# Solubility and Solubility Curves

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• 0:05 Solubility
• 2:05 Solubility Curve
• 3:41 Temperature and Pressure
• 5:51 Lesson Summary

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Lesson Transcript
Instructor: Amy Meyers

Amy holds a Master of Science. She has taught science at the high school and college levels.

Learn what solubility is as well as the definitions of 'saturated,' 'unsaturated' and 'supersaturated.' Learn how to determine the solubility of a substance in water by using a solubility curve. Discover the effects of pressure and temperature on the solubility of liquids, solids and gases.

## Solubility

You have previously learned about dissolution and how a solution is made, but let's look at some of the vocabulary again. When a solute in a solvent forms a solution, it is called dissolution. A solute is the primary substance that is dissolved in a liquid called the solvent. So, what this means is dissolution is when something dissolves in something else. There are many factors that affect the rate at which a solute will dissolve, including pressure and temperature, among others.

So, you've learned about dissolution and how a solution forms. In this lesson, we are going to talk about solubility. Solubility is the amount of a solute needed to form a saturated solution at a specific temperature and specific solvent amount. Solubility is referred to in three different terms: saturated, unsaturated and supersaturated.

A saturated solution is a solution that contains the maximum amount of dissolved solute. If a solution does not contain the maximum amount of solute, it is unsaturated. A supersaturated solution is a solution that contains more than the maximum amount of dissolved solute than a saturated solution under the same conditions. I know this sounds impossible, but you will see what I mean.

Do you drink iced tea? Do you like sugar in your iced tea? What happens if you put too much sugar in your tea? (Other than it gets too sweet to drink.) You often find some undissolved sugar at the bottom of your glass when you are finished with the drink. This is because your tea was already saturated with sugar, and it couldn't take any more into solution.

You could have made the tea supersaturated, though. You would do this by heating the tea. When the temperature of the tea is increased, it can absorb more sugar because it has a higher saturation point. When you cool the tea, the sugar stays in solution and is supersaturated at a lower temperature than it could have been if you had just started with a cool solution.

## Solubility Curve

Solubility is measured by determining the maximum mass of a solute that can be dissolved in 100 g of a solvent at a given temperature. Scientists plot the relationship between the temperature and solubility of a particular solute in a solubility curve. In general, the solubility of a compound will increase as temperature increases.

Here is an example of a solubility curve:

It has temperature plotted against grams of solute per 100 g water. This is what you will typically see on a solubility curve. The lines on the solubility curve indicate a saturated solution - a solution that has the maximum amount of solute dissolved in 100 g of water. Any amount below the line for a particular substance shows an unsaturated solution, and anything above the line shows a supersaturated solution.

As you can see from the curve above, NH3 and Ce2(SO4)3 are two compounds whose solubility decreases with increasing temperature because their slope is downwards.

From the solubility curve, you can determine the solubility of a substance at any temperature between 0 and 100 degrees C. For example, what is the solubility of KNO3 at 0 degrees C? Around 13 g per 100 g water. What about at 50 degrees C? It is 80 g per 100 g water. So, the solubility of KNO3 increases more than six times between 0 and 50 degrees C.

## Temperature and Pressure

Solubility is sensitive to both temperature and pressure. In general, solubility increases with temperature. When a solid is dissolving in a liquid, the solid needs to essentially 'melt' or break apart in order to dissolve. Heat, or energy, is needed to break the bonds and 'melt' the solid. When you increase the temperature of a solvent, you increase the kinetic energy, or energy of movement of the molecules, and this greater energy helps 'melt' or dissolve more of the solute molecules. The exceptions to this, such as NH3, which decreases in solubility with temperature, is because when these molecules dissolve or break apart, heat is given off, so adding more heat to this reaction has a negative impact.

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