Water boils at 100 degrees Celsius under standard conditions.

Water boils at 100 degrees Celsius under standard conditions.

• A. 0°C
• B. 100°C
• C. 50°C
• D. 212°C

Answer: (B)

The key idea is the boiling point at standard atmospheric pressure. At standard conditions, which is about 1 atmosphere of pressure (roughly 101.3 kPa), water boils when its vapor pressure equals the surrounding pressure. For water, that occurs at 100°C, so it begins to turn into steam at that temperature. The other temperatures don’t match this condition under standard pressure: 0°C is the freezing point, so water would be solid rather than boiling; 50°C is well below the boiling point at 1 atm; 212°C would require much higher pressure to keep water liquid, not standard conditions.

The key idea is the boiling point at standard atmospheric pressure. At standard conditions, which is about 1 atmosphere of pressure (roughly 101.3 kPa), water boils when its vapor pressure equals the surrounding pressure. For water, that occurs at 100°C, so it begins to turn into steam at that temperature. The other temperatures don’t match this condition under standard pressure: 0°C is the freezing point, so water would be solid rather than boiling; 50°C is well below the boiling point at 1 atm; 212°C would require much higher pressure to keep water liquid, not standard conditions.