Four Mystery Packets: Which Way Does the Energy Go?

Investigation question: Four unlabeled packets go into water. Which ones release energy to the surroundings, which absorb it, and which do neither, and what evidence tells you?

Scientific purpose

Students measure the temperature of the water before and after each of four teacher-measured packets is added, classify each change as exothermic, endothermic or close to neutral from the data, and then use their evidence to match each packet to a description on a manufacturer's spec sheet. Two of the packets fizz but move the temperature in opposite directions, which separates 'gas is produced' from 'energy is released' and makes the thermometer, not the eye, the deciding instrument.

Why this helps students with this chapter

Students arrive believing that a change which looks energetic (fizzing, foaming) must be giving off energy, and that cold is something a cold pack makes. Measuring four changes with a thermometer, including two that fizz in opposite directions and one that does almost nothing, forces the classification to rest on where the energy went. Having to identify each packet from a spec sheet turns the measurements into evidence for an argument instead of numbers in a table.

Standards connections

MS-PS1-6 (practice, characterizing the chemical processes that a warming or cooling device could use): Before students can design a pack in Lab 2 they need to know which safe processes release thermal energy and which absorb it, and how much. This lab produces that evidence with the same reagents, so it is practice toward the design PE rather than evidence of it.

MS-ETS1-2 (supports, matching measured performance to a written specification): Matching each packet to a spec-sheet description from evidence is a small version of evaluating whether a material meets a stated requirement, which students do fully in Lab 3.

Roles (rotate each session)

• Safety captain and mixer: Checks that everyone's goggles are on, pours the measured water, adds the packet on the timer's signal, and stirs gently with the stick. The only person who touches reagents.
• Timer and caller: Starts the stopwatch the instant the packet goes in and calls out 'read' every 30 seconds for three minutes.
• Thermometer reader: Holds the probe in the middle of the liquid without touching the cup bottom and reads the temperature aloud to one decimal place at each call.
• Recorder: Writes every reading and observation in the table, computes the temperature change, and leads the group's spec-sheet matching.

Materials

Estimated cost: 20 to 35 per class of 8 groups, excluding thermometers (an estimate; Estimate. A 10 lb bag of calcium chloride ice melt (about $15) and a 1 lb jar of citric acid (about $8) supply dozens of classes; baking soda, table salt, a bottle of 3% peroxide and a packet of yeast total under $6; foam cups about $4. Digital kitchen thermometers run $8 to $12 each if the room has none.)

Safety

• Goggles on from the first packet to the last rinse. Citric acid, calcium chloride and hydrogen peroxide are all eye irritants.
• Only the safety captain touches reagents, and with dry hands. Calcium chloride pellets warm when wet and can irritate skin; if any touches skin, rinse with water for a minute.
• 3% hydrogen peroxide can sting cuts and bleach fabric. Keep it in the cup, on the tray. The yeast foam may overflow the cup; that is why the tray is there.
• Never cap or seal any cup. Stations B and D make gas; a sealed container can burst.
• Nothing is tasted, nothing is sniffed up close, and no cup is held near the face. Hands are washed at the end.
• The amounts are chosen so that no station should pass about 40 °C. If a cup feels hot, set it on the tray and tell the teacher; do not pick it up.
• Disposal: all four solutions are drain-safe in these amounts. Pour them down the sink with running water; cups and sticks go in the trash.
• No flames, no hot plates, no sealed bags. This activity has not been physically tested by the authors; pilot it before class.

Teacher preparation and setup

1. Weigh the packets the day before on a kitchen or lab balance: 3 g calcium chloride, 3 g citric acid plus 3 g baking soda mixed, 3 g salt, and about 1 g yeast per group. Fold each in paper or put it in a small labeled cup. Keep calcium chloride sealed; it pulls water from the air and clumps.
2. Pilot station A and station D yourself with your actual products. If 3 g of your calcium chloride in 50 mL warms the water less than 5 °C, raise packet A to 5 g. If the peroxide and yeast warm less than 3 °C in three minutes, use fresh peroxide (it loses strength once opened) and a fresh yeast packet.
3. Pour 30 mL of 3% hydrogen peroxide into each cup D just before class and keep the cups on the trays; do not let students pour it.
4. Print the spec sheet (it is on the recording sheet) and the data table; set a pitcher of water out the night before so it is at room temperature.
5. Write the three-minute reading schedule on the board: 0, 30, 60, 90, 120, 150, 180 seconds.

Investigation design

Student procedure

1. Put on goggles. Set the four labeled cups on the tray. Read the spec sheet on your recording sheet so you know the four fillings you will be trying to identify.
2. The safety captain measures 50 mL of room-temperature water into cups A, B and C. Cup D already holds 30 mL of a clear liquid that is not plain water; do not add water to it.
3. For station A: the thermometer reader places the probe in the water, waits ten seconds, and reads the starting temperature. The recorder writes it in the 0 s row.
4. The timer says 'go'; the safety captain tips packet A into the water and stirs gently twice. The timer calls 'read' every 30 seconds for three minutes while the reader reads and the recorder writes. Everyone also notes what they see and hear: fizzing, foam, color change, pellets disappearing.
5. Rinse and dry the probe. Repeat steps 3 and 4 for station B (add packet B to the water) and station C (add packet C).
6. For station D: read the starting temperature of the liquid in cup D, then add packet D (the yeast) on 'go' and record for three minutes. Expect foam; keep the cup on the tray.
7. Compute the temperature change for each station (final minus starting; a drop is a negative number). Classify each station as exothermic, endothermic or close to neutral and write the evidence.
8. Plot all four stations on the one graph, temperature against time, with a different symbol for each station. Label each line with its letter.
9. Match each station to a spec-sheet filling (1 to 4). Write the evidence for each match. Then answer the analysis questions individually and compare within the group.

Expected observations

• Station A: pellets disappear within a minute; the water warms steadily, typically by 7 to 10 °C with anhydrous pellets (less with hydrated ice-melt products), peaking by about 90 seconds; no gas.
• Station B: immediate fizzing and a hiss; the water cools, typically by 3 to 6 °C, most of it in the first minute; the fizzing slows as the baking soda is used up.
• Station C: salt dissolves slowly with no fizzing; temperature changes by about 1 °C or less, usually a slight drop (3 g of salt can cool 50 mL of water by up to about 0.9 °C).
• Station D: foam builds over a minute and may overflow; the liquid warms by several degrees over three minutes, more slowly than station A, because the breakdown continues as long as peroxide and enzyme remain in contact.

Likely difficulties

• The probe touches the cup bottom or sits in foam at station D and reads the wrong thing; coach readers to hold the probe mid-liquid and below the foam.
• Groups read station A too early and miss the peak, or read station B too late and see the water already warming back toward room temperature; keep to the 30-second schedule and use the 180 s reading as 'final' for everyone.
• Students classify by how the cup feels rather than by the numbers; require a number in the evidence column.
• The negative sign on station B's change confuses some students; model 'final minus start' on the board with a drop.
• Old peroxide or dead yeast makes station D flat; pilot with your actual products and replace them if the rise is under 3 °C.

Structured debrief

1. Pool every group's temperature change for each station on the board. Ask what the class agrees on: A and D warm, B cools, C barely moves. Point out that the spread within a station (a few degrees) is much smaller than the differences between stations, which is why the classification holds.
2. Put stations B and D side by side: both fizzed, opposite directions. Ask students to state the rule they will carry into Lab 2: gas tells you matter left the system; only the thermometer (or light and sound) tells you which way energy went.
3. Reveal the identities: A is calcium chloride, which dissolves exothermically; B is citric acid and baking soda, a genuine reaction that makes carbon dioxide and absorbs energy; C is table salt, which dissolves with almost no energy change; D is hydrogen peroxide breaking down into water and oxygen, sped up by an enzyme in the yeast. Note that A and C are dissolving processes, not reactions in the strict sense, while B and D form new substances.
4. Return to the cold-pack question from the slides: at station B the water cooled first, then the cup, then the hand. Energy moved from hand to cup to water to the reacting system. No cold moved anywhere.
5. Preview Lab 2: the hot-pack teams will use filling A and the cold-pack teams filling B, with criteria for temperature and time, so today's data are their starting point.

Recording sheet: what students fill in

Lab 1 recording sheet: Four mystery packets

You are a quality tester at a company that makes warming and cooling packs. Four candidate fillings arrived without labels. Measure what each one does in water, then use your evidence to work out which is which.

Analysis questions: answer key

Individual evidence of learning

Each student submits their own completed data table, observation and classification table, graph, boundary drawings and written answers to q1 through q9. The classification table and q4 are the primary evidence for reading energy direction from data; the boundary drawings and q7 are the evidence for the system-and-surroundings model.

Assessment criteria

Accessibility adaptations

• Assign the recorder or timer role to students who should not handle reagents; the lab works with one pair of hands on the cups.
• Use a thermometer with a large digital display or a talking thermometer; allow readings to be called to a partner who records.
• Provide a data table with the 0 s row and the sign rule ('final minus start; a drop is negative') pre-printed, and a graph with the axes already scaled.
• Offer a bilingual card for exothermic, endothermic, system, surroundings and evidence, and accept q4 and q7 as recorded spoken answers.
• Students with a chemical sensitivity or respiratory condition can sit away from station D's foam and complete the classification from the group's data.

Low-cost alternative

Run the four stations as a single teacher demonstration under a document camera with one probe thermometer, with students calling the times and recording the readings. Table salt and baking soda alone (without citric acid) give a smaller but still measurable cooling for station B if citric acid is unavailable; vinegar (30 mL in place of water) with 3 g baking soda also cools by a few degrees.

Model limitations

A cup of water is a stand-in for a pack: the pack's own water, plastic and your hand are the real surroundings, and the cup loses energy to the air faster than a sealed pouch would, so the measured changes are smaller than a pack would show. Stations A and C are dissolving processes rather than chemical reactions in the strict sense, which the debrief says plainly. The temperature change depends on the amounts, so '+9 °C' describes this cup, not calcium chloride in general. This activity has not been physically tested by the authors; the expected changes are estimates from the reagents' known behavior and should be piloted.

Teacher notes

Expect the classification to be unanimous even when the sizes vary; that agreement is the lesson. If a group's station B barely cooled, the baking soda was probably old or the water warm; the sign still should be negative. For q6, some students will write 'yeast' as a reactant at D; use it to introduce the word catalyst without assessing it. Keep the pooled class table for Lab 2, where the hot-pack teams start from station A's result and the cold-pack teams from station B's.