Build a Better Pack: Design, Test and Improve a Hot or Cold Pack

Investigation question: Can your team build a pack that warms (or cools) by a set amount within two minutes and is still working at ten minutes, without crossing the safety, mass and cost limits, and what single change should each new prototype make?

Scientific purpose

Each team receives a client brief for either a warming pack (calcium chloride dissolving in water) or a cooling pack (citric acid reacting with baking soda in water), with criteria for temperature at two minutes and at ten minutes and constraints on maximum temperature, amount of reagent, venting and cost. Teams build a first prototype from teacher-measured packets, record its temperature every 30 seconds and then every minute for ten minutes, score it against the criteria and constraints, change exactly one feature (amount of reagent, volume of water, acid-to-soda ratio, or insulation), and test again, for three prototypes in all. The final product is a data-backed justification of the best design and of the one change they would make next.

Why this helps students with this chapter

Students who have only read about the hand-warmer engineers tend to think a design either works or does not. Building a pack that reaches the two-minute target but fades by ten minutes, or that lasts but never gets warm enough, shows that criteria can pull against each other and that a prototype which fails is still a source of data. Having only one feature to change per loop forces the question the lesson keeps asking: what caused the difference? Measuring the water, not the hand, keeps the evidence on the thermometer and keeps the energy language honest: the reagents release energy to the water or absorb it from the water; nothing is created.

Standards connections

MS-PS1-6 (assesses, the whole PE: construct, test and modify a device that releases or absorbs thermal energy by chemical processes): Students build a real device from safe reagents, test it by amount of substance, time and temperature (the PE's assessment boundary), and modify it twice on the basis of their own data. The clarification statement names dissolving calcium chloride as a qualifying chemical process, and the citric acid reaction is a genuine chemical reaction, so both pack types produce evidence for the PE.

MS-ETS1-4 (practice, iterative testing and modification using data from a working prototype): The prototype pack is the model that generates temperature-versus-time data, and each new prototype is modified from that data. The criteria and the test procedure are supplied by the brief rather than developed by students, so this builds toward the PE rather than fully assessing it.

MS-ETS1-1 (supports, distinguishing criteria from constraints): Students restate the brief's criteria and constraints in their own words and write how each will be tested, which supports the PE without asking them to define a problem from scratch.

Roles (rotate each session)

• Design lead: Reads the brief aloud, runs the criteria-and-constraints discussion, makes sure the plan for each prototype names exactly one change and the data reason for it, and keeps the team to the ten-minute test schedule.
• Safety captain and builder: Checks goggles, measures the water, assembles the pack (bag in cup), adds the reagent packet on the timer's signal, and handles every reagent and used bag. The only person who touches chemicals; never picks up the bag by hand once the test has started.
• Timer and reader: Starts the stopwatch as the packet goes in, calls out each reading time, holds the probe in the middle of the liquid without touching the bag bottom, and reads the temperature aloud to 0.1 °C. Calls 'stop' if a reading reaches 48 °C.
• Data recorder: Writes every reading, computes the changes at two and ten minutes, fills in the scorecard and the cost line, and leads the team's decision about what to change next.

Materials

Estimated cost: 15 to 30 per class of 8 groups, excluding thermometers (an estimate; Estimate. If Lab 1 supplies are on hand, the extra cost is mostly zip bags (about $3 for 50), foam cups (about $5 for 50) and foil. Bought fresh: a 10 lb bag of calcium chloride ice melt about $15, a 1 lb jar of citric acid about $8, baking soda about $2. Each group uses at most 27 g of calcium chloride or 36 g of the two cold-pack reagents across three prototypes. Digital thermometers run $8 to $12 each if the room has none.)

Safety

• Goggles on from the moment the first packet is opened until the last bag is in the trash. Calcium chloride and citric acid are eye irritants.
• Only the safety captain touches reagents, with dry hands. Calcium chloride pellets warm when wet and can irritate skin; if any touches skin, rinse under running water for a minute.
• The 50 °C ceiling is a real safety limit, not a game rule. The reader calls 'stop' at 48 °C; the team records that the prototype failed the constraint, leaves the pack in its cup on the tray to cool, and tells the teacher. If the reading keeps climbing past 50 °C, only the teacher moves or empties the pack. No one picks up a warm bag. Teams may touch the outside of the foam cup with one fingertip to feel the warmth; never the bag.
• No bag is ever fully sealed. Cold packs make carbon dioxide gas; a sealed bag can burst. The bag zip stays open at least 2 cm on every prototype, and the pack stands inside its cup on the tray at all times.
• Cold-pack foam can rise over the bag top; that is why the pack is in a cup on a tray. Wipe spills with a paper towel; the solutions are drain-safe in these amounts.
• Nothing is tasted or sniffed up close. No cup or bag is held near the face. Hands are washed at the end of each session.
• No flames, no hot plates, no microwaves. Insulation means paper, foil and foam only.
• Disposal: pour liquids down the sink with running water; bags, cups and sticks go in the trash. Unused packets go back to the teacher. This activity has not been physically tested by the authors; pilot the maximum hot pack and one cold pack before class.

Teacher preparation and setup

1. Weigh packets the day before on a 0.1 g balance: 3 g calcium chloride per packet (nine per hot-pack group), 2 g citric acid and 2 g baking soda per packet (nine of each per cold-pack group). Fold each in a small square of paper or a labeled mini cup; use two colors of paper for the two cold-pack reagents.
2. Pilot the hottest possible hot pack with your actual product: 9 g of your calcium chloride in 50 mL of room-temperature water in a bag inside two nested foam cups with a foil lid (more insulation than any team can buy within the budget, so this is the worst case). Record the peak. If it reaches 48 °C or more, reduce the largest packet count to two (6 g) for the whole class and tell the hot-pack teams the constraint is 6 g. Hydrated or lower-purity ice melt usually peaks well under 45 °C; anhydrous pellets can go higher. Use pellets rather than flakes: flakes are the hydrated form and release less than half as much energy per gram, which can put the +10 °C criterion out of reach.
3. Pilot one cold pack at 6 g citric acid plus 6 g baking soda in 50 mL water to see how much foam your bag produces; if it overflows the bag top, tell teams to leave the bag zip fully open and to use the tray.
4. Decide which teams build hot packs and which build cold packs (half and half works), or let teams choose with a cap on each type. Print the recording sheet, which carries both briefs and the price list.
5. Write the reading schedule on the board: 0, 30, 60, 90, 120 s, then 3, 4, 5, 7 and 10 minutes. Set the pitchers of water out the night before.
6. Plan the two sessions: session 1 is the brief, criteria and constraints in the team's words, prototype 1 built and tested, and the first change decided; session 2 is prototypes 2 and 3, the scorecard and the justification.

Investigation design

Student procedure

1. Session 1, read the brief. Put on goggles. The design lead reads your team's client brief (hot pack or cold pack) from the recording sheet. In the criteria-and-constraints table, rewrite each criterion and each constraint in your own words and write how your team will test it with the thermometer, the timer, the balance list or the price list.
2. Plan prototype 1. Choose the amount of reagent (hot: 1, 2 or 3 packets; cold: any combination of acid and soda packets up to 12 g), the volume of water (50, 75 or 100 mL; never less than 50 mL), and the insulation (none, one paper-towel wrap, a foil lid with a probe hole, or a second cup). Price the design using the price list and write the plan and the predicted temperature at two minutes in the prototype-plan table.
3. Build and test prototype 1. The safety captain measures the water into a fresh bag standing in a foam cup on the tray, adds any insulation, and the reader places the probe in the water through the open top. Read and record the starting temperature. On 'go', the captain tips in the reagent packet(s), stirs twice with a stick, and the timer starts. Record the temperature at 30, 60, 90 and 120 seconds, then at 3, 4, 5, 7 and 10 minutes. Note foam, fizzing, and whether pellets have disappeared.
4. Score prototype 1. Compute the change at two minutes and at ten minutes (reading minus start; a drop is negative). On the scorecard, mark each criterion met or not met with the number, check every constraint, and write the cost. If the reading ever reached 48 °C, the prototype fails the temperature constraint no matter what else it did.
5. Decide the one change. Using the scorecard, choose the single feature to change for prototype 2 and write the data reason (for example: 'reached only +6 °C at two minutes, criterion is +10, so we add one packet'). Everything else stays the same. Rinse the probe; the captain puts the used bag in the trash and the cup on the tray to dry.
6. Session 2, build and test prototype 2 exactly as in step 3, with the one change. Score it. Then plan prototype 3 with one more single change, informed by prototype 2's data, and build, test and score it the same way.
7. Plot all three prototypes on one temperature-versus-time graph with a different symbol for each, label the start temperature and draw a horizontal line at each criterion level (start plus or minus the required change) so the graph shows at a glance when each prototype was inside the target.
8. Choose the best design. In the justification box, name the prototype that met the most criteria within every constraint, give the numbers, explain what each change did, and state the one change you would make for a prototype 4 and what data tells you to expect from it.
9. Draw your final pack as a system-and-surroundings diagram and complete the analysis questions individually. Clean up: liquids to the sink with running water, bags and sticks to the trash, unused packets back to the teacher, hands washed.

Expected observations

• Hot packs: 3 g in 50 mL typically rises 7 to 10 °C by 2 minutes and peaks by about 2 minutes; 6 g rises about 14 to 20 °C; 9 g in 50 mL can rise 20 to 30 °C with anhydrous pellets and is the design most likely to trigger the 48 °C stop. In a single foam cup the water loses about 3 to 6 °C of its rise by 10 minutes; a foil lid or second cup keeps roughly 2 °C more.
• Cold packs: 3 g acid plus 3 g soda in 50 mL drops about 3 to 6 °C with most of the drop in the first minute; 6 g plus 6 g in 50 mL drops about 7 to 11 °C with strong foam that can climb the bag; by 10 minutes the water has usually recovered 2 to 4 °C toward room temperature.
• Doubling the water with the same reagent roughly halves the change at 2 minutes and makes the 10-minute change a larger fraction of the 2-minute change, because more water stores more thermal energy for the same loss to the room.
• Teams that change ratio alone on a cold pack see smaller effects than teams that change total mass; a mix far from about 1 g citric acid to 1.3 g baking soda leaves one reagent unreacted and cools less per gram.
• Insulation changes the 10-minute number by a few degrees and the 2-minute number hardly at all, which is the cleanest evidence in the lab that time and temperature are separate criteria.

Likely difficulties

• Teams want to change two things at once after a bad prototype 1. Hold them to one change; the design lead's plan table requires the single change and its data reason before packets are handed out.
• The probe touches the bag bottom or sits in foam; coach the reader to hold it mid-liquid and under the foam, and to keep the same depth for all three prototypes.
• Hot-pack teams go straight to 9 g in 50 mL and trigger the stop; treat it as data (the constraint was broken) and expect the next prototype to add water or remove a packet. If your pilot showed 9 g is too hot for your product, cap packets at two before the lab starts.
• Cold-pack foam leaves the bag with 12 g in 50 mL; the tray catches it, the test counts as failed on K4, and the team learns that more water or less reagent is the fix.
• The negative sign on cold-pack changes and the 'start plus or minus' criterion lines on the graph confuse some students; model one example on the board before session 2.
• Teams forget to price insulation; the cost column on the plan table is checked by the design lead before building.

Structured debrief

1. Pool the scorecards on the board by pack type: for each prototype, the feature changed and the changes at 2 and 10 minutes. Ask the class to find the pattern: amount of reagent moves the two-minute number most; water volume and insulation move the ten-minute number most; cold-pack ratio changes matter less than total mass unless a team went far from about 1 g of citric acid to 1.3 g of baking soda.
2. Ask every team for one prototype that failed and what that failure told them. Land the point from the lesson: the engineers' first pouch reached only 28 °C and that number is what sent them to double the iron. A failed test with good data is a successful experiment.
3. Return to energy language. Hot packs: the calcium chloride separated into ions and water surrounded them, and the net result released energy to the water (the surroundings), so the water's particles sped up. Cold packs: breaking apart the citric acid and baking soda cost more energy than forming the new bonds in sodium citrate, carbon dioxide and water paid back, so the system absorbed energy from the water, which cooled. Nobody made or destroyed energy; it moved across the boundary.
4. Ask what the 48 °C stop rule models: a constraint from the world (skin burns), not from the client's wishes. Criteria are targets; constraints are walls. Any team that hit the stop should explain what they would change, and the usual answer (more water, fewer packets) shows that the same features control temperature in both directions.
5. Preview Lab 3: the class will now sit on the other side of the table and judge six finished hand-warmer prototypes from test data, for three customers with different criteria, and combine the best features into a new design.

Recording sheet: what students fill in

Lab 2 recording sheet: Build a better pack

Your team is a small design company. A camping-gear client wants a single-use pack that works with a squeeze: no plug, no flame, no freezer. You will build three prototypes, test each one the same way, and change exactly one feature between prototypes. Your final report must show the data behind every decision.

Analysis questions: answer key

Individual evidence of learning

Each student submits their own criteria-and-constraints table, prototype-plan table, data table, graph with criterion lines, scorecard, the q2 justification, the system-and-surroundings drawing and individual answers to q3 through q9. The plan table (one change with a data reason) together with the scorecard and q2 is the evidence for MS-PS1-6; q4 and the drawing are the evidence for the energy-flow targets.

Assessment criteria

Accessibility adaptations

• Assign the data recorder or design lead role to students who should not handle reagents; one pair of hands on the chemicals is the rule for everyone.
• Provide a large-display or talking thermometer and allow readings to be called to a partner; provide the data table with the time column pre-printed and the sign rule written at the top.
• Offer a graph with axes pre-scaled and the start line drawn, and a decision frame for the plan table: 'Prototype ___ reached ___ °C; the criterion is ___ °C; so we will change ___ because ___.'
• Accept q2 as a recorded one-minute pitch to the client and q4 as a labeled diagram with captions; provide a bilingual card for criterion, constraint, prototype, system, surroundings, release and absorb.
• Students with chemical sensitivities or a respiratory condition can sit away from cold-pack foam, complete the scoring and justification from the team's data, or do W9, the paper version of the same design task.

Low-cost alternative

Run one hot-pack line and one cold-pack line as teacher demonstrations under a document camera, with the class voting on the single change for each new prototype and recording the readings. If calcium chloride is unavailable, Epsom salt (magnesium sulfate) that the teacher has dried in a kitchen oven warms water noticeably as it dissolves, because drying removes most of the water built into its crystals (undried Epsom salt cools water slightly instead); if citric acid is unavailable, 30 mL of vinegar in place of the water with 3 g of baking soda cools by a few degrees. Foam cups can be replaced by paper cups inside a second cup.

Model limitations

A bag of water in a foam cup is a stand-in for a pack against skin: the cup loses energy to the room air, while a real pack trades energy with a hand that keeps supplying or taking energy, so working times and peaks in a real pack differ from the lab's. The hot pack uses dissolving, which is an exothermic process rather than a chemical reaction in the strict sense (no new substance forms), while the cold pack is a genuine reaction; the state standard's clarification statement accepts both for the design task. Amounts are capped for safety, so the packs are weaker than commercial ones. The expected temperature changes are estimates from the reagents' known behavior; this activity has not been physically tested by the authors, and the teacher's pilot of the 9 g hot pack is required before class.

Teacher notes

Check the criteria-and-constraints table and the prototype-1 plan before handing out any packets; the lab's evidence for MS-PS1-6 lives in the plan table's single change with a data reason and in the scorecard. The chemically matched cold-pack recipe is about 1 g of citric acid to 1.3 g of baking soda; teams that go far from that ratio will find that total mass matters more than ratio, which is a legitimate finding. If a hot-pack team triggers the 48 °C stop, record it as a constraint failure, cool the pack on the tray and let the team continue with a safer plan; the pilot of 9 g in 50 mL with your actual product decides whether three packets are allowed at all. Keep the pooled scorecards; Lab 3 asks students to apply the same amount-time-temperature logic to six finished hand-warmer prototypes.