In general physical chemistry, which of the following substances has the lowest freezing point among the options given?

Introduction / Context:
Freezing point is the temperature at which a substance changes from liquid to solid under standard pressure. Different elements and compounds have widely different freezing points depending on their intermolecular forces and physical properties. This question asks which of the listed substances has the lowest freezing point. Understanding these differences is useful in cryogenics, materials science, and general physical chemistry.

Given Data / Assumptions:
- The substances listed are mercury, helium, cesium, and aluminium. - We must identify which has the lowest freezing point. - We assume standard atmospheric pressure unless stated otherwise.

Concept / Approach:
Metals such as aluminium and cesium have relatively high melting and freezing points compared to light elements like helium. Mercury is a liquid metal at room temperature and has a low freezing point of about minus 39 degrees Celsius, but helium remains liquid down to extremely low temperatures close to absolute zero. In fact, helium does not solidify at normal pressure even at temperatures near zero kelvin and requires high pressure to freeze. This makes helium effectively the substance with the lowest freezing point among those listed.

Step-by-Step Solution:
Step 1: Recall that aluminium has a high melting point of around 660 degrees Celsius, so it clearly does not have the lowest freezing point. Step 2: Recall that cesium is a soft alkali metal that melts at around 28 degrees Celsius, higher than room temperature but still far above the temperatures where helium remains liquid. Step 3: Note that mercury freezes at about minus 39 degrees Celsius, which is low but still much higher than the extremely low temperatures involved with helium. Step 4: Recognise that helium remains liquid even at temperatures very close to absolute zero at normal pressure and requires pressure to solidify, making its freezing point lower than that of the other substances. Step 5: Conclude that helium has the lowest freezing point among the options given.

Verification / Alternative check:
Reference data for physical properties show that mercury freezes at about minus 39 degrees Celsius, while aluminium and cesium freeze at much higher temperatures. Helium, on the other hand, does not become solid under normal atmospheric pressure, even when cooled to almost zero kelvin, and solid helium is obtained only under high pressure. These facts clearly demonstrate that helium has the lowest freezing point or is the most difficult to freeze of the listed substances.

Why Other Options Are Wrong:
Option A (Mercury) has a low freezing point compared to many metals, but helium remains liquid at far lower temperatures, so mercury does not have the lowest freezing point. Option C (Cesium) melts and freezes at temperatures near room temperature, which are much higher than the temperatures required to freeze helium. Option D (Aluminium) has a very high melting and freezing point relative to all the other options, so it is clearly not the substance with the lowest freezing point.

Common Pitfalls:
Students may focus on mercury because it is a liquid metal at room temperature and therefore seems unusual. However, being a liquid at room temperature does not mean it has the lowest possible freezing point. Another pitfall is not remembering the special behaviour of helium at extremely low temperatures. To answer correctly, it is important to recall that helium is used in cryogenic applications precisely because it remains liquid at extremely low temperatures where most other substances are already solid.

Final Answer:
The correct answer is: Helium.