Can foods such as salt, sugar, honey, etc. be added to hot drinks and food on Shabbat? What about adding cold water to cool off hot water?
The problem that might exist with adding salt, sugar etc, to hot drinks and food on Shabbat is that one may be cooking them. This depends on two factors - firstly is the thing we are adding (the salt etc) allready cooked, and as such halachically there is no prohibition of "re-cooking" it, or are they still not fully cooked, and the prohibition of cooking it applies. Secondly, is the drink or food that they are being added to hot enough, and halachically "strong" enough to cook the additives, even if they are uncooked?
There are different types of salt - some cooked in their preparation and some not. This is true of honey and many other types of additives. If we know they have indeed been cooked already they could be added to any hot drink or food. (There are those who are strict and do not add things that will dissolve, such as salt and sugar, to a "first vessel", that is the pot that the food was cooked in on the fire,and also do not pour directly from the Shabbat kettle onto these "dissolvables", but only add them to the drink after it has been poured out into a cup or bowl. This is a stricture that is widely accepted). For example, instant coffee and sugar are cooked during their manufacture, and may be added to a cup of hot water.
However, if you are not certain that any given product was fully cooked in its manufacture, then you should only add it to a "third vessel" - that is, pour the water from the kettle into a cup or jug (=the second vessel) and from there into an empty cup (=the third vessel) into which you can now add anything you like - cocoa powder etc. Even though there are opinions that are either more lenient or stricter - this opinion is widely accepted. (See Shmirat Shabbat chapter 1, 53-56).
If you want to add such things to a hot solid food, there are opinions that treat the hot food as a first vessel, and as such do not put raw substances on it. If however the food is cooled sufficiently as to be comfortably touched, all foods may be put in. All salt may also be put on all hot solid foods, (See Shmirat Shabbat 1, 58).
Cold water may be added to hot water to cool it off if the hot water is in a second vessel - that is if you have poured hot water from the Shabbat kettle into a cup, you may now add any amount of cold water (or even an ice-cube) to the cup.