Understanding Melting and Boiling Points
Melting points and boiling points are specific temperatures at which a substance changes its state from solid to liquid and from liquid to gas, respectively. However, not all substances exhibit fixed temperatures for these transitions.
Substances with Fixed Melting/Boiling Points
- Ethanol: Ethanol has a defined melting point of around -114.1°C and a boiling point of about 78.37°C, making it a substance with fixed phase transition points.
- Oxygen: Oxygen also has specific melting and boiling points, with melting occurring at -218.79°C and boiling at -182.96°C.
- Gold: Gold, a pure metal, has a fixed melting point of approximately 1064°C and a boiling point of about 2856°C.
Why Air Does Not Have Fixed Melting/Boiling Points
- Composition: Air is a mixture of gases, primarily nitrogen (78%) and oxygen (21%), along with trace amounts of other gases like carbon dioxide and argon.
- Variable Properties: Because air is not a pure substance, its melting and boiling points are not fixed. The transitions depend on the specific composition and pressure conditions.
- Phase Changes: Different gases in air transition at different temperatures, leading to a range of melting and boiling points rather than a single fixed point.
Conclusion
In summary, while substances like ethanol, oxygen, and gold have defined melting and boiling points, air does not possess fixed transitions due to its nature as a mixture of various gases. This characteristic is the reason why air is the correct answer to the question posed.