Design Challenge: A Hand Warmer for the Snowshoe Club

Warm Hands on the Trail

1The snowshoe club at Ridgeview Middle School hikes forest trails on winter Saturdays, when the air is often colder than −5 °C. Several members complained that their fingers went numb by the halfway point, even inside thick gloves. Their science teacher, Ms. Kowalczyk, turned the problem into a design challenge: build a small pouch that warms up on its own. If the pouches were cheap enough to make in class, every member could carry two on each hike.

2The club agreed on criteria and constraints. The pouch had to reach at least 40 °C and stay above 40 °C for at least 60 minutes. The school nurse advised that a pouch held against skin should stay at or below 55 °C. Each pouch also had to fit inside a glove and use only materials the teacher approved.

3Many hand warmers work by a chemical reaction: iron powder reacts with oxygen from the air and forms iron oxide, a reddish-brown solid also called rust. Salt dissolved in water is known to speed up rusting. Ms. Kowalczyk measured out every powder, and students wore goggles and gloves while they filled the pouches.

4Every pouch held 20 g of iron powder and 5 g of damp sawdust, which holds the water the reaction needs. The pouches were made of medium-weave cotton and stored in sealed zip bags so that no air could reach them. To run a test, a student opened the zip bag in the 20 °C classroom, taped a thermometer probe to the pouch and wrapped it in a glove. The students recorded the highest temperature and the number of minutes the pouch stayed above 40 °C.

5In Test 1, the club changed only the amount of salt (Table 1). In Test 2, they kept the 2 g salt recipe and tried three cloth fabrics with different weaves (Table 2 in Section 2).