Density of Water

The term 'density' is a characteristic property of every object and forms a basic topic of study in elementary science classes. An interesting anecdote that is related to density concept is the running of Archimedes naked in the streets of Greece, shouting 'Eureka, Eureka' after he discovered a method to determine the volume of an object with an irregular shape. This discovery indirectly paved the path for study and analysis of volume and density in the subsequent years. Water, with its unique physical and chemical properties, has various interesting characteristic features and one of them is its density. Density of water is a very fluctuating parameter that depends on various factors like mass of water considered, volume of water taken, temperature and atmospheric pressure. In chemistry, density of water at room temperature is used as a parameter to compare the density of various other objects. Does a substance float on water or sink in water? That gives us an idea about the density of substance. For instance, a log of wood floats in water but a rock sinks in it concluding the fact that density of water at room temperature or normal average temperature is more than wood and less than rock. Put forward more simply, wood is less dense than water and rock is denser than water. Before we delve deep into various details about density of water, knowing about the definition of density would be more meaningful.

What is the Density of an Object?
Density qualitatively can be defined as relative heaviness of various objects with constant volume. Mathematically stated, density is mass per unit volume of a substance. Although mass and weight are generally interchangeably used terms, there is a fundamental difference between them and understanding this minute difference is necessary to avoid any confusion. Mass, a measure of quantity of matter an object contains, is independent of any external factor. That is, mass is a fixed quantity all over the Universe. Weight on the other hand, is proportional to mass but it depends on the position of the object. Weight is the gravitational force exerted on the object (generally by Earth). In physics, specific gravity or relative density is another term used to define the density of a substance relative to water.

Density (ρ) = Mass (m)/Volume(V),

where,
m = mass of the body
V= volume occupied by the body

Density of an object depends on temperature as an increase or decrease in temperature causes expansion or contraction respectively in the size of the object leading to changes in volume and hence density. According to the SI system of units, the unit of density is kilogram per cubic meter or Kg/m³. This SI unit can be easily derived by putting the SI units of mass in the density formula, that are kilogram and cubic meter. In CGS system, the unit of density is gram per cubic centimeter or g/cm³.

What is the Density of Water?
At room temperature, that is at 20°C, it is 1 g/cm³ or 1000 kg/m³. The density of pure water, devoid of any impurities is constant at any particular temperature and it does not depend on the volume of water sample considered. That is, density at any particular temperature is an intensive property. However, with changes in temperature, pressure and addition of impurities, the density of water changes. Water after freezing expands, increasing its volume by 9% and that is why the density of ice is less than that of water and hence ice floats on water. An interesting fact about water is that it is the only substance where the density in solidified form is less than its liquid form. Besides that, water is the only substance on Earth, to exist in all the three general phases of matter, that is, solid, liquid and gas!

Density of Water at Different Temperatures
It is highest at 4°C (39.2°F) and equals to approximately 1000 kg/m³. On different temperatures, here's how it varies. Observe the trend carefully as it helps in deducing some important conclusions about water density.


Observing the table above must have given you an idea about the shape of curve you will get if you plot a graph for density vs temperature. Except for a point at 4 °C, the density of water decreases with increase in temperature, that is water has highest density at this temperature. This was a summarized information about density of water that shed some light on various aspects of water and its relation to temperature.