Learning target: I can graph temperature data, calculate before-and-after differences in solubility, and use those numbers to decide which cup made a new substance and which only looked like it did.
Both tables are teaching datasets: the numbers were invented to be realistic, not copied from a real trial. Part A asks you to graph temperature and calculate how much each cup warmed or cooled. Part B gives the results of a recovery test on the solids. Show every calculation with units. Write a sign (+ or −) on every change. Answer in complete sentences where lines are given.
You will need: This worksheet, Pencil, Ruler, Calculator (optional)
Mr. Delgado's class repeated the three-bag demonstration in open cups so that a thermometer could stand in each one. Cup A held 2.5 g of baking soda (half as much, so that the vinegar was enough to use all of it up), and 50 mL of white vinegar was poured in. Cup B held 5.0 g of calcium chloride pellets, and 50 mL of water was poured in. Cup C held 5.0 g of Epsom salt, and 50 mL of water was poured in. Every liquid and every cup started at 21.0 °C, and the room stayed at 21 °C the whole time. Cup A fizzed for about two minutes. In Cups B and C the solids disappeared within a minute. The class read each thermometer at the times shown.
| Time (minutes) | Cup A: baking soda + vinegar (°C) | Cup B: calcium chloride + water (°C) | Cup C: Epsom salt + water (°C) |
|---|---|---|---|
| 0 | 21.0 | 21.0 | 21.0 |
| 1 | 18.9 | 31.4 | 19.9 |
| 2 | 17.8 | 36.2 | 19.2 |
| 3 | 17.5 | 37.0 | 19.0 |
| 5 | 17.9 | 35.1 | 19.3 |
| 10 | 19.4 | 30.6 | 20.2 |
The class left each cup uncovered on a warm shelf for three days, until the liquid was gone. They scraped the dry solid out of each cup and tested it beside a fresh sample of the solid that had gone into that cup. Every solubility test was done at 20 °C by stirring the solid into 100 mL of water a little at a time until no more would dissolve. When the class repeated a solubility test on the same powder, the two results were always within 2 g per 100 mL of each other.
| Cup | Solid tested | Appearance | Solubility at 20 °C (g per 100 mL) | Fizzes when vinegar is added? | Odor |
|---|---|---|---|---|---|
| A | Baking soda (before) | white powder | 9.6 | yes, strongly | none |
| A | Solid recovered (after) | white, slightly glassy crust | 121 | no | none |
| B | Calcium chloride (before) | white pellets | 74 | no | none |
| B | Solid recovered (after) | white crust | 75 | no | none |
| C | Epsom salt (before) | clear, colorless crystals | 71 | no | none |
| C | Solid recovered (after) | clear, colorless crystals | 70 | no | none |
A change is always 'after minus before'. If the answer is negative, the cup cooled or the value dropped; if it is positive, the cup warmed or the value rose. For q1, run your finger down each cup's column, mark the reading farthest from 21.0, and subtract 21.0 from it. For q5, Table 2 comes in pairs: the 'after' row sits right under the 'before' row for the same cup, so subtract the upper number from the lower one.
Reference values: table salt (sodium chloride) dissolves 36 g per 100 mL of water at 20 °C and 39 g per 100 mL at 100 °C. Imagine a Cup D with 5.0 g of table salt and 50 mL of water. Predict its two rows of Table 2 (before and after) and its temperature change in Table 1, with a reason for each prediction. Then explain the trap: a student measures the 'before' solubility in 20 °C water and the 'after' solubility in freshly boiled water. What numbers would that student get, what would they wrongly conclude, and how does the 2 g per 100 mL repeat-test spread help you decide whether a 3 g difference is real?