A student constructs a cold pack which becomes cold to the touch when activated. The cold pack is activated when an inner water pouch is squeezed until it bursts, allowing the water to mix with a solid substance. She constructs the device as shown below.

Question

The solid she uses in the cold pack is ammonium chloride. She wants to modifiy her device to make it colder when it is activated. She conducts some tests and collects the data shown below.

Solid
Substance Mass Added to
100 mL Water
(g) Initial
Temperature
(°C) Final
Temperature
(°C)
ammonium chloride 10 20 8
ammonium nitrate 10 20 2
urea 10 20 9
calcium chloride 10 20 32
sodium chloride 10 20 20

Based on the test results, which solid should she use that would best modify her cold pack?
A.
ammonium nitrate
B.
sodium chloride
C.
urea
D.
calcium chloide

Answer 1

To determine which solid would make the cold pack colder when activated, we can analyze the temperature change caused by each substance. The effectiveness of a cold pack depends on the ability of the solid to decrease the temperature of the solution when dissolved.

Here’s a review of the temperature changes from the student’s data:

• Ammonium chloride: Final temperature = 8 °C (Temperature drop of 12 °C from 20 °C)
• Ammonium nitrate: Final temperature = 2 °C (Temperature drop of 18 °C from 20 °C)
• Urea: Final temperature = 9 °C (Temperature drop of 11 °C from 20 °C)
• Calcium chloride: Final temperature = 32 °C (Temperature increase of 12 °C from 20 °C, not effective as a cold pack)
• Sodium chloride: Final temperature = 20 °C (No temperature change)

The best candidate for making the cold pack colder is the one with the greatest temperature drop.

Among the solids tested:

• Ammonium nitrate caused the most significant drop (from 20 °C to 2 °C).

Thus, the best choice for the solid that would modify her cold pack to make it colder when activated is:

A. ammonium nitrate