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Where Does the Copper Go? Etching Circuit Boards at a Makerspace

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Where Does the Copper Go? Etching Circuit Boards at a Makerspace

Use mass data and an atom model to follow copper atoms from a circuit board into a liquid and back.

Etching the badges

Read the passage and study Table 1. Then answer the questions.

Blinking Badges for the Library Fair

1The teen club at the Riverside Community Makerspace builds a project for the city library's fair every year. This year, members Sakura Ellison and Dmitri Volkov designed name badges with two blinking lights. A badge needs a small circuit board: a thin, stiff sheet of plastic and glass fibers with a layer of copper metal on one side. Electric current flows through narrow copper lines, called traces, that connect the lights, the battery and a tiny chip.

2To make the traces, Sakura drew the circuit on each board with a special marker whose ink protects the copper under it. Then the makerspace's lab manager, Mr. Teodoro Balangue, wearing goggles and gloves, placed the boards one at a time in a covered tray of brown etching liquid on a scale in a vented corner. The students watched from behind a clear screen. The liquid is iron chloride dissolved in water. Wherever copper was not covered by ink, it slowly disappeared from the board, and after about 20 minutes only the inked traces were left. Mr. Balangue then rinsed off the ink.

3Mr. Balangue explained the reaction with this model: Cu + 2 FeCl₃ → CuCl₂ + 2 FeCl₂. A copper atom (Cu) from the board reacts with iron chloride (FeCl₃). The products, copper chloride (CuCl₂) and a different iron chloride (FeCl₂), both stay dissolved in the liquid. No gas forms. Dmitri weighed each inked board just before it went into the tray and again after it was lifted out, drained over the tray and patted dry, before the ink was rinsed off. Mr. Balangue read the scale under the covered tray before each board went in and after it came out.

4Copper dissolved in water can harm fish and other water life, so the makerspace never pours used etching liquid down the drain. Instead, it saves the used liquid from a whole month in a covered bucket. Then Mr. Balangue drops in pads of clean steel wool, which is mostly iron. Over a few days, the steel wool gets thinner, and a reddish-brown powder of copper metal collects at the bottom of the bucket. The model for this reaction is Fe + CuCl₂ → FeCl₂ + Cu. Last month, the boards etched at the makerspace lost a total of 24.0 g. After the powder was rinsed and dried, it had a mass of 23.6 g. Some copper powder stayed stuck to the steel wool and the bucket.

Table 1. Masses Before and After Etching (one tray for all three boards)
BoardBoard before (g)Board after (g)Tray before (g)Tray after (g)
Badge A9.859.10852.30853.05
Badge B9.809.40853.05853.45
Large sign26.1524.85853.45854.75
1.
Use Table 1.

During etching, the Badge A board lost g, and the tray of liquid gained g.

2.
Which explanation of the mass the boards lost fits the model in paragraph 3 best?
Question 2 options
3.
Use the large sign in Table 1. Add the mass of the board and the mass of the tray.

Total mass before etching: g. Total mass after etching: g.

4.
Sakura claims that no copper was destroyed during etching. Which result from Table 1 is the best evidence for her claim?
Question 4 options
5.
Why was it important that the etching reaction forms no gas and that the tray was covered?
Question 5 options

Counting the atoms

Use the etching model in paragraph 3: Cu + 2 FeCl₃ → CuCl₂ + 2 FeCl₂. Each FeCl₃ has 1 iron atom and 3 chlorine atoms.

6.

The reactants hold chlorine atoms, and the products hold chlorine atoms. Counting every kind, each side of the model has atoms.

7.
Dmitri builds the products with beads. He joins 1 copper bead to 2 chlorine beads. Then he makes 2 more groups, each with 1 iron bead joined to 3 chlorine beads. What is wrong with his product model?
Question 7 options
8.
Think about one copper atom that is etched off a board and later collected in the bucket. Sort each place by whether the copper atom is in it before etching, after etching, or after the steel wool is added.

Getting the copper back

Use paragraph 4 and your answers above.

9.

Last month, the dried copper powder had a mass g less than the total mass the boards lost.

10.
Which is the most reasonable explanation of the difference you found in question 9?
Question 10 options
11.
Use the model Fe + CuCl₂ → FeCl₂ + Cu. Why does the steel wool get thinner in the bucket?
Question 11 options
12.
A visitor at the library fair says, “The etching liquid destroyed the copper. You can see it vanish from the board, and the liquid still just looks brown.” Do you agree? Write a claim, use evidence from Table 1 and paragraph 4, and use the atom model in paragraph 3 to explain what happened to the copper atoms.