Prelims 2011 · archive
EasyThe surface of a lake is frozen in severe winter, but the water at its bottom is still liquid. What is the reason?
Answer & explanation
Answer: (c) The density of water is maximum at 4°C
Water is densest at 4 °C, not at 0 °C. As a lake cools in winter, the 4 °C water sinks and stays at the bottom. Colder water, which is lighter, stays on top and freezes first. The lake therefore freezes from the top down, and the bottom stays liquid.
- ✓ (c) Water's density peaks at 4 °C. Surface water that cools below 4 °C becomes lighter and stays on top, so the 4 °C water settles at the bottom. Ice (at 0 °C) is lighter still and forms a layer on the surface.
- ✗ (a) Ice does conduct heat poorly, and the ice layer insulates the water beneath it. But this only slows further freezing once ice has formed on top. Why the ice forms on top, with the denser 4 °C water at the bottom, comes from water being densest at 4 °C, so this is not the reason asked for.
- ✗ (b) The lake loses heat continuously to the colder winter air. The surface freezes because heat is lost, not because none is lost.
- ✗ (d) Statement (c) is correct, so 'none of the above' cannot be right.
Remember · Water is densest at 4 °C. Ice floats and the 4 °C water sinks, so lakes freeze from the top down and aquatic life survives below the ice.
📘 Read it in NCERT: Class 8 Curiosity, Ch 9 (practise this chapter)
Sources
- NCERT Class 8 · Curiosity, Chapter 9 “Water has a special property that its density is highest at 4 °C. As the temperature drops, and water turns into ice at 0 °C, it undergoes a change in structure”
Question: UPSC Civil Services (Preliminary) 2011, General Studies paper. Explanation: Minimalist IAS, checked 25 Oct 2026 (how we verify). ·