Correct Answer: (c) the density of water is maximum at 4°c
Solution:Generally all liquid expands on heating but water is an exception to this rule. If water is heated, its volume gradually decreases (This decrease in volume continues till the temperature rises to 4°c .At temperatures over 4°c water starts expanding. It then keeps expanding with the further rise in temperature, till finally at 100 deg * C and turns into steam. In other words, at 4°c water has the least volume (occupies the least amount of space) and maximum density (is at its heaviest). This irregular expansion of water is called anomalous expansion. This anomalous expansion plays an important role by only freezing the upper layer in lakes and rivers. During winter months in colder countries, the outside or atmospheric temperature is very low-it drops to below freezing and the upper layers of water in the lakes and ponds start cooling. When the temperature of the surface layers falls to 4°c the water body acquires maximum density and sinks down. The water that sinks down displaces water below, and the lower layers of water simultaneously rise up. This also gets cooled to 4°c and again sinks down. When the temperature of the water body finally goes below 4°c the density or heaviness of water decrease and as a result water does not sink down.The surface water finally freezes at 0°c while the lower part still is in 4°c.The light frozen layer of ice floats on top.
