A student constructs a hand warmer that becomes hot to the touch when activated. It is activated when a pouch filled with 100 mL of water bursts and combines with a solid substance in a dry bag, as in the model shown below.

Question

The student wants to continue using the same solid. However, he finds that the hot packs he constructs do not become warm enough, so he wants to modify his design. He conducts a test using the same solid but with different concentrations, as shown in the table below.

Mass of Solid
(g) Initial
Temperature
(°C) Final
Temperature
(°C)
20 20 51
30 20 63
40 20 73
50 20 86
60 20 96

After further research, he found that temperatures above 70°C are too hot for human touch. He wants the hot pack to be as warm as possible, but not too hot to touch. Based on the information given, how much of the substance should the student use to best modify his hot pack?
A. 35 g
B. 25 g
C. 45 g
D. 40 g

Answer 1

To determine the best amount of the solid to use in the hot pack, we want to find a mass that produces a final temperature that is warm but not too hot to touch (i.e., above 70°C but below that limit).

From the data provided in the table:

• 20 g results in a final temperature of 51°C
• 30 g results in a final temperature of 63°C
• 40 g results in a final temperature of 73°C
• 50 g results in a final temperature of 86°C
• 60 g results in a final temperature of 96°C

Among these results:

• 40 g produces a final temperature of 73°C, which is just slightly above the threshold.
• 30 g produces a final temperature of 63°C, which is below the threshold.
• 50 g produces a final temperature of 86°C, which is too hot.

Since the goal is to find the highest possible temperature that is still considered safe to touch, the best option is to use 40 g of the solid, since it generates a final temperature of 73°C.

Thus, the answer is: D. 40 g