ILTS Science: Chemistry (240) common mistakes
ILTS Science: Chemistry (240) · Understand the traps and how to avoid them.
Treating the independent variable, dependent variable, and controlled variables as interchangeable labels when reading a described experiment, then selecting a conclusion the design cannot support.
Candidates memorize the vocabulary but never practice mapping it onto a described procedure, so they default to whichever variable is mentioned most often or is numerically largest.
What to do instead
Before answering, write a one-line skeleton: 'Experimenter changes ___ (IV); measures ___ (DV); holds ___ constant.' Then check that the proposed conclusion speaks only to the IV-to-DV relationship and does not generalize beyond the described population or conditions.
Confusing precision with accuracy and choosing the wrong description of a measurement set, or accepting a graph whose axis scale or error bars misrepresent the data.
Everyday usage treats the words as synonyms, and candidates rarely compute a mean versus a true value or examine axis intervals, so they judge by whether the numbers 'look close' rather than by spread versus deviation from accepted value.
What to do instead
Apply a fixed test: precision = clustering among replicates (small spread, small standard deviation); accuracy = closeness of the mean to the accepted value. For graphs, verify equal axis intervals, labeled units, and that error bars represent the stated quantity before endorsing a claim.
Counting subatomic particles from mass number and charge without adjusting for charge state, and predicting periodic trends by memorizing 'increases across, decreases down' as a single rule for all properties.
The shortcut rule works for some properties (electronegativity, ionization energy) but reverses for atomic radius, and candidates rarely write electron configurations to check, so ions such as a +2 cation get the neutral atom's electron count.
What to do instead
For any ion, compute protons = Z, electrons = Z minus charge, neutrons = A minus Z, and write the configuration explicitly. For trends, derive the answer from effective nuclear charge and shielding rather than reciting a direction, and check radius, ionization energy, and electron affinity separately.
Drawing Lewis structures and predicting geometry that ignore formal charge, expanded octets, or lone-pair repulsion, then reporting bond angles and polarity inconsistent with the structure drawn.
Candidates stop at 'all atoms have an octet' and skip the formal-charge check and VSEPR lone-pair count, so structures with avoidable charge separation or wrong central-atom electron domains slip through.
What to do instead
Use a fixed sequence: total valence electrons, skeleton, octets, then formal charges (minimize separation, negative charge on the more electronegative atom). Count electron domains including lone pairs, assign the geometry from that count, then subtract lone-pair repulsion to state the angle and use vector cancellation for polarity.
Balancing equations and doing stoichiometry without identifying the limiting reactant, or converting between moles, mass, and particles using the wrong molar quantity (molar mass versus Avogadro's number versus molarity).
Textbook problems often supply a single reactant amount, so the limiting-reactant step feels optional; unit labels are also dropped mid-calculation, hiding the mismatch.
What to do instead
Always carry units through every conversion factor and cancel them explicitly. For multi-reactant problems, convert each to moles, divide by its coefficient, and let the smallest quotient set the theoretical yield before computing percent yield or excess remaining.
Applying Le Chatelier predictions or equilibrium calculations without checking whether the system is at equilibrium, and confusing a rate change with a position-of-equilibrium change (for example, claiming a catalyst or pressure change on a constant-volume system shifts K).
Candidates conflate kinetics and thermodynamics and treat any disturbance as a shift, and they plug into K expressions without confirming the reaction quotient Q relative to K.
What to do instead
Compute Q and compare it to K first: Q < K shifts right, Q > K shifts left, Q = K no net change. Then ask separately whether the disturbance changes the rate (catalyst: yes, both directions equally) or the equilibrium position (only changes in concentration, partial pressure, or temperature; only temperature changes K).
Calculating pH or pOH without tracking whether the given concentration is of a strong acid, weak acid, or a salt, and misassigning the conjugate acid-base pairs or buffer behavior in a neutralization or titration context.
Candidates apply pH = -log[H+] universally, including to weak acids where dissociation is partial and to bases where the given value is Kb, and they memorize 'acid plus base gives neutral' regardless of strength.
What to do instead
Classify the species first (strong/weak, acid/base/salt), then choose the tool: strong acid gives direct -log; weak acid needs Ka and an ICE approximation checked against the 5% rule; buffers need Henderson-Hasselbalch; equivalence-point pH depends on the conjugate species formed. Label each conjugate pair explicitly before answering.
Naming or drawing organic structures that violate valence or IUPAC priority rules, and predicting reaction products without tracking the mechanism (nucleophile/electrophile, Markovnikov orientation, substitution versus elimination).
Candidates pattern-match functional groups to memorized reactions and skip the step of identifying which atom is electron-rich, so regiochemistry and stereochemistry are guessed rather than derived.
What to do instead
For nomenclature, find the longest parent chain containing the principal functional group, number to give it the lowest locant, then alphabetize substituents with lowest-locator tiebreaks. For reactions, mark electron-rich and electron-poor sites, identify the mechanism class, and only then apply orientation rules; check stereochemistry at each new stereocenter.
Selecting or describing laboratory procedures that ignore safety data (SDS), incompatible waste streams, or PPE and engineering controls, and reporting data without significant figures, uncertainty, or a stated calibration check.
Safety and technique are studied as isolated lists rather than as decisions embedded in a procedure, so candidates answer what sounds cautious rather than what the hazard class and equipment require.
What to do instead
For any procedure, state the hazard class and required controls (fume hood for volatiles, splash goggles and gloves for corrosives, no open flame near flammables), segregate waste by compatibility, and specify the glassware and calibration used. Report results with correct significant figures and an explicit uncertainty or replicate check.
Designing instruction or assessment that states chemistry content correctly but omits the phenomenon, the student investigation, or the elicitation of prior conceptions, and writing learning objectives that name an activity ('students will do a titration') instead of an observable outcome.
Candidates with strong content knowledge default to lecture-and-verify labs and to objectives copied from textbook chapter titles, because they have not practiced translating a standard into evidence of three-dimensional learning.
What to do instead
Write each objective as 'students will [observable verb] [specific content] by [practice],' then build the lesson around a phenomenon students explain or a question they investigate, with a planned elicitation step (probe, prediction, or pre-assessment) and a rubric that scores the explanation, not the procedure.