Imagine you are drinking a cup of tea. You take a sip, grimace at how bitter it is, then grab some sugar. As you stir in the sugar, you watch it disappear as it dissolves into your now sweeter tea. The ability of sugar to dissolve is based on its solubility.
Fig.1-When dissolving sugar in tea, we are observing its solubility. Pixabay
In this article, we will understand what factors affect solubility and why certain solids are soluble while others aren't.
- This article is about solubility.
- We will look into how temperature affects solubility based on Le Chatelier's Principle.
- Then we will look at how solubility curves graph the change in solubility based on temperature
- Then we will review the solubility rules for ionic solids
- Lastly, we will calculate the solubility equilibrium constant (Ksp) to understand what we consider "slightly soluble"
Solubility Definition Chemistry
Let's start by looking at the definition of solubility.
Solubility is the maximum concentration of solute (a substance that dissolves in a solvent) that can be dissolved in the solvent (dissolver).
In our tea example, sugar is the solute being dissolved in the solvent (tea). Initially, we have an unsaturated solution, meaning that we haven't met the concentration limit and sugar can still dissolve. Once we add too much sugar, we end up with a saturated solution. This means we have met the limit, so any added sugar won't dissolve, and you'll end up drinking straight sugar granules.
Solubility and Temperature
Solubility is a function of temperature. When a solid is being dissolved, bonds are broken down, which means heat/energy is required. However, heat is also released when new bonds between the solute and solvent are made. Typically, the heat required is greater than the heat released, so it is an endothermic reaction (net gain of heat). However, there are some cases, like in Ca(OH)2, where the heat released is greater, so it is an exothermic reaction (net loss of heat).
So, how does this affect solubility? Depending on whether a reaction is endothermic or exothermic, the solubility can change based on Le Chatelier's Principle.
Le Chatelier's Principle states that if a stressor (heat, pressure, concentration of reactant) is applied to a system at equilibrium, the system will shift to try and minimize the effect of the stress.
Back to our tea example for earlier, let's say you really wanted your tea sweet, but aren't a fan of having to drink the solids bits. Would you need to increase or decrease the temperature to increase the solubility of sugar? Let's look at the reaction:
$$C_{12}H_{22}O_{11\,(s)}+\text{solvent}+\text{heat} \rightleftharpoons C_{12}H_{22}O_{aq}$$
The dissolution of sucrose (table sugar), is endothermic, so heat is a reactant. According to Le Chatelier's Principle, the system wants to minimize stress, so if we increase the temperature (i.e. add heat), the system wants to make more product to "use up" the heat added. This means that undissolved sugar will now be able to dissolve. We use solubility curves to graph the change in solubility based on temperature.
Fig.2- The solubility of sucrose increases with temperature
The curve above shows how solubility increases with temperature. Curves typically are based on how much solute dissolves in 100g of water, since it is the most common solvent. For solutes that have exothermic dissolving reactions, this curve is flipped.
How many more grams of sucrose can be dissolved if the temperature is increased from 40 to 50 °C? (Assume 100g of water)
Based on our curve, at 40 °C, about 240 g of sucrose can be dissolved. At 50 °C, it is about 260 g. So, we can dissolve ~20 g more sucrose if the temperature is increased by 10°
The fact that more solute can be dissolved at a higher temperature is used to form supersaturated solutions. In a supersaturated solution, a solution has more solute dissolved than its equilibrium solubility. This happens when more solute is dissolved at a higher temperature, then the solution is cooled without precipitating (returning to a solid) the solute.
Reusable hand warmers are supersaturated solutions. The hand warmer contains a supersaturated solution of sodium acetate (solute). When the metal strip inside is bent, it releases tiny bits of metal. The sodium acetate uses these bits as sites for crystals to form (it is going from dissolved back to a solid).
As the crystals spread, energy is being released, which is what warms our hands. By placing a hand warmer in boiling water, the sodium acetate is redissolved, and it can be reused.
Solubility Rules
Now that we've covered how solubility changes with temperature, it's now time to look at what makes something soluble in the first place. For ionic solids, there are solubility rules which determine whether they will dissolve or form a precipitate (i.e. stay a solid).
In the next section is a solubility chart with these rules.
Solubility Chart
| Soluble | Exceptions |
| Slightly Soluble | Insoluble |
| Group I and NH4+ salts | None | None |
| Nitrates (NO3-) | None | None |
| Perchlorates (ClO4-) | None | None |
| Fluorides (F-) | None | Mg2+, Ca2+, Sr2+, Ba2+, Pb2+ |
| Halides (Cl-, Br-, I-) | PbCl2 and PbBr2 | Ag+, Hg2+, PbI2, CuI, HgI2 |
| Sulfates (SO42-) | Ca2+, Ag+, Hg+ | Sr2+, Ba2+, Pb2+ |
| Acetates (CH3CO2-) | Ag+, Hg+ | None |
| Insoluble | Exceptions |
| Slightly soluble | Soluble |
| Carbonates (CO32-) | None | Na+, K+, NH4+ |
| Phosphates (PO42-) | None | Na+, K+, NH4+ |
| Sulfides (S2-) | None | Na+, K+, NH4+, Mg2+, and Ca2+ |
| Hydroxides (OH-) | Ca2+, Sr2+ | Na+, K+, NH4+, Ba2+ |
As you can see, there are many solubility rules. When determining whether an ionic solid is soluble, it's important to reference your charts!
Categorize these compounds as either soluble, insoluble, or slightly soluble.
a. MgF2 b. CaSO4 c. CuS d. MgI2 e. PbBr2 f. Ca(CH3CO2)2 g. NaOH
a. While fluorides are typically soluble, when it is bonded to Mg, it is insoluble.
b. Sulfates are also typically soluble, but when bonded to Ca, it is slightly soluble.
c. Sulfides are typically insoluble, and Cu is not one of the exceptions, so it is insoluble.
d. Halides are typically soluble, and Mg is not an exception, so it is soluble.
e. Bromine is typically soluble, but with Pb, it is slightly soluble.
f. Acetates are typically soluble, and Ca is not an exception, so it is soluble.
g. Hydroxides are typically insoluble, but when bonded to Na, it is soluble.
Ksp and Temperature
Another way we can determine solubility is based on the solubility constant (Ksp).
The solubility constant (Ksp) is the equilibrium constant for solids dissolving in an aqueous (water solvent) solution. It represents the amount of solute that can dissolve. For a general reaction: $$aA \rightleftharpoons bB + cC$$
The formula for Ksp is: $$K_{sp}=[B]^b[C]^c$$
Where [B] and [C] are the concentrations of B and C.
The calculation uses the concentration of the ions, which is called their molar solubility. This is expressed in mol/L (M).
So, when we are referring to something that is “slightly soluble”, we mean that it has a very low Ksp. Let's look at a problem to further explain.
What is the Ksp for PbCl2, when the concentration of Pb2+ is 6.7 x 10-5 M?
The first thing we need to do is write out the balanced equation
$$PbCl_2 \rightleftharpoons Pb^{2+} + 2Cl^-$$
Since we know the concentration of Pb2+, we can calculate the concentration of Cl-. We do this by multiplying the amount of Pb2+ by the ratio of Pb2+ to Cl-.
$$6.7*10^{-5}\,M\,\cancel{Pb^{2+}}*\frac{2\,M\,Cl^-}{1\,M\,\cancel{Pb^{2+}}}=1.34*10^{-4}\'M\,Cl^-$$
Now we can calculate Ksp
$$K_{sp}=[Pb^{2+}][Cl^-]^2$$
$$K_{sp}=(6.7*10^{-5})({1.34*10^{-4}})^2$$
$$K_{sp}=1.20*10^{-12}$$
The Ksp of HgSO4 at 25 °C is 7.41 x 10-7, what is the concentration of SO42- that will be dissolved?
We first need to set up the chemical equation, then we can set up the equation for Ksp.
$$HgSO_4 \rightleftharpoons 2Hg^+ + SO_4^{2-}$$
$$K_{sp}=[Hg^+]^2[SO_4^{2-}]$$
Now that we have set up our equation, we can solve for the concentration
$$7.41*10^{-7}={[Hg^+]^2}{[SO_4^{2-}]}$$
$$7.41*10^{-7}=[x]^2[x]$$
$$7.41*10^{-7}=x^3$$
$$x=9.05*10^{-3}\,M$$
One thing to note is that even insoluble compounds can have a Ksp. The value of Ksp is so small, however, that the molar solubility of the ions is negligible in solution. This is why it is considered "insoluble" despite some of it actually dissolving.
Also, Ksp, like solubility, is dependent on temperature. It follows the same rules as solubility, so Ksp will increase with temperature. It is standard that the Ksp is measured at 25 °C (298K).
Solubility - Key takeaways
- Solubility is the maximum concentration of solute (dissolvee) that can be dissolved in the solvent (dissolver).
- If the dissolution of a compound is exothermic, increasing temperature will decrease solubility. If it is endothermic, an increase in temperature will increase solubility.
- Solubility curves graph how solubility changes with temperature.
- We can look at the solubility rules to determine if a compound is soluble, slightly soluble, or insoluble.
- Ksp is the equilibrium constant for solids dissolving in an aqueous (water solvent) solution. It shows how soluble a compound is and can be used to determine molar solubility (concentration of solute dissolved).
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