Hey, I totally get the confusion here—vapour and steam look like they mean the same thing on the surface, but there are key distinctions that make them different. Let me break this down for you clearly:

• Vapour (Vapor)
  • This is a broad, general term for the gaseous form of any substance that's typically a liquid or solid at standard room temperature and pressure. It's not exclusive to water—think things like ethanol vapour from an open bottle, gasoline vapour at a gas station, or even iodine vapour when solid iodine is heated.
  • Vapour can form at temperatures well below the substance's boiling point. For example, the water vapour in our ambient air (humidity) exists even when it's 20°C, which is way lower than water's boiling point of 100°C.
• Steam
  • This term specifically refers to the gaseous state of water—it's a subset of vapour, but tied strictly to H₂O.
  • You've probably seen the white "steam" coming off a boiling kettle, but technically that's condensed steam (tiny water droplets). Pure, invisible steam is just gaseous water; it only becomes visible when it cools and mixes with cooler air, forming those tiny clouds.
  • While steam can form via evaporation (like water vapour), it's most commonly used in contexts involving boiling water, industrial processes, or power generation (think steam turbines).

Quick Recap #

All steam is vapour, but not all vapour is steam. Steam is water-specific, while vapour covers any liquid/solid that's turned into gas under normal conditions.

内容的提问来源于stack exchange，提问作者Puskar Chhetri