Lighter Cup, Heavier Pad: Where Did the Mass Go? · Chapter 13

Learning target: I can observe two reactions whose balance readings change in opposite directions, record what I notice and wonder, and write a first explanation that says where mass could have come from or gone.

Directions

Read the two demonstrations and study the figure. The numbers are a teaching dataset written to be realistic; your teacher may show the real demonstrations instead. Write what you notice before you try to explain anything. Then answer the questions in complete sentences. There are no wrong first explanations on this sheet; the goal is to write one you can test later.

You will need: This worksheet, Pencil

Two demonstrations, two balances

Demonstration 1: chalk in vinegar. A plastic cup of vinegar sits on a balance that reads to 0.01 g. The cup and vinegar together read 86.40 g. A 2.00 g piece of white chalk made of calcium carbonate, broken into small lumps, is dropped in, so the balance should read 88.40 g. The chalk fizzes at once. Tiny bubbles stream up and pop at the surface for about twenty minutes. When the fizzing stops, no chalk is left and the balance reads 87.52 g. The cup is open to the air the whole time.

Demonstration 2: a copper pad in the heat. A new copper scouring pad is shiny and orange-pink. On the balance it reads 2.40 g. The teacher sets it on a very hot plate for ten minutes, using tongs. The pad slowly turns dull, then black. After it cools, it is placed back on the balance: 2.58 g. Nothing was added to the pad by hand. It sat in ordinary air the whole time.

Teaching data: the readings are invented to be realistic for a 2.00 g piece of chalk and a small copper pad. They are not measurements from a real classroom.

Part A: Notice and wonder

1.Write three things you notice in the demonstrations or the figure. Stick to observations: things you could see or read on the balance, not explanations.

2.Write two things you wonder. Start each one with 'I wonder' and make it something an experiment could answer.

Part B: What you already know

3.From the last lesson, what evidence tells you that a chemical reaction (not just a melt or a mix) happened in each demonstration? Name one piece of evidence for each.

4.Calculate the change in reading for each demonstration and write it with a sign (+ or −). Then write one sentence saying why the two changes are surprising when you put them side by side.

Part C: Your first explanation

5.Write your first explanation. Where could the 0.88 g that 'left' the cup have gone? Where could the 0.18 g that 'joined' the copper pad have come from? Use the word atoms at least once, and say whether you think any matter was created or destroyed.

6.Suppose Demonstration 1 were repeated inside a tightly sealed bottle instead of an open cup, with the whole bottle on the balance. Predict the reading after the fizzing stops and explain your prediction in one sentence.

Sentence starters for the explanation

If q5 is hard to start, use these frames. 'The reading in the cup went down because something ______ the cup. I think it was ______.' 'The reading for the pad went up because something ______ the pad. I think it was ______.' 'I think matter was / was not created or destroyed because ______.' Remember that air is matter even though you cannot see it.

Design the sealed test

Sketch an experiment that would test your q6 prediction fairly. Name the container, say what goes on the balance, list two things you would keep the same between the open and sealed versions, and state exactly what reading would support your explanation and what reading would force you to change it. Then explain one reason a sealed container could still give a slightly different reading even if your explanation is right.