Praxis Physical Science 5485 common mistakes
Praxis Physical Science 5485 · Understand the traps and how to avoid them.
When asked what a scientific theory is, the candidate selects 'an educated guess' or 'a hypothesis that has been supported once,' and ranks a law above a theory on a hierarchy of certainty.
Everyday English uses 'theory' to mean speculation, and many candidates memorized a 'hypothesis → theory → law' ladder that is not how science works; the exam instead tests the nature of scientific knowledge.
What to do instead
Learn the functional definitions: a hypothesis is a testable prediction, a law describes a consistent relationship (often mathematical, often narrower in scope), and a theory is a broad, well-supported explanatory framework that generates predictions. Practice sorting statements such as 'F = ma', 'plate tectonics', and 'if fertilizer increases growth then...' into the three categories, and note that theories are never 'promoted' to laws.
The candidate reads an experimental scenario and mislabels the independent and dependent variables, or fails to identify the control condition - for example, calling light intensity the dependent variable when plant height is what is measured.
Scenarios are written in prose rather than with labeled axes, and candidates rush to the graph or the conclusion without first parsing what the experimenter deliberately changed versus what was measured.
What to do instead
Before answering, write three short phrases: 'changed on purpose' (independent), 'measured as a result' (dependent), 'held the same' (controlled variables). Then ask what the comparison group received (no treatment, placebo, or standard condition). Practice on unfamiliar scenarios - e.g., enzyme concentration vs. reaction rate, ramp angle vs. cart acceleration - until the mapping takes under 30 seconds.
The candidate states that the normal force on a book resting on a table and the book's weight are an action–reaction pair, or that they 'cancel' because they are equal and opposite.
Both pairs are equal in magnitude and opposite in direction, so the surface pattern match is tempting; candidates do not check which objects the forces act on.
What to do instead
Apply two tests. First, third-law pairs act on two different objects and are always the same type of force (gravity pairs with the book's pull on Earth, not with the normal force). Second, forces that balance act on the same object and need not be the same type. For any force pair on the exam, name the object each force acts on before judging it.
In free-fall or projectile problems, the candidate substitutes g = +9.8 m/s² for upward motion (or -9.8 for downward), or flips the sign of velocity mid-problem without redefining the positive direction.
Candidates treat the sign as attached to 'gravity' rather than to the chosen coordinate axis, and they switch axes between the upward and downward legs of a problem.
What to do instead
State the axis in writing first - e.g., 'up is positive, so a = -9.8 m/s²' - and keep it for the whole problem. Remember that at the top of a projectile's path velocity is zero but acceleration is still 9.8 m/s² downward. Check the answer physically: an object thrown upward must have negative acceleration and a velocity that decreases, crosses zero, then becomes negative.
The candidate claims that a charged particle moving in a uniform magnetic field speeds up, or that the magnetic force does work on the particle, or that the particle spirals outward while the field remains uniform.
The candidate has learned 'force causes acceleration' and assumes any force changes speed; the perpendicular geometry of F = qv × B is not visualized.
What to do instead
Fix the picture: the magnetic force is always perpendicular to velocity, so it does zero work, kinetic energy and speed stay constant, and the particle travels in a circle (or a helix if it has a velocity component along the field). Acceleration is centripetal, not tangential. Contrast this explicitly with an electric field, which can do work and change speed.
The candidate says current is 'used up' as it passes through resistors, or applies the series resistance formula to a parallel network (and vice versa), or places an ammeter across a component.
Charge-flow is treated like a consumed fluid, and the reciprocal formula for parallel resistors is memorized as symbols without the underlying idea that parallel branches share voltage.
What to do instead
Anchor on conservation of charge and energy: current is the same everywhere in a series loop and splits across parallel branches, while voltage drops across series elements and is equal across parallel branches. Derive the parallel rule once from I = I₁ + I₂ with equal V, so the reciprocal form is reconstructed rather than recalled. Memorize the instrument rules as a pair: ammeter in series with low resistance, voltmeter in parallel with high resistance.
In a limiting-reactant problem, the candidate multiplies grams of reactant directly by the coefficient ratio from the balanced equation, or computes product yield from the reactant that is in excess.
Mole ratios are applied to masses, skipping the molar-mass conversion, and candidates pick the reactant with the smaller mass as limiting rather than the one that runs out first in moles.
What to do instead
Use the same four-step chain every time: balance the equation, convert every reactant's mass to moles, divide each by its coefficient to find the smallest value (that reactant limits), then convert product moles to grams. Sanity-check with a quick estimate of molar masses so a mass-based shortcut is visibly wrong. For solutions, use moles = M × V(L) before the ratio.
The candidate decides a molecule is polar simply because it contains polar bonds - labeling CO₂, CCl₄, or BF₃ as polar - or assumes any molecule with lone pairs on the central atom is nonpolar.
Bond polarity and molecular polarity are conflated, and the vector sum is skipped because the candidate does not draw the geometry first.
What to do instead
Follow a fixed sequence: draw the Lewis structure, apply VSEPR to get the shape, use electronegativity differences to draw bond dipoles as arrows, then add them as vectors. Symmetric arrangements (linear, trigonal planar, tetrahedral with identical substituents) cancel to zero; bent or pyramidal shapes do not. Practice with a matched set - CO₂ vs. SO₂, CCl₄ vs. CHCl₃ - so the contrast is automatic.
The candidate computes the pH of a weak acid solution using [H⁺] equal to the acid's concentration, or assumes pH is unchanged when a solution is diluted by a factor of ten.
Only the strong-acid formula pH = -log[HX] has been practiced, and the difference between 'strong' and 'concentrated' has not been separated in the candidate's mind.
What to do instead
Separate the two vocabulary items explicitly: strong = fully ionized, concentrated = large amount per volume. For weak acids, set up Ka ≈ x²/[HA]₀ and solve for x before taking the log; for strong acids, [H⁺] equals concentration. For dilution, use M₁V₁ = M₂V₂ and then recompute pH, expecting roughly a one-unit rise per tenfold dilution (until water autoionization limits it). A quick check: pH of 0.1 M weak acid must be higher than that of 0.1 M strong acid.
The candidate plugs Celsius temperatures into the ideal gas law or Charles's law, or uses 22.4 L/mol as the molar volume at whatever conditions the problem states.
Gas-law problems often quote temperatures in °C and volumes in mL, and the molar volume constant is memorized without its 0 °C and 1 atm conditions attached.
What to do instead
Make 'convert to kelvin and to liters' the first written line of every gas problem, and only then choose the relationship. Attach conditions to every constant you memorize: 22.4 L/mol applies at STP (0 °C, 1 atm), not at room conditions (~24 L/mol at 25 °C). Verify with a limiting case - volume should scale with 273 K, never with 0.