Anomalous Expansion Of Water
During winter a lake freezes only at its surface while fish survive in the water below; this is best explained by which unusual thermal property of water near 4°C?
Select the correct option:
Solution
Water has maximum density at about 4°C
As described in NCERT Class 11, Chapter 11 (Thermal Properties of Matter), water shows anomalous expansion: as it cools from higher temperatures its volume decreases until about 4°C, where its density is maximum, and on further cooling toward 0°C it expands again. This unusual behaviour means the densest water sinks to the bottom while colder, lighter water rises to the surface and eventually freezes there. The layer of ice on top insulates the water beneath, keeping it liquid so aquatic life can survive. The option claiming minimum density at 4°C reverses the true behaviour. The option of uniform expansion at all temperatures ignores the anomaly entirely. The option of zero expansion below 4°C is false, since water actually expands as it approaches freezing. A consistency check confirms the everyday observation that ice floats and lakes freeze top-down, both direct consequences of water's density maximum near 4°C.
🔒 Solution Hidden from View
Submit your answer to unlock the detailed step-by-step solution.
About This Question
- Subject
- physics
- Chapter
- properties of solids and liquids
- Topic
- anomalous expansion of water
- Difficulty
- Easy
- Year
- 2025
Solution
Correct Answer:
Water has maximum density at about 4°C
As described in NCERT Class 11, Chapter 11 (Thermal Properties of Matter), water shows anomalous expansion: as it cools from higher temperatures its volume decreases until about 4°C, where its density is maximum, and on further cooling toward 0°C it expands again. This unusual behaviour means the densest water sinks to the bottom while colder, lighter water rises to the surface and eventually freezes there. The layer of ice on top insulates the water beneath, keeping it liquid so aquatic life can survive. The option claiming minimum density at 4°C reverses the true behaviour. The option of uniform expansion at all temperatures ignores the anomaly entirely. The option of zero expansion below 4°C is false, since water actually expands as it approaches freezing. A consistency check confirms the everyday observation that ice floats and lakes freeze top-down, both direct consequences of water's density maximum near 4°C.
This easy difficulty physics question is from the chapter properties of solids and liquids, covering the topic of anomalous expansion of water. It appeared in the 2025 exam.
Looking for more practice? Explore all physics questions or browse properties of solids and liquids questions on RankGuru.