Hot flames have a tendency to make problem and as the there is evaporation of solution it infrequently gets overheated and the fluid progresses toward becoming super heated. That regularly makes "bumping" in which the problem area ejects and that is probably going to scatter any solid and on the off chance that you are doing quantitative work that implies the trial is demolished and if not at least it will be a mess, that is why cool flames are important to use instead of hot flames.