The pressure exerted by a column of mercury 760 mm high at 0 degree Celsius is commonly defined as which standard unit of pressure?

Introduction / Context:
Standard reference values for pressure are widely used in physics, chemistry, engineering and meteorology. One classical way of defining a standard pressure is using a mercury barometer, which balances atmospheric pressure with the pressure due to a vertical column of mercury. Historically, a specific height of mercury in a barometer tube has been used to define one atmosphere of pressure. This question asks which named pressure unit corresponds to the pressure exerted by a 760 mm column of mercury at 0 degree Celsius, a value that appears frequently in gas laws and thermodynamics problems.

Given Data / Assumptions:
• Height of the mercury column is 760 mm (0.76 m). • Temperature of the mercury column is 0 degree Celsius. • The arrangement is a standard mercury barometer used to measure atmospheric pressure. • The options include pascal, atmosphere, bar and poise.

Concept / Approach:
The classical definition of one atmosphere of pressure is based on the height of a mercury column in a barometer at sea level under standard conditions. One atmosphere is defined as the pressure exerted by a column of mercury 760 mm (or 76 cm) high at 0 degree Celsius under standard gravity. In SI units, this pressure is approximately 1.013 × 10^5 pascals. The pascal is the SI unit for pressure, defined as one newton per square metre. The bar is a metric unit equal to 10^5 pascals, close to one atmosphere but not exactly the same by definition. Poise is a unit of dynamic viscosity, not pressure. Therefore, the name associated directly with 760 mm of mercury is one atmosphere, not pascal, bar or poise.

Step-by-Step Solution:
Step 1: Recall that pressure due to a fluid column is given by p = rho * g * h. Step 2: Understand that a mercury barometer balances atmospheric pressure with the pressure from a column of mercury. Step 3: Note from standard conditions that at sea level, atmospheric pressure supports a column of mercury approximately 760 mm high at 0 degree Celsius. Step 4: Recognise that this condition has historically been used to define one atmosphere of pressure. Step 5: Compare the given description with the options and identify one atmosphere as the matching named unit. Step 6: Confirm that other units listed serve different roles or approximate values and are not defined directly by 760 mm of mercury.

Verification / Alternative check:
Looking at gas law problems and tables for standard temperature and pressure, you will often see standard pressure given as 1 atm = 760 mm of mercury = 1.013 × 10^5 pascals. The bar, defined as exactly 10^5 pascals, is numerically close but slightly different from 1 atm. The pascal itself is a very small unit, and many practical pressures are expressed in kilopascals or bars. Poise appears in fluid mechanics as a unit of viscosity, not pressure. These equivalences and definitions confirm that 760 mm of mercury at 0 degree Celsius is associated specifically with one atmosphere of pressure.

Why Other Options Are Wrong:
Option A, 1 pascal, is far smaller than atmospheric pressure; 1 atm is over one hundred thousand pascals. Option C, 1 bar, is close to one atmosphere but defined independently as exactly 10^5 pascals, not by the 760 mm mercury column. Option D, 1 poise, measures dynamic viscosity in the cgs system and has no direct relation to pressure. None of these descriptions matches the classical definition based on a 760 mm mercury column.

Common Pitfalls:
Students sometimes mix up atmosphere, bar and pascal because their numerical values are similar. Another common error is to assume that any unit used for gases must be atmosphere, without checking the precise definition. It is helpful to remember the key identity 1 atm = 760 mm Hg and to keep separate in your mind the SI standard unit pascal and the convenient near equivalent bar. Distinguishing these clearly prevents unit conversion mistakes in gas law calculations and fluid mechanics problems.

Final Answer:
The correct choice is 1 atmosphere, because by definition one atmosphere of pressure is the pressure exerted by a 760 mm column of mercury at 0 degree Celsius under standard gravity.