Design Challenge: A Flameless Soup Warmer for a Power Outage

Hot Soup Without a Stove

1Last winter, an ice storm knocked out the power at Aaliyah Grant's home for two days. The electric stove did not work, and the family ate cold canned food. Aaliyah, a seventh grader, and her grandfather, Desmond Grant, decided to design a flameless warmer for their emergency kit. The plan was simple: an insulated outer cup holds water, a sealed pouch of soup sits in the water, and something added to the water makes it warm up on its own. Some camping meals come with a heater pouch, but Aaliyah wanted to understand how such a heater works and build one from things sold at a grocery store.

2They set three rules for their design. After 15 minutes, a 150 g pouch of soup must be at least 40 °C. The water in the outer cup must never go above 65 °C, so no one burns a hand lifting out the pouch. Each use must cost no more than $0.75.

3First they tested four powders from the kitchen and the medicine cabinet. For each one, Mr. Grant stirred 20 g of the powder into 100 mL of 20 °C water in a foam cup, and Aaliyah read the thermometer after 2 minutes. Both wore safety goggles and gloves. Some of the baking soda stayed at the bottom of its cup without dissolving. Aaliyah recorded each reading in Table 1.

4Next they built the warmer using calcium chloride, a food-grade salt sold for keeping pickles crisp. Each test used 200 mL of 20 °C water in a foam-lined outer cup and a 150 g pouch of soup that started at 20 °C. They changed only the mass of calcium chloride. The calcium chloride costs 1 cent per gram. Mr. Grant poured the powder in each time, and Aaliyah recorded the hottest water temperature and the soup temperature after 15 minutes.