Displacement of metals from salt solutions

Displacement of metals from salt solutions is one of the clearest ways to see a redox reaction happen in a single test tube. It sits early in the Form 5 Redox Equilibrium chapter because it links the reactivity of metals to the idea of electron transfer, and the same reasoning returns later in voltaic cells and corrosion. Work through the method below, then use the Paper 3 section to practise the exact answers the marking scheme rewards.

Aim

To investigate the displacement of a metal from its salt solution by a more electropositive (more reactive) metal, and to explain the change in terms of the transfer of electrons, that is, as a redox reaction.

Apparatus and materials

• Test tubes and a test tube rack
• Measuring cylinder (for the salt solution)
• Spatula and forceps
• Sandpaper (to clean the metal surfaces)
• Strips or pieces of zinc, iron, copper and magnesium
• Solutions of copper(II) sulfate, iron(II) sulfate, zinc sulfate and magnesium sulfate (all of the same concentration)
• Safety goggles and gloves

Procedure

1. Clean a strip of zinc with sandpaper until the surface is shiny, to remove the oxide layer.
2. Using the measuring cylinder, pour about 5 cm³ of copper(II) sulfate solution into a clean test tube.
3. Note the colour of the solution and the appearance of the zinc strip before the reaction.
4. Use forceps to place the cleaned zinc strip into the copper(II) sulfate solution so that it is fully immersed.
5. Leave the test tube undisturbed in the rack for about 20 to 30 minutes.
6. Observe any change on the surface of the metal strip and any change in the colour of the solution.
7. Repeat steps 1 to 6 with other combinations, for example, a copper strip in zinc sulfate solution, an iron strip in copper(II) sulfate solution, and a magnesium strip in iron(II) sulfate solution, using a fresh, clean test tube each time.
8. Record every observation in a results table, pairing each metal with each salt solution.

Expected observations

• When zinc is placed in copper(II) sulfate solution, a reddish-brown solid deposits on the zinc strip and the blue colour of the solution gradually fades. The test tube may feel slightly warm.
• When magnesium is placed in a solution of a less reactive metal salt, a solid deposit again forms on the magnesium and the reaction is usually more vigorous.
• When copper is placed in zinc sulfate solution, there is no visible change: no deposit forms and the colourless solution stays as it is.

Describe what is seen, a deposit forming, a colour fading, warmth, rather than quoting invented numerical values, because Paper 3 rewards accurate qualitative observation.

Inference and conclusion

A more electropositive metal displaces a less electropositive metal from its salt solution. Where zinc displaces copper, the zinc atoms lose electrons and are oxidised to zinc ions, while the copper(II) ions gain those electrons and are reduced to copper atoms, which appear as the brown deposit. Because oxidation and reduction occur together, the change is a redox reaction. Where no change is seen, copper in zinc sulfate, the added metal is less electropositive than the metal already in the salt, so it cannot displace it. Ranking the metals by which displaces which reproduces the electrochemical (reactivity) series: magnesium above zinc, zinc above iron, iron above copper.

Science process skills (Paper 3 style)

• Hypothesis. A suitable hypothesis is: “A metal higher in the electrochemical series displaces a metal that is lower in the series from its salt solution.” State a testable relationship, not just a fact.
• Variables. The manipulated variable is the type of metal added (its position in the reactivity series); the responding variable is whether displacement occurs, judged by the deposit forming and the solution colour fading; the controlled variables are the concentration and volume of the salt solution, the temperature, the surface area of the metal and the reaction time.
• Tabulating data. Build a table with columns for the metal, the salt solution, the observation, and the inference. A tidy table lets you compare pairs at a glance.
• Making an inference. From a brown deposit forming on zinc in copper(II) sulfate, infer that zinc is more electropositive than copper and has displaced it.
• Operational definition. “Metal X is more electropositive than metal Y if X displaces Y from a solution of a salt of Y.” This defines the idea by what you actually observe.

Safety precautions

• Wear safety goggles, because copper(II) sulfate and the other salt solutions are irritant and harmful if splashed into the eyes.
• Handle sandpaper carefully and away from others, so that grit and metal filings do not enter the eyes.
• Do not touch the solutions with bare hands, and wash your hands after the experiment, because the metal salts are harmful.
• Dispose of the used solutions in the labelled waste container rather than down the sink, because they contain heavy-metal ions.

Common errors

• Not cleaning the metal strip first: an oxide layer stops proper contact, so the displacement is slow or does not appear to happen.
• Using solutions of different concentrations across the combinations, which makes the comparison unfair.
• Reading the colour of the solution against a poor background, so the fading of the blue is missed.
• Reusing an unwashed test tube, which contaminates one solution with traces of another and gives a false deposit.
• Not leaving enough time, then wrongly concluding that no reaction occurs when the change is simply still too small to see.

How we help

In our online one-to-one SPM Chemistry lessons, taught in English, our teachers rehearse this experiment as a redox story, which metal loses electrons, which ion gains them, so that you can write the half-changes and the reactivity-series conclusion under exam pressure. Fees start from RM50 per hour, and we offer a paid one-hour trial lesson so you can see how the practical is broken down before you commit. Because displacement, cells and corrosion all rest on the same electron-transfer idea across SPM Chemistry, mastering this practical pays back through the whole Redox Equilibrium chapter and into Paper 3.