The constant temperature at which a liquid change into gaseous state is known as

Step 1: Analyze the question The question asks for the name of the constant temperature at which a liquid changes into a gas.

Step 2: Explain the concept When a liquid is heated, its temperature rises until it reaches a certain value. At that temperature, bubbles of vapour form throughout the liquid, and it changes to a gas. While this change happens, the temperature stays constant, because the heat supplied goes into breaking the bonds between molecules (latent heat of vaporization) rather than raising the temperature. This temperature is called the boiling point.

Step 3: Check each option

• A. Evaporation point: Not a standard physics term. Evaporation occurs at any temperature, only at the surface, so there is no fixed "evaporation point."
• B. Boiling point: The fixed temperature at which a liquid changes to gas throughout its body. For pure water at standard atmospheric pressure it is 100°C.
• C. Melting point: The temperature at which a solid changes to a liquid.
• D. Dew point: The temperature at which air becomes saturated and water vapour condenses into liquid droplets (the reverse direction, gas to liquid).

Step 4: Answer The correct answer is B. boiling point.

Tip: Remember the direction of each change:

• Melting point: solid → liquid
• Boiling point: liquid → gas
• Dew point: gas → liquid (condensation)

Also remember that boiling happens at a constant temperature and throughout the liquid, while evaporation happens at any temperature and only at the surface.