The freezing point constant can be compromised by various causes, for example, the amount of solute that dissolves, its nature, and the amount of ions into which the solute can dissociate. The water that comes from the tap and that you could use to prepare the solutions contains various ions, such as fluoride ions, chloride ions, etc., which would not exist in distilled water. The presence of these ions previously causes the freezing point to decrease, therefore, using tap water in this experiment would increase the kf value (greater than the real one) for this system.
The Kf (Freezing Point Depression Constant) is been affect by freezing point among many other factor now using tap water for this experiment would mean that there would be salt present in the solution in that tap water is usually added fluoride which would react with the natural potassium present in the water to form a salt this salt would depress the freezing point of the water hence the value of Kf would be bigger that the actual value
