Question

What is less dense water?

Answer 1

We know that the density is the amount of mass present in a specific volume of a substance. It depends upon the structural arrangement of the substance. The substance in which molecules and atoms are far apart from each other is said to be less dense

Complete answer:
Water is an essential part of our lives. It covers up to $70$ of the earth’s surface. It exists in solid, liquid and gaseous phases within a low range of temperature and pressure. Each water molecule is made up of two hydrogen atoms and one oxygen atom. These molecules of water are bonded to each other by hydrogen bonds in liquid state. The density of a substance is defined as the mass present in a unit volume of that substance. Another useful notion is number density, mostly used in the study of gas theory. It is given by the number of molecules present in unit volume. Density is defined as the mass per unit volume of a substance. The density of water at room temperature is approximately. But the density of water changes with the change in temperature.
When water gradually cools, its mass remains constant but volume decreases. Since volume is inversely proportional to the density, thus density of water should increase with decrease in temperature. But when the temperature of water and the density of water is maximum at $3.98{}^\circ C$ and has the least density at $0{}^\circ C.$ When water freezes at $0{}^\circ C,$ hydrogen-bonded molecules form a rigid open lattice like a web. This open structure makes the ice denser than water.

Note:
Remember that the substance with lower value of density tends to float upon the substance with higher density. Since the density of water at zero is less than the density of water at room temperature, this makes the water at zero, that is ice, float upon the liquid water. This is also why icebergs and ice cubes float on the surface of liquid water.