Final answer:
Sodium chloride is not used as a de-icing agent at -20°F because it is ineffective at such low temperatures due to its limited ability to lower the freezing point of water.
Step-by-step explanation:
The student's question is asking at what temperatures sodium chloride (NaCl), commonly known as table salt, is not used for de-icing purposes.
Using sodium chloride for de-icing is based on the principle of freezing point depression, where a solution's freezing point is lowered when a solute (in this case, salt) is added. Sodium chloride is effective in melting ice on roads and sidewalks due to this phenomenon. However, its effectiveness decreases at very low temperatures.
According to the provided information, sodium chloride has a limited capacity to lower the freezing point of water.
Comparing the given temperatures, sodium chloride would be least effective at -20°F because it is much colder than sodium chloride's effective range for de-icing. In fact, sodium chloride's de-icing capabilities significantly diminish below 15°F (-9.4°C), making it ineffective at -20°F. Hence, sodium chloride is not used as a de-icing agent at -20°F.