The gemara[2] says that one must make a beracha before eating because one may not benefit from the world before thanking HaShem. However, Chazal, who instituted the specific rules and formulae of berachot, distinguished between different types of benefit. For benefits received from food, there are berachot; for medicinal benefits, no berachot were instituted.[3]
The primary property of a food item that is responsible for generating the obligation to make a beracha on it is its taste. Because of the beracha-related importance of taste, if one eats a food for medicinal reasons but also enjoys its taste, he recites the food’s regular beracha.[4] Water is an exception to the rule in this respect since it is assumed to lack a positive taste. If so, why do we ever make a beracha on water?
The mishna[5] says that one recites a beracha on water when it serves to quench his thirst. The gemara[6] says that this comes to exclude a case where one drinks water to rinse down something that is caught in his throat. The gemara does not say what to do if one drinks water neither because of thirst nor to get something out of his throat. Nevertheless, the consensus of the poskim is that one makes a bracha on water only if it serves to quench thirst or something equivalentHaHH.[7] Therefore, if one drinks water to swallow a pill, he does not make a beracha before or after drinking.[8]
Generally, water’s thirst-quenching property is equivalent to other foods’ property of taste. Therefore, it is logical that if one drinks water for medicinal purposes but also happens to be thirsty, then he does make a beracha, as the Mishna Berura[9] confirms. However, there is a difference between water and a tasty food or drink: When someone eats a tasty food for medicinal purposes, our standard assumption is that he will also get the benefit of its taste. However, the standard assumption is that if one is not consciously thirsty, drinking water for medicinal reasons will not provide the type of thirst-quenching benefit that warrants a beracha.[10]
Regarding the procedure you describe, although the water containing dissolved powder is drunk for medicinal purposes,[11] since the examinees fast for several hours before drinking, one can assume that they are thirsty. Thus, unless one notes that he is not thirsty, he should make a beracha before and after drinking the solution. If one is not sure, he cannot solve the problem by your suggestion of drinking some plain water first: water that is consumed in order to solve a halachic problem is not water that is consumed for thirst and, therefore, does not warrant a beracha.[12] This is, in general, an important concept to remember. Often, a person does not know what beracha to make on a particular food and wants to solve the problem by making a Shehakol on water and thereby covering the food in question. Based on what we have seen, if he is not thirsty, the beracha on the water is itself a beracha l’vatala.[13]
If the examinees are allowed to eat a small amount of something else, that would be a way of removing doubt. After the test, one can eat and drink something (other than water) that warrants the same beracha acharona.[14] Nevertheless, excluding the case where one notes he is not thirsty, one can confidently make the berachot before and after.