Contest Difficulty Ratings

Posted by Brian Lee, 2025-10-09

We’ve added difficulty ratings to all USNCO contests to help you choose the right practice materials for your skill level.

The Rating System

🌶️ : Basic application of equations and definitions. The specific equation or definition to be applied is usually obvious. Honors Chemistry, AP Chemistry, and early (< 2017) USNCO Local exams typically fall under this category, differing only in how much material they cover.

Example problem:
```
Which atom has the smallest first ionization energy?

(A) Na (B) K (C) Mg (D) Ca
```
🌶️🌶️: Compound application of equations and definitions. At least two equations, principles, or definitions need to be correctly chained together to answer a problem. The choice of which equations, principles, or definitions to be applied is usually clear but might be nontrivial. Late (>=2017) USNCO Local exams and early (<2017) USNCO National exams fall under this category.
```
Excess solid silver chloride is added to water and the mixture stirred until equilibrium is achieved.
Addition of which substance will increase the concentration of chloride ion in this solution?

(A) AgNO₃ (B) AgCl (C) NH₃ (D) H₂O
```

🌶️🌶️🌶️: Nontrivial application of equations. A scenario requiring the correct identification of the right chemical equation/principle to analyze. The correct choice of equation/principle will usually require a principled, fundamental understanding of the topic. Late (>=2017) USNCO National exams fall under this category.

Example problem:

A sample of cyclohexane (C6H12, bp = 81 °C) is introduced into an evacuated container at 40 °C.
One third of the cyclohexane by mass is in the form of the liquid and two-thirds is in the form
of the vapor. The volume of the container is then doubled, maintaining the temperature at 40 °C.
What happens to the pressure as the volume is doubled?

(A) The pressure remains the same.
(B) The pressure decreases by 25%.
(C) The pressure decreases by 33%.
(D) The pressure decreases by a factor of two.

🌶️🌶️🌶️🌶️: Serious calculation or puzzle-solving aspects. A scenario with several (3+) plausible interpretations or possibilities, requiring accurate and careful consideration to disambiguate the possibilities. May require obscure knowledge (e.g. advanced organic reagents), but even without the specific knowledge, the capable student should be able to reverse engineer the missing knowledge through general chemical principles – e.g. if you’ve never seen sodium cyanoborohydride, you can probably deduce that it functions similarly to sodium borohydride, but less reactive due to the electron-withdrawing nature of the cyano group muting the electron-rich hydride. Typical of the USNCO Study Camp or International Chemistry Olympiads. You may occasionally find questions in the USNCO National reaching this level of difficulty.
```
What is the final pH when  0.1 mol of NaHCO₃ is added to 1L of a solution containing 0.05M Ca²⁺?

Ksp for CaCO₃ is 3e-9, while pKa1 = 3.5 and pKa2 = 10.3 for carbonic acid.
You may neglect any equilibria involving CO₂.
```
🌶️🌶️🌶️🌶️🌶️. Requires extended calculation or puzzle-solving aspects or extremely thorough understanding of some chemical principle. May also require obscure knowledge without which the problem is not actually solvable. You might get one or two of these questions on the IChO, but in general, I’ve only seen this type of questions in the 2013 Russian Preparatory Problems, which were never intended to be solved under time constraints. Example problem: Synthesis of \(\ce{B11H14-}\).

What’s with the early/late USNCO split?

In 2016, Seth Brown, one of the IChO team members from the US’s very first participation in the IChO in 1984, took over as chair of the USNCO exam-writing committee. At the time, the recent release of ChemWOOT (2013) had driven a major upskilling of competitors and so over the next year or two, the exams became significantly more difficult. I’ve picked 2017 as the dividing line; USNCO contests after this year have +1 🌶️ compared to before 2017.