Strange science question
By owatagoosiam
@owatagoosiam (751)
United States
January 5, 2008 9:43pm CST
Ok, related to cooking. If you throw salt in a pot it increases the temperature at which it boils. Food cooks faster. Ok, ok.
There is this home remedy for which I throw I hot pot of boiling water down a stuck up toilet to help it clear. Today, I added Salt to the water to increase the boiling temperature to increase the 'clear the clog' properties. Following me?
So, I add a cup of rock salt to the water. Why use table salt? I'm just going to throw it in a toilet. The water does this interesting thing. It forms bubbles at the bottom, but these bubbles never reach the top. I don't get a rolling boil.
Interesting, I say to myself. I try to imagine what is happening with the water. The best I can think, is that water boils at the bottom and then rises up through salt-water. The salt water is cooler than the layer at the very bottom of the pot and the gas of the 'boiled' water re-condenses before it reaches the surface. The bubbles on the bottom never break the surface, so this is my best theory.
Anyway.. anyone have a chemistry background or cooking background and care to explain this? Anyone use this as a cure for a stopped up toilet. Just idle curiosity.
2 responses
@opinione (749)
• Italy
7 Jan 08
From a chemical point of view the first thing you mentioned is a so called colligative property of solutions: if you add a solute (NaCl, the cooking or the rock salt) to a solvent (H2O, tap water) you are influencing the so called ebullioscopic delta...
An this is just the beginning.
Of course, the bubbles you are describing are made up of a volatile substance which is no more in the solute phase, and is confined in the spherical form at the bottom of your pot in order to minimize the interphace area with the surrounding fluid.
An important relation by Clapeyron of basic (and cooking) chemistry, based mainly on the law of Boyle-Mariotte and on Avogadro principle is the following (which is only partially apt to solve your question, but it is a first step in the right direction if you wanna go further in your 'closet chemistry' research):
PV=nRT
(pressure X volume) = (n°of moles of volatile substance X constant X absolute temperature of the system)
This relation gives you the hint that a volatile substance can have different volumes depending on T but also on pressure on it.
Moreover, if you have enough pressure on your volatile substance you can resolubilize it in the solvent: think to CO2 in a closed can of mineral sparkling water, the gas begins to develop from the water just when you open the can, changing the pressure, or you mix it, adding some mechanichal energy to the system, which enhances the thermal movement of molecules and the T of the system.
So, in my most humble opinion, one of the elements influencing the possibility that our beloved bubbles are reaching or not the ... 'top of the pot' is the pressure of the column of water upon them.
You could prepare such a domestic experience:
a set of pots of different size filled of water at the same temperature ('coeteris paribus': all the surrounding conditions have got to be the same) just at different levels of water.
When you drop your salt (possibly the same quantity in all the pots...) you can measure the level of water making the difference between a pot where the bubbles reach or not the top.
Of course you can use the same pot in more different occasions filled with different quantities of water...
good bye and happy new year
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