Just wanted to share some information on building water from scratch.
Mr. Beer kits are generally comprised of HME which when dissolved and diluted in water create a wort with a certain pH.
pH is a measure of acidity or alkalinity and has a scale of 0 - 14. A pH of 7 is neutral (neither acid or alkaline). On this scale, acidity is less than 7 and alkalinity is greater then 7.
When the HME is made it's pH is set during the brewing process. All-grain brewers will normally adjust their mash pH to 5.2-5.6 to maximize extract and minimize tannins and other unwanted organics. The boil pH is also adjusted separately from the mash pH. It is set at the beginning of the boil to optimize hop oil extraction (5.2) and at the end of the boil to accommodate the amount of acidity added by the yeast while fermenting - such that the final beer arrives at a pH of approximately 4.2 - 4.8 or for a sour beer 3.5 or less.
Why is that important? Well if you dissolve your HME in neutral water (distilled or deionized) the HME will bring the pH of that water to approximately that which was intended by the brewery. The problem is that there are no minerals in the neutral water thus relying on the minerals from the HME to re-mineralize. However, if the HME itself was made with mineral deplete water there's not many minerals to propagate. Thus you end up with a dull tasting beer.
Let's take the other end of the spectrum and say you use tap water. Tap water is normally alkaline and has a pH of 8 - 9. Mixing your HME with this water will result in a wort with a higher than optimal pH. Remember that the optimal end of boil pH is 5.2, however in these circumstances you would end up with a pH of 6 - 7, perhaps with a darker HME and if you're lucky, it may be in the high 5's. After adding the yeast you'll wind up with a beer that has a much higher than optimal pH.
This is where the long conditioning times come into play and why darker HME's are more successful than lighter ones. During the room temperature conditioning period, the yeast are trying to clean up their by-products as well as to bring the pH of the beer down to where they're comfortable living, though they do tire out and eventually drop out of suspension. Darker HME's are more acidic than the lighter ones therefore the alkalinity of the tap water has less of a negative effect when making those types of kits.
Let's talk about water.
Types of water:
Tap - City tap water is normally treated with chlorine or chloramines. Chlorine can be eliminated by boiling, setting the water out for a period of time and campden tablets which remove both chlorine and chloramine.
Well - Well water can have chlorine if it has been "shocked" to remove iron or rust bacteria, however, most well water should be free from chlorine and chloramines.
RO - Reverse Osmosis water has been run through a RO filter which removes most of the minerals. Before running through the RO filter this water must be run through a chlorine/chloramine filter.
RO/DI - Reverse Osmosis and Deionized water has been through the RO setup and through Deionization filters which remove the remaining minerals. Deionized water is the most pure water you can obtain.
Distilled - Distilled water has been run through a distillation unit and is almost as pure as DI water because certain oils can make it through the distillation process.
Softened - Softened water has been run through a water softener which removes certain mineral ions (mostly calcium and magnesium) as well as metal cations by exchanging them with sodium. Softened water should be used cautiously for brewing as it may contain slightly higher sodium or potassium content which may adversely affect the beer. This is not to be confused with "soft" water which is simply water that has very little mineral content.
Alkalinity vs. Hardness
Alkalinity is a measure of resistance to change in the pH of the water. Alkalinity is neutralized using acid additions.
Hardness is a measure of the amount of mineral content in the water. Hardness is eliminated by using a RO/DI system or RO/DI, distilled or naturally soft spring water.
Saying that you have "hard" water means there's a lot of minerals in it (good and/or bad). Saying that you have "alkaline" water means that it is very resistant to change in pH.
Both can adversely affect beer. High alkalinity buffers against pH change by the grain in the mash and the yeast in the fermentation. High mineral content adversely affects the flavor of the beer but also plays a smaller role in pH.
Chloride - provides a fullness and perceived sweetness to the beer
Sulfate - provides a dryness and sharpness of flavor of the beer
Magnesium - provides a sour bitterness to the beer
Sodium - enhances the flavor of the beer in limited quantities
Calcium - provides for yeast health during fermentation (as well as enzyme activity during mashing)
Minerals are added to water by using "water salts":
Calcium Sulfate (Gypsum)
Magnesium Sulfate (Epsom Salts)
Sodium Chloride (Kosher table salt)
Any of these salts are available at your LHBS or in your local pharmacy or grocery store.
Acids are used to reduce or neutralize alkanity in water during the brewing process:
Lactic acid - a very popular acid that adds the lactate ion to the beer
Phosphoric acid - a very popular and neutral acid does not add flavor to the beer
Hydrochloric acid - not very popular on the homebrew scene, requires great care and caution as it is very strong
Sulfuric acid - not very popular on the homebrew scene, requires great care and caution as it is very strong
Lactic and phosphoric acids are available at your LHBS.
Let's suppose that I want to make the Classic American Light HME and I've done it before using my tap water and it just didn't turn out. We've previously seen that this is due to high alkalinity in the tap water and not much acidity in the light colored HME. This time we have a choice, use the tap water and neutralize the alkalinity using an acid or start with distilled water. We'll start with distilled because we haven't had our tap water tested to determine it's mineral content.
Following the Mr. Beer instructions we add the four cups of distilled water to our pot and we add 1/8 teaspoon of calcium chloride and an 1/8 teaspoon of calcium sulfate and continue to bring to a boil. Knowing that these minerals will enhance our perception of fullness and sweetness and increase the effect of the small amount of hops in the HME. We follow the rest of the Mr. Beer instructions using distilled water to complete the brew. Perhaps the next time we make this can of HME we'll try a 1/4 teaspoon of calcium chloride or calcium sulfate and none of the other to determine which we like best.
If we had a pH meter we could even add lactic or phosphoric acid until we hit a target pH of approximately 5.2 (before fermentation).
How much water salts to add:
Water calculators or spreadsheets were made for this purpose and most of them center around all-grain brewing. They can be used, however, to determine how much of each salt needs to be added to reach a certain mineral level in the brewing water. They also provide different water profiles for various brewing regions (not that you should attempt to match or even brew with those).
Bru N Water - a popular spreadsheet for homebrewers - https://sites.google.com/site/brunwater/home/files
EZ Water - http://www.ezwatercalculator.com/
Getting your water tested:
If you're going to continue to use your tap water and want to experiment with using an acid to neutralize alkalinity while building upon your current water, then you'll want to get it tested to determine it's mineral content and alkalinity.
Many labs will do this for you and there are home test kits available.
You'll also want to purchase a good pH meter for testing purposes.
Water is a complex subject until you "get" it - after which point it becomes mostly second nature.
While this topic will probably not be that popular with Mr. Beer users and most will continue to just use their tap water, it is interesting to note that excellent beer can be produced by adding minerals back to neutral water (or even some tap waters).
For those that are interested, there are many excellent resources, both books and online, concerning water and brewing:
Thanks for reading and hope some will continue to learn more and experiment with their water.