Metals, Reactivity and the Rock Cycle
The reactivity series ranks metals by how vigorously they react with water, acid and oxygen. A common order from most to least reactive is potassium, sodium, calcium, magnesium, aluminium, zinc, iron, copper, silver and gold. A more reactive metal will displace a less reactive one from its compound, which is how displacement reactions and extraction work: metals below carbon in the series can be extracted by heating their oxide with carbon, while more reactive metals need electrolysis. Corrosion is a reaction between a metal and its surroundings; rusting is the specific corrosion of iron, and it needs both oxygen and water. The rock cycle describes how the three rock types change into one another. Sedimentary rocks form when fragments settle in layers and are compacted and cemented, and they may contain fossils. Metamorphic rocks form when existing rock is changed by heat and pressure without melting. Igneous rocks form when molten rock cools and solidifies; slow cooling underground gives large crystals and fast cooling at the surface gives small ones.
Method
- For a reactivity question, first place both metals in the series. The one higher up is more reactive and takes the place of the one lower down.
- Write a displacement word equation as metal plus compound goes to new compound plus displaced metal, and check that the more reactive metal ends up in the compound.
- Predict observations, not just outcomes: a colour change in the solution, a coating forming on the metal, a temperature rise, or no reaction at all if the added metal is less reactive.
- For extraction, compare the metal with carbon. Above carbon means electrolysis; below carbon means reduction by heating with carbon.
- For rusting experiments, remember that the control needs both air and water; a boiled and sealed water tube removes oxygen and a tube of dry air with a drying agent removes water. Each missing condition prevents rust.
- For the rock cycle, describe the process rather than naming the rock only: fragments transported and deposited then compacted, or heat and pressure without melting, or molten rock cooling and crystallising.
Worked example
A student adds a strip of zinc to blue copper sulfate solution. Predict and explain what happens, write the word equation, and state what would happen if copper were added to zinc sulfate solution instead.
- Place both metals in the reactivity series. Zinc is above copper, so zinc is the more reactive metal.
- The more reactive metal takes the place of the less reactive one in the compound, so zinc displaces copper.
- Predict the observations: the blue colour of the solution fades because copper sulfate is being used up, and a red-brown coating of copper builds up on the zinc strip. The mixture also warms up.
- Write the word equation: zinc + copper sulfate goes to zinc sulfate + copper.
- Now reverse the pair. Copper is below zinc, so it is less reactive and cannot take zinc's place in the compound.
- State the outcome: no reaction. The solution stays colourless and the copper strip is unchanged.
Practice questions
Try each question, then tap to reveal the answer.
Q1Place these metals in order from most to least reactive: iron, magnesium, copper, calcium.Show answer
Answer: Calcium, magnesium, iron, copper.
Q2Write a word equation for the reaction between magnesium and iron oxide, and explain why it works.Show answer
Answer: Magnesium + iron oxide goes to magnesium oxide + iron. Magnesium is more reactive than iron, so it displaces iron from its oxide.
Q3Explain why aluminium is extracted by electrolysis but iron is extracted using carbon in a blast furnace.Show answer
Answer: Aluminium is above carbon in the reactivity series, so carbon cannot displace it from its ore and electrolysis is needed. Iron is below carbon, so heating iron oxide with carbon reduces it to iron, which is cheaper.
Q4State the two substances needed for iron to rust, and name one method of preventing it.Show answer
Answer: Oxygen and water. Prevention: painting, oiling or greasing, galvanising with zinc, or coating with plastic.
Q5Give one way to tell a sedimentary rock from an igneous rock by looking at it.Show answer
Answer: Sedimentary rock shows layers and may contain fossils or rounded fragments. Igneous rock is made of interlocking crystals with no layers and no fossils.
Q6Two igneous rocks form from the same magma, one underground and one at the surface. Explain the difference in their crystals.Show answer
Answer: The rock that cooled slowly underground has large crystals, because the particles had time to arrange themselves into big regular structures. The rock that cooled quickly at the surface has small crystals.
Q7Explain why fossils are found in sedimentary rock but not in metamorphic rock formed from it.Show answer
Answer: Fossils form as remains are buried in layers of sediment that are then compacted. When that rock is later changed by heat and pressure into metamorphic rock, the structure is deformed and recrystallised, so the fossils are destroyed.
Exam-style questions
Written in the style of a KS3 Science exam paper, with a full mark scheme.
A student sets up four test tubes, each containing a clean iron nail. Tube 1: tap water and air. Tube 2: boiled water with a layer of oil on top. Tube 3: dry air with a drying agent, stoppered. Tube 4: salty water and air. After a week only tubes 1 and 4 show rust, and the nail in tube 4 has rusted most. (a) Explain the results in tubes 2 and 3. (b) Explain what tube 4 shows. (c) Suggest why the experiment was left for a week rather than an hour.
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Describe how a piece of sedimentary rock could be changed into an igneous rock and then back into a sedimentary rock, naming each process in the rock cycle.
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Free printable worksheet
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