Density of Water

The density of water is maximum at 4 ℃. If you cool water at room temperature, it shrinks and the density increases. The density reaches a maximum value at 4 ℃. If you cool water below 4 ℃, it expands and the density decreases.

Procedure

You need a vessel fitted with two thermometers, lid, cold water, ice-pieces.

1. Put cold water in the vessel so that the upper thermometer is dipped in water.
2. Put some ice pieces in water and close the vessel with its lid.
3. Note the temperature of the two thermometers at regular internal of say 2 minutes.

Discussion

As ice floats on water, the top layer of water gets cooler. As the temperature decreases, water density increases and the colder water goes down. This sets convection current and the whole water cools down.

However, when the temperature decreases upto 4-5 ℃, the water density reaches its maximum. The water at the bottom is denser than the cooler water at the top. The convection current slows down. The water in the bottom layer remains at 4-5 ℃. The top layer is cold but the bottom layers remains warm.

If the temperature above water is negative, the top layer may even freeze but the lower layer remains liquid. That saves aquatic species.

Structure of the Atom: there are several experiments of the why the density is maximum at $$4\,{}^o C$$. one simple model is that at $$4\,{}^o C$$, the thermal motion comes to such a low level that bonding tendency to form solid ice crystal can start here and there. Such tiny ice crystals will make and break maintain a statistical equilibrium. As ice has lower density than water, the net average density will decrease. As you lower the temperature from $$4\,{}^o C$$, the fraction of these tiny ice crystal increases and hence density keeps decreasing at 0 degree celsius, thermal energy is too small and ice start forming at a much larger scale.

Variant: Good

This is a simple experiment which can be given to students of middle school level to get a feel of variation of density of water with temperature. The experiment simply consists of measuring the mass and volume of the liquid and calculating the density.

You need a beaker with a mark to fix a volume of 40 ml, hot and cold water, an electronic balance, syringe.

1. Look at the electronic balance and get familiar with it.
2. Put the beaker on the balance and TARE it.
3. Remove the beaker from the balance. Panel reading will become negative. Fill cold water in it up to the mark at the volume 40 ml. You can take help of the syringe to put or remove small amounts of water. Do it carefully, look at the mark only after bringing it at the same horizontal level as the eye. Make sure that the whole water surface is horizontal. One way is to ask help from your partner and look at the mark from opposite sides.
4. Put the beaker back on the balance. Note the reading. This is the mass of the water.
5. Calculate the density of water.
6. Measure the temperature of the water in the beaker. Write the density and temperature in the table.
7. Do the same for warm water.
8. Calculate the average fractional change $$\Delta\rho/\rho$$ in density per degree change in temperature for water.

Why does density decrease with increasing temperature? This is due to the asymmetry in the molecular potential as a function as a function of separation between.

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