Which statement is true about boiling?

• A. Boiling occurs at a specific temperature for a given substance.
• B. Boiling occurs at any temperature if pressure is high.
• C. Boiling requires no heat input.
• D. Boiling is the same as freezing.

Answer: (A)

Boiling happens when the liquid’s tendency to become vapor (its vapor pressure) matches the pressure acting on the liquid from outside. For any given substance at a fixed pressure, there is a specific temperature where this balance occurs, so the liquid boils at that particular temperature. That’s why a substance like water boils at about 100°C at 1 atmosphere of pressure, and why changing the surrounding pressure changes the boiling temperature: lowering the pressure makes boiling occur at a lower temperature, while increasing the pressure raises the boiling temperature.

It’s also important to note that boiling involves heat input. You must supply energy to reach the boiling temperature and, once boiling starts, to provide the latent heat of vaporization as the liquid turns into gas. That’s why the statement that boiling requires no heat input isn’t correct. And the other options don’t fit because boiling is not determined by high pressure allowing any temperature, and boiling is not the same as freezing.