Under standard conditions, at what temperature does water boil?

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

Answer: (A)

Boiling point at standard conditions is determined by atmospheric pressure. At 1 atmosphere (about 101.3 kPa), liquid water changes to steam at 100°C. The other temperatures either correspond to the freezing point (0°C), are simply below the boiling point (50°C), or are not the Celsius boiling point (212°C is the Fahrenheit value 212°F, and at 1 atm water would already be steam). Remember, boiling point depends on pressure: higher pressure raises it, lower pressure lowers it.