Understanding Super-cooled Liquids
Super-cooled liquids are materials that are in a liquid state but have not yet crystallized, even when they fall below their freezing points. Glass is one of the most commonly cited examples of a super-cooled liquid.
Why Glass is Considered a Super-cooled Liquid
- Amorphous Structure:
- Glass lacks a crystalline structure. Instead, it has a disordered arrangement similar to liquids, which allows it to flow very slowly over time, even at room temperature.
- Transition from Liquid to Solid:
- When glass is cooled, it does not form a regular crystal lattice as ice does. Instead, it transitions into a solid state without a clear phase change, making it appear as a super-cooled liquid.
- Viscosity:
- Glass has a high viscosity, meaning it does not flow easily. However, over long periods, it can exhibit flow characteristics, which is a key feature of super-cooled liquids.
Comparison with Other Options
- Ice-cream:
- Ice-cream is a colloidal suspension and not a super-cooled liquid; it contains crystalline ice and air, which gives it a solid-like texture.
- Ammonia:
- Ammonia is a gas at room temperature and becomes a liquid under pressure. It does not exhibit super-cooling properties like glass.
- Wood:
- Wood is an organic material composed of cellulose and lignin, which does not belong to the category of super-cooled liquids.
Conclusion
In summary, glass is classified as a super-cooled liquid due to its unique amorphous structure, transition characteristics, and high viscosity. This distinguishes it from other materials listed, making option 'C' the correct answer.