Water, density & temperature

An important property of water is the relationship between temperature and density. Those who live in colder regions of the world are aware of this property - water freezes from the the top down!

A dramatic example of this property of water is LAKE VOSTOK, which lies below a sheet of Antarctic ice!  To understand exactly why this is the case, we will examine the relationship between water's density and temperature.

Temperature (T) is a property of a population of molecules; a single molecule does not have a temperatureA single molecule has kinetic energy, a function of its mass and speed.

Density is the mass of an object divided by its volume.  Under "standard" conditions, mass is independent of T.

We can therefore determine changes in density (mass/volume) by measuring changes in volume.



Before we start we will want to insure that we are measuring the properties of pure water.

Water can be contaminated by a number of dissolved solids, gases and other molecules.

The presence contaminants can alter the properties of water.

There are number of methods to prepare pure water.   We will use the simplest and most time-tested, a "still".

Most (but not all) impurities can be removed by distillation.

For example, salts dissolved in the water will not be present in the water vapor– they will concentrate in the solution left behind.

On the other hand, volatile compounds will evaporate, but they may not condense as efficiently, and so their concentration in the final distilled solution will be reduced.

Use Wikipedia | revised 17 December 2004