 We have learned about the solubility rules in a different lesson. These rules help us predict whether or not a salt is soluble in water. For salts that are soluble in water, some are more soluble than others at a given temperature. In this lesson, we will learn about solubility curves, which is a graphical representation of the solubility of a certain salt over a temperature range. If we were to compare copper-2 sulfate and potassium sulfate, we will find that one is bright blue and the other is white. If we try to dissolve them in water of equal temperature, we will find that copper-2 sulfate is a lot more soluble than potassium sulfate. In other words, copper-2 sulfate has a higher solubility in water. This is actually a physical property of a substance, much like a substance melting point and boiling point. The solubility of a salt is usually measured as grams of salt per, let's say, 100 grams of water. Here is a challenge for you. Do you think solubility increases or decreases with temperature? Pause, think, and continue when you have an answer. The answer is that solubility usually increases with increasing temperature. So, if we dissolved copper-2 sulfate and potassium sulfate in hotter water, we will find that more of each salt is able to dissolve. This observation is only applicable to a solid dissolving in a liquid. We can measure how much of a given salt can dissolve in 100 grams of water at a certain temperature. We can plot solubility as a function of temperature to give a solubility curve. Here are our solubility curves for copper-2 sulfate and potassium sulfate. We can see that for all temperatures, copper-2 sulfate is more soluble than potassium sulfate. From a solubility curve, it is also possible to find solubility at a certain temperature. Let's say we wanted to find the solubility of copper-2 sulfate at 90 degrees Celsius. Take a ruler and draw a vertical line starting at the x-axis at 90 degrees Celsius until it reaches the curve. Then, draw a horizontal line from the point on the curve until it reaches the y-axis. At 90 degrees Celsius, approximately 68 grams of copper-2 sulfate is soluble in 100 grams of water. To compare at the same temperature, only 24 grams of potassium sulfate would be soluble in water. Using the provided graph, try to find the solubility of potassium sulfate at 60 degrees Celsius. Pause, try, and continue when ready. If your answer is around 18 grams of potassium sulfate for 100 grams of water, then you would be correct. When a maximum amount of a certain salt is fully dissolved, the resulting solution is known as a saturated solution. In a saturated solution, no more solute is able to dissolve. If we took the saturated solution and let it cool, it would become a super saturated solution. This means that there is more solute than the maximum amount able to dissolve. When this happens, some of the solute may crystallize or come out of solution.