Density of Water

Introduction

Density is the physical quantity which tells us the amount of mass per unit volume. So if I say that the density of iron is 7.874 $\mathrm{gram/centimeter^3}$, it means that 7.874 grams of iron can be placed in 1 $\mathrm{cm^3}$ of volume. And as we know that the molecules of iron are very closely packed and more molecules will be composed in 1 $\mathrm{cm^3}$ of volume.

The molecular formula of water is $\mathrm{H_2O}$.

It is very important to learn all the properties of water as it is essential for the existence of life on earth. Properties are as follows:

• Molar mass = 18.01528 gram/mole

• Boiling point at atmospheric pressure = 100°C

• Freezing point at atmospheric pressure = 0°C

• Density of water at room temperature

  $\mathrm{(22^{\circ}C) = 997.77 kg/m^3.}$

Temperature and Density of water

Take one beaker of liquid water and a few cubes of ice. Put the cubes of ice in the beaker containing the water. You can see that the ice cubes start floating. But the ice cubes are solid, which means as per our perception the density of ice must be higher than the density of liquid water. But if it is true then how the ice is floating on the top of water?

So here we will study about the variation of density of water with temperature and some interesting facts about this. As the temperature of water decreases then the water molecules will start coming closer and the thermal motion of the water molecules will be decreasing. So the density will increase as the temperature decreases. But below 4°C the water molecules come closer and since the thermal motion is less at this temperature then the water molecules start forming hydrogen bonds which will prevent molecules from coming closer. And at the 4°C there is a balance between the making of hydrogen bonds and the thermal agitation which results in the highest density at 4°C.

Now let us check with an important graph which shows this variation and let us understand this theory with the graph.

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Figure - 1

From the graph (Figure - 1), we can see that when the temperature of the water is near 80°C then the density of the water is around 0.97 $\mathrm{g/m^3}$ and as we are decreasing the temperature then the density of the water is decreasing. It becomes maximum around 4°C and if we are decreasing the temperature again the density of water starts decreasing.

Density of Water and Pressure

Density of water is directly proportional to the pressure exerted on the water surface. As the pressure increases the molecules come closer and as a result the density increases. But since water is not compressible as gasses. So after some time the density will become constant.

Specific gravity

Specific gravity or relative density of a liquid is defined as the ratio of density of that liquid with respect to the density of the water.

For example the specific gravity of petrol is approximately 0.8, it means that the density of petrol will be approximately 800 $\mathrm{kg/m^3}$.

Density of Water Experiment

Let us understand the density of water with a small experiment. Let us take a transparent long

Step 1: Pour a one-quarter cup of coloured water gently on top of the honey.

Step 2: Pour a one-quarter cup of coconut oil on top of the coloured water glass tube having 3 labels which divide the whole glass tube in three equal parts. First, pour honey till first label then coloured water over the top of honey. Then again pour coconut oil.

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Figure - 2

Now you can see that the three layers formed in the tube. This is based on the density difference between the oil, water and honey. So we can say that the water is less dense than honey and denser than oil.

Conclusion

We have now learned that water is one of the most available liquids on the planet and essential for the existence of life on Earth. It is because water is a very neutral material which has no odor, no taste, no color and is also transparent. The density of water is taken as a reference to define the density of other materials.

FAQs

Q1. Does water have absolute density?

Ans. No, water is not having absolute density. Its density varies with the change in its temperature.

Q2. When we place ice over water, it starts floating. Give a reason for this.

Ans. Ice floats over the water because the density of the water is higher than the solid ice.

Q3. What is the density of water at room temperature (22°C)?

Ans - The density of water at room temperature is 997.77 $\mathrm{kg/m^3}$.

Q4. When we decrease the temperature the density of water increases till 4°C and becomes maximum at 4°C. Why?

Ans . The density of water is maximum at 4°C because there is a balance between the making of hydrogen bond and thermal agitation. So the density will become maximum at 4°C.

Q5. What is specific gravity?

Ans. Specific gravity or relative density of a liquid is defined as the ratio of density of that liquid with respect to the density of the water.