[paul.fr]

i just filled a small pan with water, added lots of blue food colouring and stirred until the colouring was well mixed in and the water had turned blue.

I then boiled the water, why then did i not get blue steam?

[daveshorts]

Two main reasons

1) the food colouring was probably less volatile than the water so it stayed in the pan and didn't evaporate

2) Steam is strongly white because it is made up of lots of tiny droplets and you get reflections and refractions very strongly at each air - water interface, these reflections and refractions are not dependent on the colour of the liquid in the drop. They are so strong that most light that is reflected back out probably only goes through a very few droplets, and as the droplets are very small it passes through very little actual water. You would need an awful lot of food colouring to make <.5mm of water look strongly coloured.

[paul.fr]

Two main reasons

1) the food colouring was probably less volatile than the water so it stayed in the pan and didn't evaporate

i though of that, so boiled the pan dry. there was nothing left in the pan.

2) Steam is strongly white because it is made up of lots of tiny droplets and you get reflections and refractions very strongly at each air - water interface, these reflections and refractions are not dependent on the colour of the liquid in the drop. They are so strong that most light that is reflected back out probably only goes through a very few droplets, and as the droplets are very small it passes through very little actual water. You would need an awful lot of food colouring to make <.5mm of water look strongly coloured.

this i did not think of, thanks Dave.

[Batroost]

Steam is strongly white

Actually no. True Steam is invisble because it consists of separate water molecules. What you're describing is water vapour which, as you say, is made up of droplets (lots of molecules clumped together in a liquid state).

Another thing to consider is whetehr the food colouring has a colour when it isn't disolved in water? Some common chemicals are colourless until you get them wet.

[daveshorts]

True, steam is invisible - although so is water vapour, which is gaseous water below it's boiling point http://en.wikipedia.org/wiki/Water_vapor .

I guess cloud is the correct term for a suspension of water droplets which you can see - although I think this is implied by cloud of steam... or cloud of water vapour.

[Batroost]

Fair point - sloppy on my part.

One of the best illustrations of the invisible nature of steam that I saw was when the High Pressure Safety Relief Valves lifted at Hinkley Point 'B' Power Station. You knew it had happened because of the roaring noise but you couldn't see the steam unless you looked way up! I'm guessing that the steam didn't condense enough to be visible as droplets until it was about 50m above the power station. MInd you it starts out at 600 degrees Celsius so it has a lot of cooling down to do...

Another thought, did teh blue colouring fade as teh water boiled? Might it have been unstable at high temperature?

[lyner]

The blue dye in the water will appear blue because the light is passing through  a significant   number of dye particles. The effective thickness of the dye in the air occupied by the 'steam' or, rather,  tiny water droplets would be much less. What you would see would be dominated by the whiteness caused by scattering of light by these droplets. The effective density of the dye (even if it still were  still blue in its vapour state) would be very low compared to the density whilst in solution.
 You do get 'coloured steam' on occasions, when there is enough pollutant in the steam cloud.