AIChE SU SC· 2018 ·

Seems weird but still inside sciences box

Can you drink a cup of tea with a temperature of 69 ºC?

Think well before answering! One of the conditions that must be met to make Cuba from the tea is to boil the water. Is boiling temperature of water at 69 ° C? Think well!! Fluids have a very important characteristic, such as density and viscosity, which is a vapor pressure feature The liquid in the normal state consists of molecules, some of which try to escape from the surface and release from the liquid state of the gas or vapor state, which is more random, If the molecule succeeds in this task, thanks to its kinetic energy, The particles released from the surface of the liquid and transformed from the liquid phase to the gas phase cause pressure to be exerted on the surface of the liquid ... When the liquid and vapor are stabilized and reach to equilibrium, meaning that (No more particles evaporate) at a certain temperature, The pressure of the transformed molecules of the steam ,and exerting on the liquid surface is called Vapor pressure.

Vapor pressure: pressure of vaporized molecules on its liquid surface at equilibrium between liquid and vapor at a certain temperature.

As the temperature increases; the kinetic energy increases and the liberated particles increase. Thus the vapor pressure of the liquid increases.

So, what if air pressure is less than 1atm?!

Does the vapor pressure increase in this case or decrease? When the barrier that blocks the molecules is reduced in the way of the liberation and transformation of the gaseous state, what prevents the particles from being released more and more into the gas state? Thus, vapor pressure will increase.

So, the vapor pressure is increased by increasing the temperature or by decreasing pressure on the surface at a certain temperature, when all particles have kinetic energy that enables them to be released and can be transformed into a gaseous state. This degree is called the boiling point, for water; it is equal to 100°C at atmospheric pressure, at this degree - boiling point – the vapor pressure increases to be equal to the atmospheric pressure, what inhibits the molecules during the transformation of the whole vaporization? The answer is nothing.

Therefore, increasing the pressure increases the boiling point until reaching the critical point where the properties of liquid and gas are in dynamic equilibrium.

The boiling point cannot be higher than the critical point, the boiling point decreases by reducing air pressure until reaching the triple point. The boiling point cannot be less than this point.

How does air pressure less than 1atm? When we rise from the sea level, at the top of Mount Everest, which is about 9 km above sea level, the air pressure drops by a large margin and the boiling point of the water is reduced to 69°C and hence you can drink a cup of tea with boiling water at this rate.

But it will cost you the rise of a high mountain, which is massive exhausting task!! and full of danger.

Only now can you explain alone why the water boils at 110ºC at the Dead Sea, which is 350 meters below sea level.

Written by :

Ahmed Saad Farhat

Operation Engineer
Tci Sanmar Company


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