Chem/Phys is the section test takers most often name as the hardest, and it is usually the one they prepare for least accurately. The name puts physics front and centre, so people spend weeks on kinematics and circuits, then open the exam and find passages about buffers, enzymes, and thermodynamics of binding. The section is chemistry-heavy, biochemistry-heavy, and only about a quarter physics.
The other misconception is that it is a math section. There is arithmetic, but the exam gives you no calculator precisely because it does not want precise arithmetic. It wants you to recognise the relationship, estimate, and pick the answer that is in the right region.
What is actually in the section
The Chemical and Physical Foundations of Biological Systems section is 59 questions in 95 minutes, ten passages plus fifteen standalone questions. The AAMC's approximate discipline mix, which varies a little between forms, looks like this.
| Discipline | Approximate share |
|---|---|
| General chemistry | About 30 percent |
| First-semester biochemistry | About 25 percent |
| Introductory physics | About 25 percent |
| Organic chemistry | About 15 percent |
| Introductory biology | About 5 percent |
Read that table as a study plan. General chemistry and biochemistry together are more than half the section, and the biochemistry overlaps directly with what you are learning for Bio/Biochem, which makes it the best-value studying on the exam. Physics matters, but it is a quarter, and it is tested conceptually far more than computationally.
The equations worth memorizing
There is no formula sheet on test day, but the useful list is much shorter than the ones sold as downloads. The exam rarely asks you to recall an equation in isolation. It asks you to recognise which relationship governs a situation, which means what you actually need is a small set you know cold plus the ability to read what each variable does to the others.
| Area | Worth knowing cold |
|---|---|
| Acids and bases | pH = pKa + log([A-]/[HA]); pH = -log[H+]; Ka x Kb = Kw |
| Thermodynamics | dG = dH - T dS; dG = -RT ln K; q = mc dT |
| Kinetics and equilibrium | Rate laws from experimental data; Q versus K to predict shift |
| Electrochemistry | dG = -nFE; a positive cell potential means spontaneous |
| Gases and solutions | PV = nRT; dilution as M1V1 = M2V2 |
| Mechanics | F = ma; W = Fd cos(theta); KE = 1/2 mv^2; p = mv |
| Fluids | P = rho g h; continuity as A1v1 = A2v2; Bernoulli as a trade between pressure and speed |
| Circuits | V = IR; P = IV; series and parallel rules for resistors and capacitors |
| Waves and optics | n1 sin(theta1) = n2 sin(theta2); 1/f = 1/o + 1/i; v = f x lambda |
Alongside every formula, write the relationship in words: 'halving the radius of a vessel raises resistance sharply' or 'a buffer is strongest when pH is near pKa'. Passages ask about directions of change far more often than they ask for a number.
The math, without a calculator
Answer choices on this exam are usually spread far enough apart that a good estimate wins. The skill is not arithmetic speed, it is knowing how much precision the question actually requires, then getting there in two or three moves.
- 1Work in scientific notation and handle the exponents first. Get the order of magnitude, then worry about the leading digits.
- 2Round deliberately and track the direction. If you rounded the numerator up, your answer is a little high, which is often enough to separate two close choices.
- 3Use log anchors. Memorize log 2 is about 0.30, log 3 is about 0.48, log 5 is about 0.70, and log 7 is about 0.85. Everything else you can build.
- 4Split negative logs into a whole number and a decimal. For example, -log(3 x 10^-5) equals 5 minus log 3, which is about 5 - 0.48, so roughly 4.5.
- 5Check the answer choices before computing. If they differ by an order of magnitude, you only need the exponent, and you can stop early.
That last habit is the one that saves the most time. A large share of Chem/Phys calculations can be finished the moment you realise the choices are 10^-3, 10^-5, 10^-7, and 10^-9, and every remaining step would only refine a digit you do not need.
Pacing: where to be, and when
Fifty-nine questions in 95 minutes averages a little over 90 seconds each, but averages mislead here. Standalone questions should take well under a minute, which buys time for the passages that need four or five. Use checkpoints instead of a per-question timer.
| Time elapsed | Roughly where you should be |
|---|---|
| 24 minutes | Around question 15 |
| 47 minutes | Around question 30, the halfway mark |
| 71 minutes | Around question 45 |
| 90 minutes | Finished the first pass, 5 minutes for flagged questions |
There is no penalty for a wrong answer, so an unanswered question is a pure loss. If a calculation is going nowhere after ninety seconds, pick the most reasonable magnitude, flag it, and take back the four questions that time would have cost you.
How to actually study it
Chem/Phys punishes passive review harder than any other section, because recognising a solved example is nothing like structuring a new problem. Build the loop around problems from the start.
- 1Start with general chemistry and biochemistry, since they are the majority of the section and the biochemistry doubles as Bio/Biochem preparation.
- 2Do physics conceptually first. Understand what each relationship means before drilling numbers, because most physics questions here are qualitative or single-step.
- 3Practise organic chemistry through recognition, not mechanisms in isolation. Functional groups, common reaction types, and reading IR and NMR data for what they identify.
- 4Do timed sets from early on. Untimed practice hides the exact failure mode this section is built to expose.
- 5Review by problem type, not by chapter. Group your misses into 'did not know the relationship', 'set it up wrong', and 'arithmetic', because those three have completely different fixes.
That last split matters. Content gaps need study, setup errors need more passages, and arithmetic errors need estimation drills. Sorting them is the same discipline described in how to review MCAT practice tests.
Mistakes that cost points here
- Studying it as a physics section. Physics is a quarter of it, and gen chem plus biochem is over half.
- Memorizing long formula sheets without knowing which situation calls for which relationship.
- Computing precisely when the choices are far apart, which spends a minute to earn nothing.
- Skipping units. Unit analysis alone eliminates wrong choices on a surprising number of questions.
- Avoiding the section in practice because it feels bad. The discomfort is the signal that this is where your points are.
The bottom line
Treat Chem/Phys as what it is: a chemistry and biochemistry section with a quarter of physics attached, tested through passages and estimated math. Know a short list of relationships cold, learn each as a sentence about cause and effect, estimate rather than calculate, and pace by checkpoints instead of by question. That combination moves this section faster than another pass through a physics review book.
Free Chem/Phys questions with molecules, circuits, and titration curves drawn on the page, and a worked explanation showing every step of the reasoning.
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