Worksheet

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

Grade 7 physical science: analyze board and tray masses from etching circuit boards and count atoms to show that mass is conserved as copper is moved, not destroyed.

Grade 7ScienceInteractive · Printable

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1 NGSS standard
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12 questions
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Details

Format: Interactive · Printable
Grade: Grade 7
Subject: Science
Question types: Fill in the blank, Multiple choice, Sorting, Short answer

About this worksheet

What students practice

Students analyze mass data to show that the total mass of a system stays the same during a chemical reaction.
Students count atoms in a reaction model to explain that atoms are rearranged, not created or destroyed.
Students trace the atoms of one element through two reactions.

How to use

Read paragraphs 1–3 together and have students say where each copper atom is before and after etching. Students can answer the rest on their own; the last question works as a written assessment.

Curriculum

NGSS standard covered

MS-PS1-5

Model how atoms are rearranged but not created or destroyed in a chemical reaction, so total mass is conserved.

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Name Date

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)
Board	Board before (g)	Board after (g)	Tray before (g)	Tray after (g)
Badge A	9.85	9.10	852.30	853.05
Badge B	9.80	9.40	853.05	853.45
Large sign	26.15	24.85	853.45	854.75

Use Table 1.

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

Which explanation of the mass the boards lost fits the model in paragraph 3 best?

A. Copper atoms left the board and became part of copper chloride in the liquid.
B. The liquid broke the copper atoms into much lighter pieces of copper.
C. The copper atoms changed into iron atoms, which are lighter than copper atoms.
D. The copper turned into a gas that stayed trapped under the tray's cover.

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.

Sakura claims that no copper was destroyed during etching. Which result from Table 1 is the best evidence for her claim?

A. The large sign lost more mass than either badge did.
B. For each board, the tray gained what the board lost.
C. Each board weighed less after etching than before.
D. The tray got a little heavier after each board went in.

Why was it important that the etching reaction forms no gas and that the tray was covered?

A. So that the scale could weigh the boards in the liquid
B. So that no atoms could leave the tray without being weighed
C. So that the copper would dissolve faster in the warm, covered liquid
D. So that the brown liquid would stay the same color

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.

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

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?

A. It has 8 chlorine beads, but the reactants had 6, and atoms are not created.
B. It has too few copper beads, because the copper should be split between the groups.
C. It has the right number of beads, but the iron beads should be joined to the copper bead.
D. It has 2 iron beads, but the iron should have been used up during the reaction.

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.

In the reddish-brown powder at the bottom of the bucketIn the copper layer on the boardIn copper chloride dissolved in the etching liquid

Before etching	After etching	After the steel wool is added

Getting the copper back

Use paragraph 4 and your answers above.

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?

A. Drying the powder turned some of the copper into heat, which left the powder.
B. Some copper atoms were destroyed in the second reaction, so they could not be collected.
C. Some copper turned into iron in the bucket, so the copper powder weighed a little less.
D. Some copper powder stayed stuck to the steel wool and bucket, so it was not weighed.

11.

Use the model Fe + CuCl₂ → FeCl₂ + Cu. Why does the steel wool get thinner in the bucket?

A. The liquid turns the iron atoms of the wool into the copper atoms of the powder.
B. The iron atoms in the wool break apart into the chlorine atoms of the liquid.
C. Iron atoms from the wool join chlorine atoms and go into the liquid as FeCl₂.
D. The copper powder grows on the wool and wears it away as the powder sinks.

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.