Temperature
The definition of solubility insists on a stated temperature. This is why.
The method
Do activities 3.1.6 and 3.1.7 again, but instead of the room-temperature water use 100 ml of hot water at about 80 °C each time. Find the mass of solute that dissolves, and see whether it has changed.
The observation
More dissolves. For each solute, a larger mass goes into the same fixed volume of water at the higher temperature than went into it at room temperature.
What it means for the number
The solubility of a substance is not one number for that substance. It is one number for that substance at that temperature. Quote the figure 53.0 g for magnesium chloride without saying 25 °C and you have not said anything precise.
Raising the temperature will increase the solubility of most solutes in a solvent.
Which is why the comparison has to be controlled
To compare how different solutes dissolve in one solvent, the masses that dissolve must be measured in the same volume of liquid and at the same temperature. Change either and you are no longer comparing the solutes — you are comparing the conditions.