Boiling point of water is defined as the temperature at which:

Options

A. vapour pressure of water equal to that of atmospheric pressure
B. bubbles are formed
C. steam comes out
D. none of the above

Explanation

To answer the question regarding the boiling point of water, we can follow these steps:

### Step 1: Understand the Definition of Boiling Point
The boiling point of a substance is defined as the temperature at which its vapor pressure equals the external atmospheric pressure.

### Step 2: Relate Vapor Pressure and Atmospheric Pressure
When the vapor pressure of water increases with temperature, it eventually reaches a point where it equals the atmospheric pressure. At this point, bubbles of vapor can form within the liquid, leading to boiling.

### Step 3: Identify the Correct Answer
Given the options, the correct answer to the question is:
- **The temperature at which the vapor pressure of water is equal to that of atmospheric pressure.**

### Step 4: Note the Normal Boiling Point
Under normal atmospheric conditions (1 atm pressure), the boiling point of water is 100 degrees Celsius. However, this boiling point can change with variations in atmospheric pressure.

### Step 5: Consider the Effect of Altitude
At higher altitudes, the atmospheric pressure is lower, which causes the boiling point of water to decrease. For example, at Mount Everest, the boiling point of water is approximately 69 degrees Celsius.

### Final Answer
The boiling point of water is defined as the temperature at which the vapor pressure of water is equal to that of atmospheric pressure.

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