The incorrect statement among the following is

Options

A. The boiling point of a liquid at one bar is called standard boiling oint of the liquid
B. The vapour pressure of a liquid is constant temperature
C. The SI unit of coefficient of viscosity of a pascal second
D. The boiling point of liquid is same external pressures.

Answer

To determine the incorrect statement among the provided options, we will analyze each statement one by one.

### Step 1: Analyze Option 1
**Statement:** The boiling point of liquid at 1 bar is called the standard boiling point of liquid.

**Analysis:** This statement is correct. The standard boiling point is defined as the boiling point of a liquid at a pressure of 1 bar (or 100 kPa).

### Step 2: Analyze Option 2
**Statement:** The vapor pressure of a liquid is constant at a constant temperature.

**Analysis:** This statement is also correct. The vapor pressure of a liquid is indeed constant at a given temperature. It does not change unless the temperature changes.

### Step 3: Analyze Option 3
**Statement:** The SI unit of the coefficient of viscosity is Pascal second.

**Analysis:** This statement is correct. The SI unit for viscosity is indeed Pascal-second (Pa·s), which is a measure of the internal friction of a fluid.

### Step 4: Analyze Option 4
**Statement:** The boiling point of liquid is the same at external pressure.

**Analysis:** This statement is incorrect. The boiling point of a liquid varies with changes in external pressure. For example, water boils at 100 degrees Celsius at 1 atm pressure, but at higher pressures, the boiling point increases, and at lower pressures, it decreases. Therefore, the boiling points of different liquids at the same external pressure are not the same.

### Conclusion
The incorrect statement among the options is **Option 4**: "The boiling point of liquid is the same at external pressure."

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