Learning target: I can graph and calculate from balance readings taken during a reaction to show that mass only leaves or joins a system when matter crosses its boundary, and that a closed system keeps its mass.
Both parts use teaching datasets: the numbers were written to be realistic for these amounts, not measured in a real classroom. Part A follows the vinegar and baking soda reaction on a balance, once in an open cup and once in a capped bottle. Part B follows steel wool pads of different sizes as they burn. Graph with a ruler, show every calculation with units, and write answers in complete sentences. When a question asks you to explain, use the words atoms and boundary.
You will need: This worksheet, Pencil, Ruler, Calculator (optional)
A teacher set up two runs of the vinegar and baking soda reaction. Open cup: an empty cup (100.00 g) with 60.00 g of vinegar sat on a balance reading to 0.01 g; at time 0 she tipped in a 2.00 g packet of baking soda. Capped bottle: a rigid 2-liter plastic bottle held 60.00 g of vinegar and a small open vial with 2.00 g of baking soda standing upright inside; she screwed the cap on tight, put the whole bottle on a second balance, and at time 0 tipped it to mix. Both mixtures fizzed. The bottle stayed capped for 180 seconds; then she unscrewed the cap, squeezed the bottle gently a few times to push out the gas, left the cap off, and read the balance again at 300 seconds: 164.52 g.
| Time (s) | Open cup (g) | Capped bottle (g) |
|---|---|---|
| 0 | 162.00 | 165.50 |
| 30 | 161.42 | 165.50 |
| 60 | 161.18 | 165.50 |
| 90 | 161.08 | 165.50 |
| 120 | 161.03 | 165.50 |
| 150 | 161.01 | 165.50 |
| 180 | 161.00 | 165.50 |
In a teacher demonstration (behind a shield; students never handle flame), four pads of fine steel wool of different masses were heated until they glowed, allowed to go dark and cool, and then every crumb was returned to the balance. In a separate described experiment, a 2.00 g pad was sealed inside a glass flask of air, the whole flask was weighed (250.00 g), the pad was heated through the glass until it darkened, and the sealed flask was weighed again: 250.00 g. When the stopper was then removed, air hissed in and the flask read 250.14 g.
| Pad | Mass before heating (g) | Mass after cooling (g) |
|---|---|---|
| A | 1.00 | 1.07 |
| B | 2.00 | 2.15 |
| C | 3.00 | 3.21 |
| D | 4.00 | 4.29 |
| Mass gained (g) | Gain per gram of pad (g/g) |
|---|---|
| Pad A | |
| Pad B | |
| Pad C | |
| Pad D | |
| Average |
Before plotting, run your finger down the open-cup column and write the drop between each pair of readings in the margin: 0.58, 0.24, 0.10, 0.05, 0.02, 0.01. The drops get smaller, so your curve should be steep at first and nearly flat by the end. The bottle column never changes, so its line is flat. For the rate in q2, rate = mass lost ÷ time, and the unit is grams per second. In Part B, 'gain per gram' means divide the gain by the starting mass, so a 2.00 g pad that gains 0.15 g has 0.15 ÷ 2.00 = 0.075 g of gain for each gram of pad.
The bottle lost 0.98 g after opening, but the chemistry predicts that 2.00 g of baking soda in this much vinegar should make about 1.05 g of carbon dioxide. Some gas stays dissolved in the liquid, the way carbon dioxide stays in a soda. Design a fair test to find out how much carbon dioxide stayed dissolved: say what you would do to the opened mixture (for example, warm it gently or stir it), what you would measure, and how you would calculate the dissolved amount. Predict the result and explain why stirring a soda makes it go flat.