Solutions on the MCAT

Table of Contents

What’s the thing that connects your Nestle lemon flavor iced tea,  the Starbucks classic syrup, and the boiling salt water that makes your pasta? As you probably guessed by the title of this chapter overview, they are all examples of solutions. 

While we usually think of solutions as merely something dissolving into something else, there are definitely — as with all other topics on the MCAT — other complexities involved with the formation and nature of solutions. 

This chapter overview will introduce you to some of the basic terms and concepts associated with solutions. Let’s get started!

Solutions on the MCAT: What You Need to Know

Topics on general chemistry will be tested on the Chem/Phys and the Bio/Biochem section of the MCAT and can appear both as passage based and fundamental discrete questions. 

We’ve calculated that there can be 5-7 possible questions that will cover solutions, concepts, and calculations. 

Introductory general chemistry accounts for 30% of the content covered in the Chemical and Physical Foundations of Biological Systems and 5% of the content covered in the Biological and Biochemical Foundations of Living Systems.

Important Sub-Topics: Solution

The great thing about studying and reviewing solutions is that you’ll apply a lot of other topics from other general chemistry topics!

From reaction equilibrium, basic stoichiometry, chemical bonding (and many more!), the concept of solutions takes a little bit from each topic and combines them together as we’ll see as we’ll go into the chapter overview!

1. Main Terms and Units of Solution

In order to make a solution, only 2 components are needed: the solute and the solvent! The solute is the component that’s dissolved into the solvent. Conversely, the solvent is the component of the solution that dissolves the solute.

Almost always, solutions involve a solid solute being dissolved into a liquid solvent. However, there are cases where a solution can be created by a gas solute and gas solvent!

Dissolving of Solute to Form Solution

There are some units we can use in order to put a number to how concentrated a solution is. As shown below, 2 important values that quantify concentration are molarity and molality!

  • Molarity (M): Ratio of the moles of solute to the liters of solution
  • Molality (m): Ratio of the moles of solute to the kilograms of solvent
Molarity and Molality Calculation

You’ll without a doubt come across molarity significantly more during the MCAT compared to molality, but it also doesn’t hurt to have this other calculation in mind! Note the difference in the denominators: molarity uses liters of solution, while molality utilizes kilograms of solvent.

One property of aqueous solutions (i.e. solutions that use water as the solvent) is that they can lower their concentration via the addition of more water — this process is called a dilution.

This makes sense because the addition of more water increases the denominator value in the molarity equation, lowering the value. We can actually use the equation below in order to solve how much water needs to be added to attain the new molarity! 
Dilution Equation

Notice how both the products on each side of the equation (M1*V1 and M2*V2) are equal to the moles of solute! It’s important to understand that despite the changes in concentration, the value for the moles of solute remains constant!

In order to find the amount of water that needs to be added to achieve the final, diluted molarity, subtract the final volume from the initial volume — this will equal the amount of water that needs to be added!

(Coming Soon!) Full Study Notes : Main Terms and Units of Solutions

For more in-depth content review for a little more on the main terms, units, and equations utilized for solutions, check out these detailed lesson notes created by top MCAT scorers. 

2. Basic Solubility Rules and Concepts

There are a couple of general rules and principles regarding the solubility of substances into aqueous solutions. Solubility simply refers to the ability of a solute to dissolve into a solvent to form a solution. 

One important generality for solubility is the “like dissolves like” rule of thumb, which heavily refers to polar and nonpolar molecules. As the name implies, polar/ionic solutes will dissolve well in polar solvents, while nonpolar solutes will dissolve well in nonpolar solvents!

This is due to the favorable intermolecular interactions that occur between “like” molecules! Likewise, the converse of the above relationships is true: mixing polar solutes into a nonpolar solvent (and vice versa) results in low solubility of the solute. 

A. High Solubility

  • Polar Solute and Polar Solvent

  • Ionic Solute and Polar Solvent

  • Nonpolar Solute and Nonpolar Solvent

B. Low Solubility

  • Polar Solute and Nonpolar Solvent (and vice versa)

  • Ionic Solute and Nonpolar Solvent

You’ll come back across this topic when covering various chromatography techniques used in the laboratory, as covered in our organic chemistry module. Have a look at the diagram below to get a better visual understanding of the “like dissolves like” rule of thumb!

Basic Solubility Rules and Concepts - A

In addition to this, there are a couple more specific general solubility rules in regard to ionic salts being dissolved in water. There are actually a lot more rules and exceptions, but the ones covered below should suffice in your MCAT prep!

1. Group 1 alkali metals and ammonium (NH4+) salts are water soluble.
2. Acetate (CH3COO-) and nitrate (NO3-) salts are water soluble.
3. Mercury, lead, and silver salts are insoluble in water. 

An important thing to note about these rules is that they are ranked in a hierarchical order! In this case, rules 1 and 2 have higher priority than rule 3. Look at the example below!

Basic Solubility Rules and Concepts - B

(Coming Soon!) Full Study Notes : Basic Solubility Rules

For more in-depth content review on the main solubility rules and principles, check out these detailed lesson notes created by top MCAT scorers. 

3. Equilibrium of Solutions

Interestingly, concepts of chemical equilibrium can also be applied to solutions, specifically in the dissolving of solutes! In the case of solutions, equilibrium refers to when the maximum concentration of a solute is dissolved.

One way to think of this is that only so much of a substance can be dissolved! At a certain point, the addition of more solute won’t dissolve further — the solute will simply remain as a solid (Note: We’ll cover the exception of supersaturated solutions).

Specifically, we use the term saturated to describe a solution at this equilibrium where no more solute can be dissolved. Similar to the equilibrium constant, we can also put a number to solution equilibrium. Let’s look at the equation and an example below!
Equilibrium of Solutions - A

9 times out of 10, solution equilibria deal with the dissolving of an ionic compound in water. Take this time also to brush up on how to form ionic chemical formulas from ions — this will help to understand the equation above a little better!

For the most part, we calculate the solubility product constant and equilibrium constant the same way; however, notice how the ionic solid reactant is not included. This is because we don’t include solid or liquid reactants or products when calculating the equilibrium constant!

A lot can be concluded from the Ksp value of a specific ionic compound: the higher the Ksp value, the more likely the ionic compound will dissolve (high solubility); conversely, the lower the Ksp value, the less likely the ionic compound will dissolve (low solubility). Let’s use the example of barium nitrate and silver (I) chloride.

Equilibrium of Solutions - B

In addition to the Ksp value, we also have a number called the ion product (IP), which is essentially analogous to the reaction quotient of a chemical reaction! Similarly, we can also make various conclusions when comparing the value of the ion product to the Ksp as shown below:

A. IP < Ksp: Unsaturated → Solute Will Continue to Dissolve

>> Why?

  • The solution hasn’t reached saturation yet (i.e. when the maximum amount of solute has been dissolved). Because of this, the solute will continue to dissolve. 

B. IP = Ksp: Saturated → Equilibrium Achieved

>> Why?

  • The solution is now at a point where the maximum amount of solute has been dissolved. When more solute is added, the solute will remain as a solid. 
  • There is an exception for supersaturated solutions, which is discussed in the next point. 

C. IP > Ksp: Supersaturated → Any Disturbance Causes Precipitation of Solute

>> Why?

  • Supersaturated solutions have more than the maximum amount of solute dissolved. This can be achieved by heating the solution to high temperatures, adding more solute, and then slowly cooling it. 
  • At this point, the supersaturated solution is very delicate and any disturbance will result in the precipitation of solute back into a solid. 

(Coming Soon!) Full Study Notes : Equilibrium of Solutions

For more in-depth content review on equilibrium of solutions, check out these detailed lesson notes created by top MCAT scorers. 

4. The Common Ion Effect

While temperature and the Ksp are key factors in determining how much of a solute will dissolve to make a solution, another key principle that affects how much solute will dissolve is the common ion effect.

Essentially, this is just another application of Le Chatelier’s principle for solutions! If there’s already an ion present in the solution that’s also present in the incoming salt (hence, the term “common ion”), the amount of salt that will be dissolved is reduced. Take a look at the example below!
The Common Ion Effect - A

In the right solution, because there’s already a common ion present in the form of bromide, less silver bromide will be dissolved as a result — again, this is just another application of Le Chatelier’s principle in the context of solutions!

The Common Ion Effect - B

In terms of calculation questions that might appear on the MCAT, those involving the calculations with the common ion effect are a bit more complicated. We cover more about the common ion effect as well as strategies to approach the calculations in this  article!

(Coming Soon!) Full Study Notes : The Common Ion Effect

For more in-depth content review on the common ion effect, check out these detailed lesson notes created by top MCAT scorers. 

5. Colligative Properties

The term “colligative properties” really just refers to the various characteristics that solutions display as a result of the dissolved solutes! While there are many different colligative properties to note, we’ll cover 3 main ones — note that the colligative properties that occur are a result of increased solute concentration!

A. Vapor Pressure Depression

>> Why?

  • Recall that vapor pressure is the pressure that’s exerted by the evaporated solution vapor on the surface of the liquid solution. 

  • When a higher concentration of solutes is dissolved, the solutes prevent the amount of solvent that can be evaporated, decreasing the vapor pressure. Think of it like a “shield” or “wall” which prevents the solvent molecules from evaporating!

Vapor Pressure Depression

B. Boiling Point Elevation

>> Why?

  • To understand the rationale behind this, we have to go back to the above colligative property — the more solute dissolved, the lower the vapor pressure.

  • Recall also that a solution boils when its vapor pressure equals the external pressure. Because increased solute concentration results in a lower vapor pressure, and thus results in a higher temperature needed for the solution to boil. Follow the reasoning below!

Boiling Point Elevation

C. Freezing Point Depression

>> Why?

  • In order to form a solid, the solvent molecules must condense and congregate to form the solid structure. However, when more solute is dissolved into the solution, this congregation is disrupted!

  • It’s kinda like trying to compress a spring with crayons trapped within the wedges — those crayons make it harder to compress the spring, just like the solute particles make it harder for the solvent particles to condense. 

Freezing Point Depression

(Coming Soon!) Full Study Notes : Colligative Properties

For more in-depth content review on colligative properties, check out these detailed lesson notes created by top MCAT scorers. 

Important Definitions and Key Terms

Below are some high yield definitions and key terms to refer to when reviewing concepts and ideas about solutions in general chemistry!




Component of the solution which dissolves in the solvent


Component of the solution which dissolves the incoming solutes


Ratio of the moles of solute to the liters of solution


Ratio of the moles of solute to the kilograms of solvent


A method to decrease the concentration of an aqueous solution by the addition of more water


Term to describe a solution at solubility equilibrium; At this point, the maximum amount of solute is dissolved in the solvent


Term to describe a solution not yet at solubility equilibrium; Because the maximum amount of solute has not been dissolved, more solute will be dissolved until equilibrium is reached


Term to describe a solution past solubility equilibrium; It’s possible to create these solutions by heating the solution to very high temperatures, adding more solute, and then slowly cooling it down

Common Ion Effect

Application of Le Chatelier’s principle in solution equilibria where the presence of a common ion in a solution decreases the solubility of an incoming salt

Colligative Properties

Properties of solutions which are dependent on the amount of solute particles that are dissolved!

Additional FAQs - Solutions on the MCAT

What are the Main Solubility Rules? – MCAT

One general solubility rule is the “like dissolves like” rule of thumb — this states that solute particles that are polar dissolve well in polar solvents, and nonpolar solutes dissolve well in nonpolar solvents!

In addition, there are 3 main solubility rules we can apply to salts to see if they’ll be soluble in water: 1) group 1 alkali metal and ammonium salts are water soluble, 2) acetate and nitrate salts are water soluble, and 3) mercury, lead, and silver salts are insoluble. Note, that the rules are listed in hierarchical order, where rules 1 and 2 take priority over rule 3!

What is Normality? – MCAT

Normality is often defined as the gram equivalent of a solute per liter of solution; however, you can simply think of it as the molarity of a specific particle of interest. To give an example, suppose you want to find the normality of the hydrogen ion in 0.1 M of H3PO4

Because there are 3 equivalents of hydrogen (as indicated by the “3” subscript, we just multiply the molarity of the entire solute by the 3 equivalents to get the normality; hence, the hydrogen has a normality of 0.3 M when the phosphoric acid dissociates.

What is an Ideal Gas — MCAT?

An ideal gas refers more to the ideal behaviors that chemists assume when studying gasses! There are 2 main assumptions that make a gas ideal: 1) there are no intermolecular interactions between the gas particles, and 2) the gas particles themselves take up no space, which results in their volume being negligible.

Is the Ideal Gas Constant on the MCAT?

Yes! Make sure you memorize the 2 main values of the ideal gas constant utilized in the ideal gas law — 0.0821 L*atm/mol*K and 8.314 J*mol/K. For the most part, the MCAT will indicate to you which constant to use!

Additional Reading Links (Coming Soon!) – Study Notes for Solutions on the MCAT

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