I PASSAGGI DI STATO

STATE TRANSITIONS

State transitions are the transformation of matter from one physical state (solid, liquid, or aeriform) to another by changes in temperature and pressure.

The change of state is a physical transformation and not a chemical one, because the composition

of the substance is not altered, only how the particles are bound together.

To understand the transition of state, let us use a very common example: water inside a pot boiling, which consequently creates water vapour.

Or, at least, most people think it is steam!

WHAT IS WATER VAPOUR?

To understand what it is, let us start with the clouds.

The process that leads to the formation of clouds occurs when a portion of moist air cools and then condenses.

And condensation is the transition from the aeriform state to the liquid state.

So the clouds are in a liquid state and are made up of trillions of microscopic water droplets.

The same thing happens in our pot: the water evaporated from the boiling pot, upon meeting the room-temperature air in our kitchen, cools and thus condenses. And the condensation process identifies the transition from the aeriform state to the liquid state, just as we have seen with clouds. Our steaming pot is emitting tiny droplets of liquid water (liquid state) and not water vapour (aeriform state)!

So, in conclusion, our cloud, the smoke from the pot, is liquid water.

What comes out of some smokestacks or aircraft is also to be considered in the same state, and in these cases can easily condense due to aggregation with the solid waste particles of the relevant combustions, the so-called pollutants.

BUT THEN, WHAT DOES WATER VAPOUR LOOK LIKE?

Just remember the 3 ‘i’s rule:

1) Water vapour is INVISIBLE: it is contained in the air, but we do not see it until it condenses into water. We can, however, measure it with hygrometers.

2) water vapour is INCOLOUR because you cannot see it.

3) Water vapour is odourless: the air has no odour but takes on any fumes from other gases.

But then, what is the difference between a cloud and rain if both are formed by water droplets?

The difference lies in the size of the droplets: the droplet of a cloud measures about 30 thousandths of a millimetre (it is very light), whereas a raindrop has an average size of between 1 and 3 millimetres and is so heavy that the air no longer supports it; hence the first light droplets remain suspended in the sky (clouds) and the other heavier droplets fall to the ground (rain).