Hot water rising


Two glasses full of water are placed mouth to mouth with a separator.  One contains coloured hot water and the other cold water.  The separator has a hole which when opened allows the two liquids to mix.  If cold water is above hot they mix rapidly and effectively.  If the hot water is above the cold the mixing is much less rapid.


  • Four glasses
  • Hot and cold water
  • Four old compact disks
  • Food colouring


Fill two glasses brim full with cold water (the colder the better) and two with hot water (the hotter the better). Colour just the hot water or just the cold water.  Offset the holes in the centres of two compact disks and put them on one of the glasses that is full of hot water.  This glass can be placed on top of a glass of cold water.  When the holes in the compact disks are aligned the liquids can mix.  Repeat with the other pair of glasses but placing the cold water on top of the hot this time.


In the majority of cases, warming a substance causes its volume to increase. Hence the same volume of substance contains less mass, in other words the substance has become less dense. This is an example of Archimedes’ Principle in action. A body partially or totally immersed in a fluid experiences an upthrust equal to the weight of fluid displaced. If we imagine taking a 1 cm³ sample of our hot water and immersing it in the cold water, it will displace 1 cm³ of cold water. We know that 1 cm³ of hot water contains less mass than the 1 cm³ of cold water it displaces, because it is less dense. In which case, the upthrust (from the cold water on the hot water) is more than the weight of the warm water, so the warm water will experience a net force upwards. The mixing of water (or not) seen here is the result.


Instead of two CDs use one CD and some sort of thin plastic sheet (such as an acetate sheet from an overhead projector) to seal the glass.   In this case the hole in the middle of the CD can be enlarged and the effect is quicker.

A demonstration of hot water rising.

A demonstration of hot water rising.


Be very careful with hot water in this experiment. If you have the correct disks a nice seal can be formed, often, however, hot water escapes when manipulating the hot glass. Do not use glass containers for this – rigid, transparent plastic glasses or similar containers are best.

Tags: density, pressure, hot water, hot water rising.