Survey of the Natural Sciences: Biology, General Chemistry, and Organic Chemistry
What this domain tests
The Survey of the Natural Sciences (SNS) is 100 multiple-choice items in 90 minutes: roughly 40 biology, 30 general chemistry, and 30 organic chemistry, pitched at one semester of college-level work in each. Biology spans cell and molecular biology, genetics, evolution, developmental biology, anatomy and physiology, and ecology. General chemistry spans stoichiometry, gases, thermochemistry, kinetics, equilibrium, acid–base chemistry, electrochemistry, periodic trends, and nuclear decay. Organic chemistry spans bonding and hybridization, functional groups, stereochemistry, substitution and elimination, aromaticity, carbonyl reactivity, and IR/NMR/mass spectrometry.
Core rules worth memorizing
- Hybridization follows steric number: 2 = sp (linear, 180°), 3 = sp² (trigonal planar, 120°), 4 = sp³ (tetrahedral, 109.5°). Lone pairs count toward steric number but not toward the atom count in the name.
- Compare the reaction quotient to K: Q < K shifts right, Q > K shifts left. Adding an inert gas at constant volume does not shift an equilibrium; changing volume does.
- Strong acids and bases ionize completely; weak ones need Ka/Kb and an ICE table. pH + pOH = 14 at 25 °C.
- Aromaticity requires cyclic, planar, fully conjugated, and 4n + 2 π electrons; planar 4n systems are antiaromatic.
- SN1 favors tertiary carbons, polar protic solvents, and weak nucleophiles; SN2 favors methyl/primary carbons, polar aprotic solvents, and strong nucleophiles. E1 and E2 compete accordingly.
Worked example
How many stereoisomers does 2,3-dibromobutane have?
C2 and C3 are each stereocenters, so the naive count is 2² = 4. Write the configurations: (2R,3R) and (2S,3S) form a non-superimposable enantiomer pair. The remaining pair, (2R,3S) and (2S,3R), are identical to each other because the molecule has an internal mirror plane through the C2–C3 bond - a meso compound. The answer is three stereoisomers, not four.
Traps that cost marks
- Counting every stereocenter combination and forgetting meso compounds.
- Confusing net ATP with total ATP produced in respiration questions.
- Applying Le Châtelier to inert gas added at constant volume (no shift occurs).
- Reporting pH without first checking whether the acid is strong or weak.
- Missing EXCEPT and NOT in question stems; several SNS items hinge on one negation.
- Assigning R/S without ranking substituents by atomic number at the first point of difference.