Water pots

Joaquín Sevilla12 Dic. 2014

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…

Why “botijos” cool water?

And why canteens wear “coats”?

It’s not only that botijos keep water fresh,

they actually lower its temperature

Surely we have seen aluminum

canteens with a cloth cover

(particularly felt)

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?

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

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!!

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

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

Going back to our questions

• Why “botijos” cool water? Because of evaporation

• Why canteens wear felt coats?

To be wet and evaporate

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/

Why do pressure cookers cook quicker?

Because they are under pressure, but…

They do indeed, between 3 and 5 times depending on the food

What is cooking?

“Cooking” is the transformation

of food though a set of

chemical reactions that take

place at high temperature

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

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

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

Higher pressure, higher temperature, less time

Pressure cookers are pressurized flasks

Typically 2 atm (absolute),

that correspond to 120ºC

Water phase diagram

The first pressure cooker, XVII

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

Funny experiments…

Where does the sound of the kettle come from?

Nucleate boiling

Nucleate boiling

Nucleate boilingConvection 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

Nucleate boilingConvection 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

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

Accumulated vapor develops pressure, and force

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

12 Dic. 2014