How to Determine Relative Reactivity of Metals: A Practical Guide
To determine the relative reactivity of metals, compare them in displacement reactions. Place each metal in solutions containing ions of the other metals. If a solid metal displaces a metal from its solution, it is more reactive. The results establish the order.
Here, reactivity means how readily a metal loses electrons and forms positive ions. In a displacement reaction, the more reactive metal loses electrons, while ions of the less reactive metal gain them and form a solid.
How to run the test
Use clean metal strips and solutions containing soluble salts, such as suitable nitrates or sulfates. Test each strip in solutions of the other metals and record whether displacement occurs. A deposit on the strip or a change in solution colour may provide visible evidence.
For a fair comparison, use fresh solutions and strips with similar exposed surface areas. Clean off surface coatings if needed, label each tube, and allow time for reactions to become visible. Handle chemicals according to your teacher’s or lab’s safety instructions.
The salt must supply ions of the metal you are testing against. For example, a zinc strip in copper sulfate solution can displace copper ions because zinc is more reactive than copper. The solution needs to contain dissolved ions for this comparison to work (Khan Academy).
One example: ranking five metals
Suppose tests with magnesium, zinc, iron, copper, and silver produce the pattern below. A check mark means the strip displaced the metal ions in the solution; a cross means no displacement was observed. This is an example based on the stated observations, not a report of an experiment.
| Strip ↓ / Solution → | Mg²⁺ | Zn²⁺ | Fe²⁺ | Cu²⁺ | Ag⁺ |
|---|---|---|---|---|---|
| Mg | – | ✓ | ✓ | ✓ | ✓ |
| Zn | ✗ | – | ✓ | ✓ | ✓ |
| Fe | ✗ | ✗ | – | ✓ | ✓ |
| Cu | ✗ | ✗ | ✗ | – | ✓ |
| Ag | ✗ | ✗ | ✗ | ✗ | – |
Magnesium displaces ions of all four other metals, so it ranks above them. Zinc displaces iron, copper, and silver, but not magnesium. Iron displaces copper and silver, while copper displaces silver. Silver displaces none of the other metals in this set.
The resulting order is:
Mg > Zn > Fe > Cu > Ag
The table shows pairwise comparisons. You may not need to test every combination if the results establish a clear chain. For instance, if A displaces B and B displaces C, those results place A above B and B above C. Test additional pairs when the available results leave the order uncertain.
Interpreting the results
A reaction in a metal-ion solution means the strip metal is more reactive than the metal in the solution. The reverse pairing should not produce displacement under the same conditions. For example, zinc in copper sulfate can produce copper metal, while copper in zinc sulfate does not displace zinc.
Visible evidence can include a metal coating on the strip or a change in the solution. Colour is useful only when the ions and products in that test have visible colours. A thin deposit or a slow reaction may be difficult to see, so a lack of visible change does not by itself establish that no displacement occurred. If a result seems inconsistent, check that you used the correct solution and that the strip was clean and uncoated. Repeat the test with a fresh sample and keep conditions similar, since concentration, temperature, and surface condition can affect the reaction rate.
Other ways to compare metals
Reactions with dilute acids can provide supporting evidence. Metals above hydrogen in the reactivity series can generally displace hydrogen from dilute acids, while metals below hydrogen generally do not. Bubbling can indicate a reaction, but its rate alone does not establish a precise order.
Water reactions can also distinguish metals, but results depend on the conditions. Some metals react with cold water, some react with steam, and others show little or no reaction with water under ordinary test conditions. Use these observations as supporting evidence, not as the main method for ranking a small set of metals.
Reference tables, such as the Royal Society of Chemistry periodic table and the IUPAC periodic table, can help you check a result. They provide context for known chemical behaviour, while displacement tests show how the selected metals compare in your experiment.
Key Takeaways
- Compare metals by placing each one in solutions containing ions of the others.
- A metal that displaces another metal from its solution is more reactive.
- Use soluble salts, clean strips, and consistent conditions. Check uncertain results before recording them as negative.
- In the worked example, the order is Mg > Zn > Fe > Cu > Ag.
- Acid and water reactions provide supporting evidence; displacement tests provide direct pairwise comparisons.
