Final answer:
Water's density increases as temperature decreases until it reaches about 4°C, at which point it is most dense. Below 4°C, as water transitions to ice, its density decreases, allowing ice to float. Thermal expansion at temperatures above 4°C results in reduced water density.
Step-by-step explanation:
The density of a substance, which is its mass per unit volume, is known to change with temperature. For water, as the temperature decreases from its liquid state, the density increases up until it reaches approximately 4°C. At this point, water is at its maximum density. If the temperature continues to drop below 4°C, water begins transitioning toward the solid state and its density decreases, which is an anomaly as compared to most substances. Normally, the density of substances reduces as temperature increases due to the phenomenon of thermal expansion. This is the case for water above 4°C, where an increase in temperature leads to more vigorous molecular motions and the subsequent increase in the average molecular separation, thus decreasing its density.
One interesting consequence of water's unique density behavior is related to ice formation. Since ice has a lower density than liquid water, this causes it to float, which is observed when ice forms at the top of a water body. Therefore, at the bottom of a lake or ocean, the water temperature is typically around 4°C, ensuring that the aquatic organisms can survive in this relatively warmer water during cold seasons.