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?

Your role today

• 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.

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.

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.

Record your observations

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