What is the freezing point of pure water on the Celsius temperature scale under standard atmospheric pressure?

Introduction / Context:
The freezing point of water is one of the most fundamental reference temperatures in science and everyday life. It is used as a calibration point for thermometers and as a benchmark in weather reports and physical experiments. On the Celsius scale, the freezing point of pure water at standard atmospheric pressure was originally chosen as one of the fixed points for defining the scale. This question checks whether you can recall the correct value of this temperature in degrees Celsius.

Given Data / Assumptions:

• The substance is pure water, not a solution containing salts or impurities.
• Freezing point refers to the temperature at which water changes from liquid to solid.
• Standard atmospheric pressure is approximately 1 atmosphere.
• The options include several temperatures on the Celsius scale, such as 0, 32 and 100 degrees Celsius.

Concept / Approach:
The Celsius temperature scale was historically defined so that the freezing point of water is 0 degrees Celsius and the boiling point is 100 degrees Celsius at standard atmospheric pressure. Although modern definitions use the Kelvin scale and more precise reference points, these values remain accurate enough for everyday and school level use. At 0 degrees Celsius under one atmosphere pressure, pure water and ice coexist in equilibrium. Therefore, the freezing point of water in degrees Celsius is 0, not 32, which is the freezing point on the Fahrenheit scale.

Step-by-Step Solution:
Step 1: Recall that on the Celsius scale, the freezing point of pure water is defined as 0 degrees Celsius. Step 2: The boiling point of water on the same scale is 100 degrees Celsius at standard atmospheric pressure. Step 3: The value 32 degrees is associated with the freezing point of water on the Fahrenheit scale, not on the Celsius scale. Step 4: Negative values such as minus 32 or minus 42 degrees Celsius are far below the freezing point and represent temperatures where water has long been solid. Step 5: The temperature 100 degrees Celsius represents the boiling point of water at one atmosphere and not the freezing point. Step 6: Therefore, the correct freezing point of water in Celsius is 0 degrees Celsius.

Verification / Alternative check:
If you place a mixture of ice and pure water in a well insulated container at normal atmospheric pressure and insert a thermometer, it will read close to 0 degrees Celsius while both phases coexist. This experiment confirms that the phase change between water and ice occurs at that temperature on the Celsius scale. Weather forecasts frequently refer to 0 degrees Celsius as the temperature at which water freezes on roads and surfaces. Science textbooks universally state that the freezing point of water is 0 degrees Celsius in standard conditions, confirming that this is the correct value.

Why Other Options Are Wrong:
Option B, 32 degrees Celsius, is incorrect because 32 degrees is a warm temperature; water is liquid and feels warm, not freezing, at this temperature. Option C, minus 42 degrees Celsius, and option D, minus 32 degrees Celsius, are far colder than the freezing point and would represent conditions where ice is already well formed. Option E, 100 degrees Celsius, is the boiling point of water at standard pressure, not the freezing point. Only option A matches the accepted freezing point of pure water on the Celsius scale.

Common Pitfalls:
A common confusion arises from mixing the Celsius and Fahrenheit scales, especially because in Fahrenheit the freezing point of water is 32 degrees and the boiling point is 212 degrees. Students sometimes mistakenly pick 32 degrees Celsius due to this mix up. To avoid this, remember that 0 and 100 are the freezing and boiling points on the Celsius scale, while 32 and 212 are the corresponding points on the Fahrenheit scale. Keeping this mapping in mind helps you quickly pick the correct temperature in exam questions and everyday reasoning.

Final Answer:
The freezing point of pure water on the Celsius scale at standard atmospheric pressure is 0 degree Celsius.