If they are single part dry
nutrients (complete), mixing them in warm water can cause another problem. While it is true that mixing in warm water will help completely dissolve the mineral salts, mixing them in small volumes of water can cause the the mineral elements to
precipitate. Taking 10 gallons of dry
nutrient mix, and mixing it in one gallon of water (warm, hot, or cold) creates a liquid concentration ten times stronger than normal. The mineral elements can bond almost emeditly, and even diluting it later in more water wont free the bonded elements once they've precipitated.
When in high concentrations (and/or high pH levels) some elements will bond to other ones, becoming un-usable to the plants. That's why most liquid nutrient concentrates come in more than one part mixes. To keep the elements that will bond to others separated. Often times the precipitation can be seen at the bottom of the reservoir as a white powdery looking substance. This can often be confused as nutrients that didn't dissolve. But is actual nutrients that DID dissolve, but then "re-solidified" when bonding to other elements because of high concentrations, high pH, or both.
P.S.
If there two, or three part dry nutrients, then pre-liquefying each part "
separately" in warm water before pouring into the full volume of water in the reservoir should be OK.