Mayim shelanu history?

Question

The water used to bake Matzah must be mayim shelanu—water that had been drawn in the evening and left to cool overnight. What is the halachic background for this requirement?

Answer 1

This halacha is brought down in the Shulchan Aruch 455 where the Mishna Beura explains what it is according to Rashi. Where Rashi explains that in Nissan spring water is hot as the sun heats up the springs (as the location of the sun in that time is close to the earth's ground). There is also another opinion who understands differently to Rashi, how the sun heats the springs up. With regards to making Matzah one is not allowed to use hot water as it will be easy to make it chametzdik. Therefore the Rabbis said one must leave this hot water in a vessel overnight to cool it down.

Answer 2

The purpose of this post is to argue for the reason of the requirement of maim shalanu. Since most people here have never brought a bucket of water from a well or from a spring, and are unfamiliar with chemistry of water which is important, when understanding reaction of chamitza, I will sketch the issues and provide a more detailed comment with references afterwards.

At issue here is purity of water, which is a very serious requirement in ability not to start the chamitza earlier and of general taharah on Pesach. Spring water is "living water" due to very thorough purification. However, spring water or well water usually contains some small particles, which need to be physically removed. These particles are invisible to the eye, however they get noticeable after 12 hours as they fall to the bottom of the bucket. Usual filtering process may do the particle removal (1), however, the chemical ability of water to "preserve memory" of the removed particles does not allow using it right away (2). The simplest solution to separate the particles is to let the bucket stay overnight covered. If there was any foreign mixture in the water of either dust, or sediment, or ions, or molecules, it usually sinks to the bottom overnight. The chemical structure of spring water (3) allows keeping the memory of them for some time and chemically act as if the foreign mixture is still in the water (4). Nevertheless, when water stands overnight, after the particles sink, water molecules reorganize to erase the memory of the mixture that was present. That's what it means that water "cools down", i.e. it no longer has any particles nor their memory to start the chamitza reaction early.

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(1) The history of water purification while going to ancient times, only in relatively recent times has allowed to solve the problem with high degree of purity.

(2) This is also how most of homeopathic remedies work: by diluting the chemical to a very low ratio in spring water, one can preserve its potency and avoid its side effects.

(3) Why does water hold memory of the mixture? The water molecule contains two hydrogen atoms and one oxygen atom. The oxygen grabs electrons from the hydrogen atoms to complete its external electron level, which turns the molecule into a dipole, i.e. one side has a positive charge and another side has a negative charge:

This way, molecules of pure water get organized into regular clusters or "Structured Water" and it behaves like a crystallic liquid, as the clusters form and break to join other clusters. For example, in clouds water particles turn into "living water" as they get purified to such a degree that they forms regular hexagonal snowflakes. Liquid living water is similar, as it keeps crystalline hexagonal structures.

(4) The crystalline properties of water have effect on the outcome of chemical reactions as shown by chemists Ralf Ludwig, Martin Chaplin, and others relatively recently. Their experiments also show how these properties change if water has presence of mixture or its memory.