Drop a strip of one metal into a solution of another metal’s compound and one of two things happens. Either the strip goes dark and the solution fades, or nothing happens for as long as you are prepared to watch. Both outcomes are informative, and neither is obvious in advance.
You have four metals and four solutions, which is sixteen combinations. By the end of the period the class produces a single ranked list, and the argument about where the ties go is the good part.
What you are trying to find out
When a metal and a dissolved metal ion are in the same beaker, they are in competition for the same thing: electrons. One of them ends up as the solid and the other ends up dissolved, and which is which appears to be decided by the identity of the metals rather than by the amounts, the temperature, or how hard you stir.
If that is true — and it is your job to test whether it is — then the metals can be put in a single order that predicts every one of the sixteen combinations from four pieces of information instead of sixteen. That is an extraordinary claim for a table to make. Test it.
The second question follows immediately: where does hydrogen sit in that order? Dilute acid supplies hydrogen ions, so a metal that displaces hydrogen from acid outranks it and a metal that does not, does not. One extra column places a non-metal in a list of metals.
What you have to work with
- Small pieces of magnesium ribbon, zinc, iron, and copper, cleaned.
- Solutions at 0.20 mol/L: magnesium sulfate, zinc sulfate, iron(II) sulfate, copper(II) sulfate.
- Dilute hydrochloric acid, 1.0 mol/L.
- A spot plate with at least twenty wells, on a tray. Fine emery paper. A stopwatch. A hand lens.
Your design decisions, written down and checked before you mix anything:
- The layout of the grid, and where your controls go. Four metals against four solutions leaves four cells where a metal meets its own ion. Those cells are not wasted — decide what you expect from them and what it would mean if you got something else.
- How much solution per well, held constant across the grid, and why holding it constant matters given the question you are asking.
- How long “no reaction” takes to establish. Pick a number of minutes before you start and apply it to every cell, including the ones you are bored of.
- Whether you clean the metal surfaces, and what you are changing when you do. Argue it out; there is a real answer and it is not obvious.
- What you will record. A colour change in the solution, a deposit on the metal, a change in the metal’s appearance, gas, warmth — decide in advance which of these count as evidence of a reaction and which do not.
Dilute acid, hydrogen gas, and copper salts down no sink
- The acid is 1.0 mol/L hydrochloric acid, the school dilution. Two or three drops per well. Eye protection on throughout, including during cleanup, which is when most splashes happen.
- Metals in acid release hydrogen, which burns. No burners, no hot plates, no flames anywhere in the room for this period. The quantities here are tiny and the rule is still absolute.
- Magnesium in acid is vigorous and the well will warm. Use a piece no longer than a centimetre.
- Copper(II) sulfate is harmful if swallowed and is toxic to aquatic life. Every solution from this lab goes to the labelled waste container. Nothing goes down the sink. Wash your hands before you leave, whether or not you think you touched any.
- Iron(II) sulfate solution stains, skin and clothing both, and the stain darkens over the following day.
- Emery paper leaves metal dust. Wipe the bench, and do not brush it off with your hand.
- Never return unused solution to the stock bottle — a returned dropper contaminates the whole bottle and the next class inherits your mistake.
- Acid on skin: running water, several minutes, and tell me. In an eye: eyewash immediately and somebody else fetches me.
Before the first drop goes in, run this out loud with your partner:
- Eye protection on, hair back, sleeves secured
- Eyewash, shower, extinguisher, blanket, and spill kit located by pointing at each
- Grid drawn on paper, cells labelled, before anything is dispensed
- Predictions written and dated
- Spot plate flat on a tray, not on a book, not near the edge
- One dropper per solution, and it never enters a second bottle
- Waste container open and within reach
- No flame lit anywhere in the room
The prediction you write first
In your journal, before you mix anything:
- Fill in the whole grid with predictions. Sixteen cells, each one reaction or no reaction. Do not leave any blank because you are unsure — an uncertain prediction that turns out right is worth knowing about, and one that turns out wrong is worth more.
- Your predicted ranking of the four metals, most reactive first, with your reason.
- Where you expect hydrogen to fall in that ranking.
- One sentence: which single cell will decide your ranking, if the others come out as you expect.
What to collect
The grid. Rows are the metal you add; columns are the solution it goes into. In each cell record what you saw and how long it took, with the resolution of your timer noted at the top of the page.
| Metal added | in | in | in | in | in dilute |
|---|---|---|---|---|---|
| Magnesium | |||||
| Zinc | |||||
| Iron | |||||
| Copper |
Then the second table, which is the actual result of the experiment:
| Rank (most reactive first) | Metal | The cell that establishes it is above the next one |
|---|---|---|
| 1 | ||
| 2 | ||
| 3 | ||
| 4 |
Every rank needs a specific cell behind it. “Zinc is above copper because zinc is more reactive” is circular. “Zinc is above copper because zinc in copper(II) sulfate darkened within 40 s and copper in zinc sulfate showed nothing in 10 min” is evidence.
The halogen demonstration
You watch this one; you do not run it. Chlorine water is handled by me, in the fume hood, and it stays there. Chlorine is a respiratory irritant, and this is not a reagent that belongs on a student bench.
Added to a solution of a bromide and to a solution of an iodide, it produces visible colour changes. Record what you see, and then answer one question: does the same kind of ordering argument work for the halogens as for the metals, and what would you have to see to say it does not?
| Solution | Colour before | Colour after chlorine water | What you infer |
|---|---|---|---|
| Potassium bromide | |||
| Potassium iodide |
What to bring to the consolidation discussion
- Your grid, complete, with times and with the negatives written in.
- Your ranking, with the deciding cell named for each step.
- The count. How many of your sixteen predictions were right, as a number. Not a feeling.
- Whether one order really does predict all sixteen cells. Test it: take your ranking, generate all sixteen predictions from it alone, and compare with your grid. Report any cell where the list fails.
- Where you placed hydrogen, and the two cells that bracket it.
The order the class agrees on gets written up as The Activity Series so that everyone is working from the same list afterwards. It is your list, not mine, and if it disagrees with the published one we will spend the time finding out why.
What you should not claim
- “No reaction” is a statement about your waiting time. You watched for a set number of minutes with the naked eye. A reaction slower than that, or one that produces a change too faint to see against a coloured solution, is indistinguishable from no reaction by this method. Say “no visible change in 10 min”.
- A darkening strip is not necessarily deposited metal. Something changed on the surface. You have not isolated it, massed it, or tested it, and a surface that tarnishes in air alone would look similar.
- Sixteen cells at one concentration and one temperature do not establish a universal order. They establish an order under these conditions. That the order is more general than that is a claim you will meet evidence for later — it is not a claim today’s data carries.
- Cleaning the metal changes the answer, and it changes it one way. An unpolished strip carries an oxide layer that has to be broken through before the metal underneath can react, so an uncleaned metal looks less reactive than it is, and looks slower than it is. Any cell where you did not clean the surface is biased toward “no reaction”, and any ranking built on such a cell inherits the bias.
- The halogen demonstration is two test tubes. It is suggestive and it is not your data. Report it as something you watched.
Curriculum connection
C2.10
plan and conduct an inquiry to demonstrate a single displacement reaction, using elements from the metal activity series [IP, PR]
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C2.5
predict the products of single displacement reactions, using the metal activity series and the halogen series [AI]
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A1.5
conduct inquiries, controlling relevant variables, adapting or extending procedures as required, and using appropriate materials and equipment safely, accurately, and effectively, to collect observations and data
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