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.
Dissolution occurs when the solute dissolves in the solvent.
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 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:
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.
The solubility of KNO3 at 50 degrees C is 80 g/100 g water.
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.
The solubility of liquids and solids isn't much affected by changes in pressure. Gas does show changes in solubility with pressure, though. This is due to Henry's law, which says that the solubility of a gas in a liquid is directly proportional to the pressure of that gas above the surface of the solution. What this means is that if the pressure above the liquid that the gas is dissolved in is increased, more of the gas will be forced into the solution. Soda is a great example of this. Soda is bottled under pressure so that more carbon dioxide is absorbed in the liquid. This carbon dioxide comes out of solution when you open the soda and the pressure is released.
Another great example of this is scuba divers. When divers are underwater, their bodies are under a lot of pressure from the weight of the water. In addition, they are breathing compressed air from their scuba tanks. This combination of increased pressure causes more nitrogen to be absorbed in their blood. As scuba divers leave the depths of the water, they need to rise slowly in order for their bodies to adjust to the lessened pressure and allow the nitrogen to leave their blood slowly. If they don't, the decreased solubility of the nitrogen gas due to less pressure causes nitrogen bubbles in the blood. This causes pain and occasionally death from a condition called the Bends.
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.
A solubility curve shows the relationship between temperature and the solubility of a substance. Temperature affects the solubility of liquids, solids and gases. Pressure generally only affects the solubility of gases.
After watching this video lesson, you will be able to:
- Define solubility and read a solubility curve
- Understand the differences between saturated, unsaturated and supersaturated solutions and identify them on a solubility curve
- Explain how temperature and pressure affect the solubility of solids, liquids and gases