During a class discussion, a student says: "A theory is just an educated guess. Once scientists prove a theory enough times, it becomes a law." Which response from the teacher best corrects the student's misconception?

A law describes a pattern in nature, while a theory explains why it happens; theories do not turn into laws.

Laws are permanent and cannot change, while theories are untested ideas waiting for evidence.

A theory becomes a law once it has passed a set number of independent experimental tests.

A theory is a hypothesis that has been tested once, and a law is a hypothesis tested many times.

A teacher plans a demonstration with a laser pointer and a clear tank of water (n = 1.33). She wants the beam to reflect completely at the water's surface with no light escaping into the air. How should she aim the laser?

From under the water, upward at the surface at an angle greater than the critical angle

From the air, down into the water at an angle smaller than the critical angle

From under the water, straight up at the surface along the normal

From the air, down into the water at an angle greater than the critical angle

A teacher shows students the light from a hydrogen gas discharge tube through a diffraction grating. Students see only four separate colored lines instead of a continuous rainbow, and one student asks why. Which explanation is consistent with the Bohr model?

Electrons exist only at certain energy levels, so only photons of specific energies are emitted.

The grating blocks most colors of light, so only four colors are able to pass through to the eye.

Hydrogen atoms contain only four electrons, and each electron gives off one color of light.

Electrons can have any energy, but the four brightest colors hide all the colors in between.

Which of the following reactions is NOT an oxidation– reduction reaction?

Ba(NO₃)₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaNO₃(aq)

Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)

2KClO₃(s) → 2KCl(s) + 3O₂(g)

CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)

A teacher drops a 1 kg steel ball and a 5 kg steel ball from the same height at the same time; air resistance is negligible. Before the drop, most students predicted the 5 kg ball would land first because "it is pulled down harder." Which explanation best accounts for what students observe?

The 5 kg ball has 5 times the weight but 5 times the mass, so both accelerate at g and land together.

The 5 kg ball has more inertia than gravity can overcome, so it lands slightly after the 1 kg ball.

The 5 kg ball has 5 times the weight, so it lands first, but too quickly for students to see the gap.

Gravity pulls equally on both balls, so they land together because their weights are the same.

Praxis Physical Science (5485)

Free Praxis Physical Science (5485) Practice Questions

10 sample questions for the Praxis Physical Science (5485), with the correct answer and an explanation for each. No account needed. Try each one before you open the answer.

Sections covered on the exam: Versus Theories, Reproducible Evidence, Controlling Variables, Accuracy and Precision, Percent Error, Significant Figures, Chemical Splash Response, Solutions and Acid–Base Chemistry, Criteria and Constraints, Iterative Refinement, Precipitation, Renewable Energy Trade-Offs, Treatment, Technology and Science, Foil Evidence, Subatomic Particle Counts, Average Atomic Mass, Configuration and Position, Life From Data, Decay Product, And Energy In Fusion, Evidence of Chemical Change, Conservation of Mass, Energy Calculation, Energy Goes, Calculating Heat, Comparing Specific Heats, Versus Temperature, Heating Curve Data, Sublimation and Phase Diagrams, Charles'S Law, Molecular Theory, Classifying Mixtures, Particle Diagrams, Chemistry, Chemical Reactions and Periodicity, Physics, Mechanics, Waves, Sound, and Light, Observation and Inference, Unsupported Hypothesis, Finding A Pattern In Data, Sources of Error, Conversion, Interpreting A Graph'S Slope, Chemical Storage, Extinguisher Classes, Science Leading To Technology, Defining A Design Problem, Ozone Layer Misconception, Nuclear Versus Coal Power, Energy Savings Calculation, Medical Imaging Choice, Cathode Ray Evidence, Spectra and Energy Levels, Notation, Configuration From Position, To Decay, Shielding Data, Homogeneous Mixtures, Compound Structure, Identifying By Properties, In An Open System, Reading An Energy Diagram, Breaking Misconception, Temperature Scales, Calorimetry, Demonstrating Radiation, Melting and Warming Ice, Cooling Curve Data, Gas Law, Pressure In A Heated Can, Chemical Composition, Bonding, and Structure, Electricity and Magnetism.

Question 1 · Versus Theories
During a class discussion, a student says: "A theory is just an educated guess. Once scientists prove a theory enough times, it becomes a law." Which response from the teacher best corrects the student's misconception?
- A.A law describes a pattern in nature, while a theory explains why it happens; theories do not turn into laws.
- B.Laws are permanent and cannot change, while theories are untested ideas waiting for evidence.
- C.A theory becomes a law once it has passed a set number of independent experimental tests.
- D.A theory is a hypothesis that has been tested once, and a law is a hypothesis tested many times.
Show answer and explanation
Answer: A. A law describes a pattern in nature, while a theory explains why it happens; theories do not turn into laws.
Laws and theories are different kinds of knowledge: a law summarizes an observed regularity (often mathematically), and a theory is a well- supported explanation of why it occurs. A theory never graduates into a law no matter how much evidence supports it. Both laws and theories can be revised when new evidence appears, so the claim that laws are permanent is also a misconception.
Question 2 · Decay Product
Uranium-238 (Z = 92) undergoes alpha decay. Which nuclide is produced?
- A.Neptunium-238, with Z = 93
- B.Thorium-236, with Z = 90
- C.Uranium-234, with Z = 92
- D.Thorium-234, with Z = 90
Show answer and explanation
Answer: D. Thorium-234, with Z = 90
An alpha particle is a helium-4 nucleus (⁴₂He). Conserving mass number and charge: ²³⁸₉₂U → ⁴₂He + ²³⁴₉₀Th, since 238 − 4 = 234 and 92 − 2 = 90. Element 90 is thorium. Raising Z by one with no change in mass number describes beta-minus decay instead.
Question 3 · Chemistry
ELEMENTAL ANALYSIS OF AN UNKNOWN SUGAR Element Percent by mass Molar mass (g/mol) C 40.0 12 H 6.7 1.0 O 53.3 16 A separate experiment shows the molar mass of the compound is 180 g/mol. What is the molecular formula of the compound?
- A.C₂H₄O₂
- B.C₆H₁₂O₆
- C.CH₂O
- D.C₃H₆O₃
Show answer and explanation
Answer: B. C₆H₁₂O₆
In 100 g: C 40.0 ÷ 12 = 3.33 mol, H 6.7 ÷ 1.0 = 6.7 mol, O 53.3 ÷ 16 = 3.33 mol, a 1 : 2 : 1 ratio, so the empirical formula is CH₂O (30 g/mol). Since 180 ÷ 30 = 6, the molecular formula is C₆H₁₂O₆. CH₂O is only the empirical formula and ignores the molar-mass data.
Question 4 · Chemistry
A student dissolves 5.85 g of NaCl (58.5 g/mol) in water and adds water to a final volume of 250 mL. What is the molarity of the solution?
- A.0.0234 M
- B.0.0250 M
- C.0.100 M
- D.0.400 M
Show answer and explanation
Answer: D. 0.400 M
Moles NaCl = 5.85 ÷ 58.5 = 0.100 mol, and 250 mL = 0.250 L, so M = 0.100 ÷ 0.250 = 0.400 M. The value 0.100 M ignores the volume, and 0.0234 M divides grams by milliliters without converting to moles or liters.
Question 5 · Physics
A ball rolls off the edge of a 20 m high cliff with a horizontal speed of 6.0 m/s. Ignoring air resistance and using g = 10 m/s², how far from the base of the cliff does the ball land?
- A.6.0 m
- B.12 m
- C.20 m
- D.24 m
Show answer and explanation
Answer: B. 12 m
The fall time comes from the vertical motion alone: 20 = ½(10)t², so t = 2.0 s. The horizontal speed stays 6.0 m/s, so the distance is 6.0 × 2.0 = 12 m. Using 1 s or 4 s for the fall time gives 6.0 m or 24 m.
Question 6 · Physics
A teacher plans a demonstration with a laser pointer and a clear tank of water (n = 1.33). She wants the beam to reflect completely at the water's surface with no light escaping into the air. How should she aim the laser?
- A.From the air, down into the water at an angle smaller than the critical angle
- B.From under the water, straight up at the surface along the normal
- C.From the air, down into the water at an angle greater than the critical angle
- D.From under the water, upward at the surface at an angle greater than the critical angle
Show answer and explanation
Answer: D. From under the water, upward at the surface at an angle greater than the critical angle
Total internal reflection happens only when light travels from a higher- index medium toward a lower-index one (water to air) and strikes the boundary at more than the critical angle (about 49° for water). Light going from air into water can always refract into the water, and a beam along the normal passes straight through.
Question 7 · Spectra and Energy Levels
A teacher shows students the light from a hydrogen gas discharge tube through a diffraction grating. Students see only four separate colored lines instead of a continuous rainbow, and one student asks why. Which explanation is consistent with the Bohr model?
- A.The grating blocks most colors of light, so only four colors are able to pass through to the eye.
- B.Hydrogen atoms contain only four electrons, and each electron gives off one color of light.
- C.Electrons exist only at certain energy levels, so only photons of specific energies are emitted.
- D.Electrons can have any energy, but the four brightest colors hide all the colors in between.
Show answer and explanation
Answer: C. Electrons exist only at certain energy levels, so only photons of specific energies are emitted.
In the Bohr model, electrons occupy only certain allowed energy levels. When an electron drops from a higher level to a lower one, it emits a photon whose energy equals the difference between the levels, so only certain wavelengths appear. A neutral hydrogen atom has just one electron; the different lines come from different transitions, not different electrons.
Question 8 · Chemistry
What mass of carbon dioxide, CO₂ (44 g/mol), contains 1.204 × 10²⁴ oxygen atoms? (Avogadro's number = 6.02 × 10²³)
- A.22 g
- B.32 g
- C.44 g
- D.88 g
Show answer and explanation
Answer: C. 44 g
1.204 × 10²⁴ ÷ 6.02 × 10²³ = 2.00 mol of O atoms. Each CO₂ has two O atoms, so this is 1.00 mol of CO₂ = 44 g. Treating 2.00 mol of atoms as 2.00 mol of molecules gives 88 g.
Question 9 · Chemistry
Which of the following reactions is NOT an oxidation– reduction reaction?
- A.Ba(NO₃)₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaNO₃(aq)
- B.Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
- C.2KClO₃(s) → 2KCl(s) + 3O₂(g)
- D.CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)
Show answer and explanation
Answer: A. Ba(NO₃)₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaNO₃(aq)
In the precipitation reaction every element keeps its oxidation number (Ba +2, S +6, Na +1, N +5, O −2); the ions only change partners. Decomposition of KClO₃ changes Cl from +5 to −1 and O from −2 to 0, magnesium is oxidized by H⁺, and combustion oxidizes carbon from −4 to +4, so those three are redox reactions.
Question 10 · Physics
A teacher drops a 1 kg steel ball and a 5 kg steel ball from the same height at the same time; air resistance is negligible. Before the drop, most students predicted the 5 kg ball would land first because "it is pulled down harder." Which explanation best accounts for what students observe?
- A.The 5 kg ball has 5 times the weight but 5 times the mass, so both accelerate at g and land together.
- B.The 5 kg ball has more inertia than gravity can overcome, so it lands slightly after the 1 kg ball.
- C.The 5 kg ball has 5 times the weight, so it lands first, but too quickly for students to see the gap.
- D.Gravity pulls equally on both balls, so they land together because their weights are the same.
Show answer and explanation
Answer: A. The 5 kg ball has 5 times the weight but 5 times the mass, so both accelerate at g and land together.
Students are right that the heavier ball is pulled harder: its weight is 5 times greater. But by Newton's second law a = F/m, and its mass is also 5 times greater, so both balls accelerate at g and hit the ground together. The weights are not equal.

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