1) Botijos and canteens with cloth covers cool water through evaporation. As water molecules gain energy and evaporate from the surface, they carry away heat in the form of latent heat of vaporization. 2) Pressure cookers cook food faster by raising the boiling point of water. At higher pressures like 2 atm, water boils at 120C rather than 100C, allowing chemical reactions in foods to occur more quickly. 3) The sound of a boiling kettle comes from the process of nucleate boiling, where vapor bubbles form on imperfections in the kettle surface and rapidly grow and collapse, creating an audible noise.

1. Water pots
Joaquín Sevilla
12 Dic. 2014

2. Everyday science
• It’s easy to appreciate the power of science in high tech
equipment
•However, in the most daily things there is also plenty of
science
• Let’s see some examples in the following…

5. Why “botijos” cool water?
It’s not only that botijos keep water fresh,
they actually lower its temperature
And why canteens wear “coats”?
Surely we have seen aluminum
canteens with a cloth cover
(particularly felt)

6. Water cools when evaporates
These fresh water flasks work because of the physical fact:
“when water evaporates it removes heat form its
surroundings”
But, what is precisely “evaporation”, is the same as boiling?

7. Obviously evaporation is not boiling
• Evaporation is a type of vaporization of a liquid that occurs
from the surface of a liquid into a gaseous phase that is not
saturated with the evaporating substance.
• Boiling is the vaporization of a liquid which occurs when a
liquid is heated to its boiling point, the temperature at which
the vapor pressure of the liquid is equal to the pressure
exerted on the liquid by the surrounding environmental
pressure.
Wikipedia definitions

8. Evaporation vs. Boiling
• Boiling takes place in the bulk of the liquid and
depends on the temperature
• Evaporation is happens in the surface and is (almost)
non dependent on the temperature
Obviously shocks do not boil
when drying!!

9. Why water cools when evaporates?
In the random
movement of the
molecules, when
one gets energy
enough, it leaves
the liquid,
and takes this
energy with it !!
This energy, averaged macroscopically can be
measured and is called: latent heat of vaporization

10. How much vaporization cools?
• Just go to the tables (or the internet) and look up
“latent heat of vaporization”
• at 0ºC 2.50 · 106 J/kg 595 cal/g
• at 20ºC 2.45 · 106 J/kg 585 cal/g
• at 100ºC 2.26 · 106 J/kg 540 cal/g
The details on the process are not simple. One can discover the variables
that are relevant thinking what to do to better dry your laundry:
increase contact surface (hanging it), dry air, renewed air (some breeze),
and warm air.
But maybe we are going too far

11. Going back to our questions
• Why “botijos” cool water? Because of evaporation
• Why canteens wear felt coats?
To be wet and evaporate

12. BTW, this effect can be precisely calculated
But we are not interested in these details.
Just keep in mind that science is also
quantitative
• See: http://www.enchufa2.es/archives/la-ecuacion-del-botijo.html y
http://www.meteored.com/ram/1797/la-cermica-del-agua-y-su-relacin-con-la-aridez/

15. Why do pressure cookers cook
quicker?
They do indeed, between 3 and 5 times depending on the food
Because they are under pressure, but…

16. What is cooking?
“Cooking” is the transformation
of food though a set of
chemical reactions that take
place at high temperature

17. What is cooking?
“Cooking” is the transformation
of food though a set of
chemical reactions that take
place at high temperature
Hot air in the oven
(bake)
200ºC
Boiling oil
(fry)
175ºC
Boiling water
(boil)
100ºC

18. Evaporation vs. Boiling
• Evaporation is a type of vaporization of a liquid that occurs
from the surface of a liquid into a gaseous phase that is not
saturated with the evaporating substance.
• Boiling is the vaporization of a liquid which occurs when a
liquid is heated to its boiling point, the temperature at which
the vapor pressure of the liquid is equal to the pressure
exerted on the liquid by the surrounding environmental
pressure.
Wikipedia definitions

19. How long does it take to cook?
• Depends on the temperature.
• The higher T, the quicker the
reactions take place
• Boiling temperature depends
on the surrounding pressure
• That’s why boiling an egg at
the top of a mountain can take
really long

20. Higher pressure, higher temperature,
less time
Pressure cookers are pressurized flasks
Typically 2 atm (absolute),
that correspond to 120ºC

21. Water phase diagram

22. The first pressure cooker, XVII

23. Funny experiments…
Esta reseña está tomada de este magnífico vídeo,
http://www.youtube.com/watch?v=TH9R0J36n7w&feature=g-wl
Parte de un curso entero de ciencia y gastronomía de Harvard que está disponible entero

26. Where does the sound of the kettle
come from?

27. Nucleate boiling

28. Nucleate boiling

29. Nucleate boiling
Convection and
nucleate boiling
require gravity. It was
demonstrated in the
ISS en 2001
This process is particularly
efficient for heat transfer.
Nuclear reactors (PWR and BWR)
are designed to work with it

30. Nucleate boiling
Convection and
nucleate boiling
require gravity. It was
demonstrated in the
ISS en 2001
This process is particularly
efficient for heat transfer.
Nuclear reactors (PWR and BWR)
are designed to work with it

33. How does the water moves from the
lower to the upper part?

34. Accumulated vapor develops
pressure, and force

36. To summarize…
• Not only high tech is full of science
• These humble water pots have a lot to show
•Water phase change from liquid to gas is more
complicated than it seemed, and really useful

37. 12 Dic. 2014