Tips to de-whelm the Chem/Phys section on the MCAT

chem/phys MCAT test prep units
By Ziad

The Chemical and Physical Foundations of Biological Systems section on the MCAT, or Chem/Phys, is the most daunting and overwhelming section on the exam for many test takers. While novel biological concepts on the test are generally similar to what is learned in class and through studying, the Chem/Phys section likes to throw complex organic or physical topics at students that elicit a fight or flight reaction from having never seen the content before. While the underlying framework of the questions is built upon the finite set of knowledge, the sheer complexity of the passages tends to throw students for a loop. 

The number one solution to overcoming the uncertainty is to introduce certainty of your own. What do I mean? Having a consistent routine that you apply to Chem/Phys passages, regardless of the content of the passage, will help you cut through the haze and apply your knowledge to the questions. To do this effectively, we want to utilize the unique aspects of this particular section to our advantage. 

Tip 1: Eliminate impossible answer choices.

Many questions in the Chem/Phys section will give answers in the format of yes/no + the reasoning behind it. Usually, the four answer choices will be split up so that half say yes, half say no, and so that the reasoning is also split up half and half. An example might look like: 

Screen Shot 2024-06-25 at 9.13.59 AM

While this example is certainly simplified, we can use it to understand what an impossible answer choice might look like. It is one where the explanation for the answer does not align with the answer provided. So, we can eliminate options B and C because the change in G values given do not match the answer of spontaneity. Ultimately, it narrows down the choices to a determination of whether the reaction is spontaneous or not, which can be deduced from the passage. This helps reduce the mental clutter by allowing you to apply your knowledge in a context that is free from any overwhelming or confusing feelings provided by the passage, and improves the odds of you being able to pinpoint the information you need to gather from it! I find that when many students predispose themselves to the notion that they do not understand the content of the passage, they are more likely to just guess out of all four options, even when they would have known that half of them were implausible. 

Tip 2: Follow the units.

Particularly useful in physics, a good understanding of the units that make up other units can be critical in guiding answer selection. Let’s look at an example: 

Screen Shot 2024-06-25 at 9.15.45 AM

Again, a more straightforward example to illustrate the point, but this process can be used in a variety of situations and complexities. We start by listing out all the units that we are given.  

  • t = Seconds (s) 
  • I = Amperes (A) 
  • z = Moles of electrons, moles of cadmium (mol e-/mol Cd) 
  • F = Coulombs, moles of electrons (C/mol e-) 

Here we can see the unit of interest: moles of cadmium. But it is currently attached to another unit, moles of electrons. Therefore, we will want to utilize the other variables to cancel out the unwanted units. A general rule of thumb to remember is to eliminate a unit, you must have two copies of it. As it currently stands we have: 

  • 1x (s) 
  • 1x (A) 
  • 2x (mol e-) 
  • 1x (C) 
  • 1x (mol Cd) 

 

Clearly, we have some work to do to eliminate a few of these units. Here is where your knowledge of units comes into play! When you have mismatched units, the best action to take is to split a “compound unit” into more fundamental units. In this example, the ampere is easily broken down into coulombs per second, or (C/s). Immediately, we can see that we’ve removed the unit (A) while also providing a second instance of both (s) and (C), priming them for elimination. Now we just need to set up an equation where these units are on opposite sides of a fraction bar. 

We start by deciding where our unit of interest should go. Since we are looking for mol Cd but our unit is given in the form of z = mol e-/mol Cd, we start by putting z on the bottom of the fraction bar so that mol Cd is in the numerator.  

Screen Shot 2024-06-25 at 9.55.15 AM

Now, to cancel the mol e- in the denominator we divide by F, which also has mol e- in the denominator, now giving us C in the denominator.  

Screen Shot 2024-06-25 at 9.55.47 AM

To cancel that we multiply by I, which now has units of C/s, leaving us with s in the denominator. 

Screen Shot 2024-06-25 at 9.56.26 AM

We finally cancel that out by multiplying by t, which ultimately just leaves us with the unit mol Cd. So our final answer is It/Fz, or D

Screen Shot 2024-06-25 at 9.57.01 AM 

By tracking units in this way we can conceptually derive an answer to most problems even if we forget the dedicated equation required for a problem. Sometimes the equation to use just isn’t immediately obvious, and sometimes there isn’t a defined equation at all! Using this strategy will help normalize your approach to mathematical problems, and can help confirm any equations you might be unsure of.  

Overall, these two tips should help you cut through the fat of some of the most daunting passages. With these strategies, you can more effectively focus on the exact information you need rather than stressing about information that doesn’t even matter in the first place! 

Ziad earned a degree in Molecular Biophysics and Biochemistry at Yale (magna cum laude, Phi Beta Kappa). He is currently working at biotech startup Atias Pharma in Cambridge, Massachusetts, with plans to pursue his MD at the University of Pennsylvania next.

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