dangerously fast — even more violently than with water — so these reactions are not carried out in school laboratories. At the other end, the metals below hydrogen — copper, silver and gold — do not react with dilute acid at all, because they are less reactive than the hydrogen the acid would have to release.
Exam Tip: Metal + acid always gives salt + hydrogen — never water, and never carbon dioxide (that comes from carbonates). A metal below hydrogen (copper, silver, gold) will not react with dilute acid; stating that on its own can earn a mark.
A third clue to reactivity is how readily a metal reacts with oxygen to form an oxide:
metal+oxygen→metal oxide
The pattern follows the series exactly. Very reactive metals such as sodium and potassium tarnish almost the moment they are exposed to air and have to be stored under oil to keep oxygen away. Magnesium burns in oxygen with a brilliant white flame to give magnesium oxide:
2Mg+O2→2MgO
Iron reacts slowly, gradually rusting over days and weeks, while gold does not react with oxygen at all — which is exactly why it never tarnishes and keeps its shine. The speed and vigour of a metal's reaction with oxygen is therefore yet another way of ranking it: the faster and brighter it reacts, the more reactive it is.
Exam Tip: Reaction with oxygen follows the same order as reaction with water and with acid. All three tests — oxygen, water and acid — put the metals in the same reactivity order, so you can use whichever evidence a question gives you.
Four metals W, X, Y and Z are each added to separate samples of dilute hydrochloric acid. W fizzes very rapidly; X gives slow bubbles; Y shows no reaction at all; Z fizzes steadily but less than W. Place the metals in order of reactivity, most reactive first.
Step 1 — the rate of fizzing measures reactivity: faster fizzing means a more reactive metal.
Step 2 — rank by what was seen: W (very rapid) is most reactive; Z (steady) comes next; X (slow bubbles) is below Z; Y (no reaction) is least reactive and is probably below hydrogen.
Step 3 — write the order: W, Z, X, Y.
Answer: most reactive to least reactive is W → Z → X → Y.
Write a word equation and a balanced symbol equation for the reaction of lithium with cold water.
Step 1 — lithium is a very reactive metal, so it reacts with cold water to give a metal hydroxide + hydrogen.
Step 2 — word equation: lithium+water→lithium hydroxide+hydrogen.
Step 3 — write the formulae (Li, H2O, LiOH, H2) and balance. The H2 needs two hydrogen atoms, so use two LiOH, and then two Li and two H2O:
2Li+2H2O→2LiOH+H2
Step 4 — check: Li 2 = 2; H 4 = 4 (four on the left in two waters; two in the hydroxides plus two in H2 on the right); O 2 = 2. Balanced.
Write the balanced symbol equation for zinc reacting with dilute sulfuric acid.
Step 1 — metal + acid → salt + hydrogen. Sulfuric acid gives a sulfate, so the salt is zinc sulfate, ZnSO4.
Step 2 — write the equation: Zn+H2SO4→ZnSO4+H2.
Step 3 — check: Zn 1 = 1; H 2 = 2; S 1 = 1; O 4 = 4. Already balanced.
Answer: Zn+H2SO4→ZnSO4+H2.
Predict whether (a) iron and (b) copper will react with dilute hydrochloric acid, and explain each.
Step 1 — find each metal relative to hydrogen. Iron is above hydrogen; copper is below hydrogen.
Step 2 — apply the rule: a metal above hydrogen reacts with acid (giving salt + hydrogen); a metal below hydrogen does not.
Step 3 — conclude: (a) iron reacts (slowly), giving iron(II) chloride and hydrogen, Fe+2HCl→FeCl2+H2; (b) copper does not react, because it is less reactive than hydrogen.
Answer: iron reacts; copper does not.
Exam Tip: To decide whether a metal reacts with dilute acid, ask one question — is it above hydrogen? If yes, it reacts (salt + hydrogen); if no (copper, silver, gold), there is no reaction.
A common practical task in this topic is to work out an order of reactivity by adding equal-sized pieces of different metals to the same acid and comparing what happens. To make the comparison fair, only one thing should change — the metal itself — so everything else must be kept the same: the same acid at the same concentration and volume, pieces of metal of the same size and surface area, and the same starting temperature. You might then judge reactivity by how fast the bubbles come off, or by timing how long a fixed-size piece takes to disappear, or by measuring the temperature rise. The more reactive the metal, the faster and more vigorous the reaction. Thinking carefully about which variables to control is exactly the kind of practical reasoning the combined-science papers reward.
Exam Tip: In a "compare the reactivity" experiment, name the control variables (same acid, same concentration and volume, same size/surface area of metal, same temperature). Saying only "keep it fair" is too vague — state what you keep the same.
| Misconception | The correct idea |
|---|---|
| "All metals react with acid" | Metals below hydrogen (copper, silver, gold) do not react with dilute acid |
| "Gold is reactive because it is precious and valuable" | Gold is the least reactive common metal — that lack of reactivity is exactly why it stays shiny and is found as the native element |
| "Metal + water always gives a metal oxide" | With cold water the product is a hydroxide; only with steam is it the oxide |
| "Metal + acid gives salt + water" | Metal + acid gives salt + hydrogen; salt + water comes from acid + base |
| "A faster reaction just means a hotter mixture" | With the same acid, faster fizzing means the metal is more reactive — the rate is the evidence for the order |
Question (6 marks): A student adds equal-sized pieces of three metals — magnesium, copper and iron — first to cold water and then to dilute hydrochloric acid, recording what they see each time. Use the idea of reactivity to predict and explain the observations, and place the three metals in order of reactivity.
Mid-band response: "Magnesium is the most reactive, then iron, then copper. Magnesium fizzes the most in acid and copper does nothing. So the order is magnesium, iron, copper."
Examiner-style commentary: The order is right and the key observation (copper does nothing) is used. To climb a band, describe what each metal does in both water and acid, and explain the link between the rate of fizzing and reactivity rather than simply stating the order.
Stronger response: "In cold water, magnesium reacts only very slowly and iron and copper do not react at all. In dilute acid, magnesium fizzes rapidly giving off hydrogen, iron fizzes slowly, and copper shows no reaction because it is below hydrogen. The faster the fizzing, the more reactive the metal, so the order is magnesium (most reactive), then iron, then copper (least reactive)."
Examiner-style commentary: A strong, well-evidenced answer that uses observations from both tests and links rate to reactivity. To reach the top band, name the products (salt + hydrogen) and explain why copper does not react, using its position below hydrogen.
Top-band response: "With cold water, magnesium reacts only very slowly (a few bubbles), while iron and copper show no visible reaction — none is reactive enough to react quickly with cold water. With dilute hydrochloric acid, magnesium fizzes rapidly, giving off hydrogen and forming magnesium chloride (Mg+2HCl→MgCl2+H2); iron fizzes slowly, forming iron(II) chloride and hydrogen; and copper shows no reaction at all because it lies below hydrogen in the reactivity series and so cannot push hydrogen out of the acid. Because the rate of fizzing is a direct measure of reactivity, the order from most to least reactive is magnesium → iron → copper, which matches their positions in the reactivity series."
Examiner-style commentary: Full marks. It describes both tests, names products with an equation, explains the absence of reaction for copper by its position below hydrogen, and justifies the order using the rate of reaction — a complete, evidence-based deduction.
This content is aligned with OCR Gateway Combined Science A (J250), Topic C4 Predicting and identifying reactions and products. Refer to the official OCR specification for exact wording.