Let's say that you are in your chemistry laboratory, and you need to figure out how much salt you can dissolve in water at 30 °C without any salt being left undissolved at the bottom of the glass. How can you do this? Let me tell you that there is a graph that is very useful in these situations, and it is called a Solubility curve.
So, if you want to learn more about Solubility curves, read on!
- This article is about the solubility curve.
- First, we will discuss solubility and define what a solubility curve is.
- Then, we will learn how to read a solubility curve.
- After, we will talk about saturated, unsaturated, and supersaturated solutions.
- Then, we will look at the solubility curve of a gas.
- Lastly, we will solve some practice problems similar to what you can expect on your exam.
Solubility Curve Definition
Before diving into solubility curves, let's review solubility. A solution is a homogenous mixture of a solute dissolved in a solvent. For example, when salt completely dissolves in water, it becomes a solution. But, how can we know how much salt can be dissolved in water? Well, this is where solubility comes into play!
Solubility is referred to as the maximum amount of solute that can be dissolved in a solvent. In other words, solubility is a measure of how much solute will dissolve in a solvent!
Solute solubility is dependent on different factors such as temperature, pressure, and nature of solute/solvent. Temperature affects solutes differently depending on their state of matter: Solid solutes increase in solubility as temperature increases, whereas gas solutes decrease in solubility with an increase in temperature.
Pressure also affects solubility. When pressure increases, the solubility of solid solutes remains the same. Now, when pressure increases, the solubility of gas solutes also increases.
Polarity is also an important factor when dealing with solubility, and it has to do with a phrase you have probably heard before: "Like dissolves like". This means that if the solute and the solvent have the same Polarity, then the solute will dissolve in the solvent.
To show the solubility of a substance at different temperatures, chemists use solubility curves. The definition of a solubility curve is shown below.
A solubility curve is a graph that shows the number of grams of solute per 100 g of solvent that can be dissolved at different temperatures.
How to Read a Solubility Curve
Let's take a look at what a typical solubility curve for a solid solute dissolved in water (H2O) looks like and how to read it. The x-axis of a solubility curve represents the temperature (°C), whereas the x-axis shows the solubility in g of solute per 100g of solvent.
In a solution, the solubility limit (or saturation point) refers to the point where no further solute can be dissolved.
Figure 1. Typical solubility curve for a solid solute in water, Isadora Santos - StudySmarter Originals.
Solubility Curve Graph
To better understand solubility curve graphs, let's take a look at the graph of calcium bromide (CaBr2) in water (H2O). Notice that at 0 °C, 122 grams of CaBr2 can dissolve in 100 grams of water. As temperatures increases, the solubility of calcium bromide increases. For example, at 50 °C, the solubility of CaBr2 in H2O is 270 grams per 100 grams of H2O.
Figure 2. Solubility curve of CaBr2 in water, Isadora Santos - StudySmarter Originals.
Calcium sulfide (CaSO3) has an interesting solubility curve graph: the solubility of CaSO3 decreases with an increase in temperature! For example, at 20 °C, 0.00590035 grams of CaSO3 can be dissolved in 100 grams of H2O. However, at 100 °C, its solubility is 0.00190004 g/ 100g H2O.
Solubility Curve: Saturated, Unsaturated, and Supersaturated
When it comes to solubility, there are three important terms you should remember: saturated, unsaturated, and supersaturated solutions.
Let's start with saturated solutions. In a saturated solution, no more solute is able to dissolve in the solvent.
In a saturated solution, the amount of solute dissolved in the solvent is at its maximum.
For example, the undissolved sugar left at the bottom of your tea cup means that your tea had already been saturated with sugar, so no more sugar could dissolve! However, if we added heating and then cooling to our tea solution, then we might be able to dissolve more sugar than the maximum amount. When this happens, we have a supersaturated solution.
A supersaturated solution is a solution in which more than the maximum amount of solution is dissolved in a solvent.
Now, if we added sugar to our tea and there is no sugar left undissolved, then we have an unsaturated solution!
In an unsaturated solution, less than the maximum amount of a solute is dissolved in a solvent.
The figure below shows the solubility curve of potassium nitrate (KNO3) in water (H2O). The regions in the solubility graph tell whether the solution is unsaturated, saturated, or supersaturated at a certain point.
- At any point below the curve, the solution is said to be unsaturated.
- At any point above the curve, the solution is considered supersaturated.
- At any point in the curve, the solution is said to be saturated.
Figure 3. Solubility curve of KNO3 in water, Isadora Santos - StudySmarter Originals.
Let's solve a problem!
If you dissolved 50.0 grams of KNO3 in water at 60 °C, what type of solution will you have?
- Unsaturated solution
- Supersaturated solution
- Saturation solution
The solubility curve above tells us that for a solution of KNO3 in H2O to be saturated, we would need to add approximately 113 grams of KNO3. Since we are only adding 50 grams, then this solution will be considered unsaturated. Therefore, the correct answer is c.
Solubility Curve of a Gas
Now that we know what a solubility curve looks like, we can explore the solubility curve of a gas. Unlike solid solutes, where solubility rises with increasing temperature, the solubility of gases in water (H2O) tends to decrease with increasing temperature! The image below shows the solubility curve of various gases in water.
Figure 4. Solubility curve of gases, Isadora Santos - StudySmarter Originals.
Gases are also more soluble at higher pressures, and this is given by Henry's law. According to Henry's law, the solubility of a gas (in mM per liter of solution) in water is proportional to its partial pressure. For example, when you open a carbonated drink for the first time CO2 bubbles come out of the solution. This happens because the partial pressure of CO2 above the solution gets reduced!
To learn more about the partial pressure of gases, check out "Partial Pressures"!
Solubility Curve Practice Problems
The best way to learn how to interpret solubility curves is by solving practice problems. So, let's do it!
Practice Problem 1. The figure below shows the solubility curve of sugar in water (H2O). At what temperature will 275 grams of sugar dissolve in 100 mL of H2O?
Figure 5. Example of a solubility curve, Isadora Santos - StudySmarter Originals.
This practice problem involves using the solubility curve to find the temperature at which 275 grams of sugar would completely dissolve in water. So, by looking at the graph, we can see that the sugar has a solubility of 275 g per 100 mL of H2O at 55 °C.
Pretty simple, right? Now, let's look at another example!
Practice problem 2: If you were to make a solubility curve graph for ammonia, would you expect the solubility to increase or decrease when the temperature increases?
We know that ammonia (NH3) is a gas. Therefore, we would expect the solubility of ammonia to decrease with increasing temperature!
Now, I hope that you feel more confident in your understanding of solubility curves!
Solubility Curve - Key takeaways
- Solubility is referred to as the maximum amount of a solute that can be dissolved in a solvent.
- A solubility curve is a graph that shows the number of grams of solute per 100 g of solvent that can be dissolved at different temperatures.
- The regions in the solubility curve graph tells whether the solution is unsaturated, saturated, or supersaturated at a certain point.
- The solubility of solid solutes in water tends to increase with an increase in temperature.
- The solubility of gases in water tends to decrease when temperature increases.
References
- Zumdahl, S. S., Zumdahl, S. A., & Decoste, D. J. (2019). Chemistry. Cengage Learning Asia Pte Ltd.
- Theodore Lawrence Brown, Eugene, H., Bursten, B. E., Murphy, C. J., Woodward, P. M., Stoltzfus, M. W., & Lufaso, M. W. (2018). Chemistry : the central science (14th ed.). Pearson.
- Swanson, J. (2021). Everything you need to ace chemistry in one big fat notebook. Workman.
- Shelton, M., Princeton Review (Firm, & Penguin Random House. (2019). Cracking the AP chemistry exam. Penguin Random House.
- N Saunders, Kat Day, Iain Brand, Claybourne, A., Scott, G., & Smithsonian Books (Publisher. (2020). Supersimple chemistry : the ultimate bite-size study guide. Dk Publishing.
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