In physical science, compared to pure fresh water, seawater containing dissolved salts freezes at which one of the following temperature conditions?

Introduction / Context:
This question tests your understanding of how dissolved substances affect the freezing point of a liquid. It uses the familiar example of seawater, which contains salts and other dissolved ions, and asks you to compare its freezing behaviour with that of pure fresh water.

Given Data / Assumptions:
• Seawater is a solution of water with dissolved salts, mainly sodium chloride and other ions.
• Fresh water is taken as pure water with no significant dissolved solutes.
• We are comparing the temperatures at which each begins to freeze under similar pressure conditions.

Concept / Approach:
The presence of dissolved solutes such as salt lowers the freezing point of a solvent. This is a colligative property known as freezing point depression. For seawater, the typical freezing point is lower than 0 degrees Celsius, around minus 1.9 degrees Celsius for average ocean salinity. Therefore, compared to fresh water, seawater freezes at a slightly lower temperature.

Step-by-Step Solution:
Step 1: Recall that pure water at standard pressure freezes at 0 degrees Celsius. Step 2: Recognise that adding a non volatile solute such as salt lowers the freezing point of the solution. Step 3: Understand that seawater contains a significant concentration of dissolved salts. Step 4: Apply the concept of freezing point depression and conclude that seawater will freeze at a lower temperature than pure water. Step 5: Select the option stating that seawater freezes at a slightly lower temperature than fresh water.

Verification / Alternative check:
Practical observations in polar regions show that ocean water remains liquid even when air temperature is slightly below zero degrees Celsius. Ice forms when water cools further, confirming that the freezing point is lower than that of fresh water. This is consistent with the general rule that dissolved solutes depress the freezing point.

Why Other Options Are Wrong:
At a slightly higher temperature than fresh water: This contradicts the principle of freezing point depression.
At exactly the same temperature as fresh water: This would only be true if there were no dissolved solutes, which is not the case for seawater.
Seawater does not freeze at any temperature: Clearly incorrect, as sea ice forms in cold regions.
Only under extremely high pressure deep in the ocean: Pressure can affect freezing point slightly, but seawater can and does freeze at the surface under normal pressure when cold enough.

Common Pitfalls:
Students sometimes recall that salt is used on icy roads to melt ice and incorrectly think that salt prevents freezing entirely. In reality, salt only lowers the freezing point, so water still freezes but at a lower temperature. Keeping the concept of freezing point depression in mind prevents this misconception.

Final Answer:
Seawater freezes at a slightly lower temperature than fresh water because dissolved salts depress its freezing point.