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.
calcium chloide
B.
urea
C.
sodium chloride
D.
ammonium nitrate

Answer 1

To determine which solid would best modify the cold pack to make it colder, we need to look at the final temperatures recorded for each substance after mixing with water. The colder the final temperature, the more effective the substance is for a cold pack.

Here are the final temperatures for each substance:

• Ammonium chloride: Final temperature = 8°C
• Ammonium nitrate: Final temperature = 2°C
• Urea: Final temperature = 9°C
• Calcium chloride: Final temperature = 32°C
• Sodium chloride: Final temperature = 20°C

From these results, we can see that:

• Ammonium nitrate produced the lowest final temperature at 2°C.
• The other substances resulted in temperatures higher than this, with calcium chloride even producing a final temperature (32°C) that is significantly warmer.

Based on these observations, the best choice for the student to use in order to make her cold pack colder when activated is:

D. ammonium nitrate.