A student argues that the kinetic molecular theory of gases will eventually be promoted to a law once enough supporting experiments have been performed. Which statement BEST corrects the student's reasoning?

Theories explain why phenomena occur, while laws describe patterns, so a theory does not become a law.

Theories and laws are interchangeable terms, so the label depends only on which scientist named the idea.

Laws are tentative hypotheses that are later upgraded to theories once a mechanism has been identified.

Theories become laws only after they are confirmed by experiments repeated in many different countries.

Chlorofluorocarbons (CFCs) were once widely used as refrigerants because they are nontoxic and nonflammable, but they were found to destroy stratospheric ozone. Their common replacements, hydrofluorocarbons (HFCs), do not harm ozone. Which issue BEST illustrates a continuing trade-off with HFCs?

HFCs are potent greenhouse gases that trap far more heat per molecule than carbon dioxide.

HFCs are highly flammable, so refrigerators that use them frequently catch on fire.

HFCs break down quickly into chlorine atoms that destroy ozone in the upper atmosphere.

HFCs are radioactive, so their leaks expose households to increased levels of radiation.

A plant is supplied with water in which the oxygen atoms are the heavy isotope oxygen-18, while the carbon dioxide it absorbs contains only ordinary oxygen-16. During photosynthesis, in which product will the oxygen-18 label first appear?

Oxygen gas released from the thylakoids

Carbon dioxide fixed by the enzyme rubisco

Three-carbon sugar made in the Calvin cycle

Glucose assembled from Calvin cycle products

Along a coastline, a deep ocean trench lies offshore, a chain of explosive andesitic volcanoes rises on the continent inland, and earthquake foci become progressively deeper with distance inland from the trench. Which type of plate boundary BEST explains these features?

Oceanic–continental convergent

Continental–continental convergent

A student records the following statements about a burnt-out campfire site. Which statement is an inference rather than an observation?

The campers left the site early this morning before the fire had cooled.

Three partly burned logs lie in the center of the ring of stones.

The ashes in the fire ring are gray and still feel warm to the touch.

A faint smell of smoke is present near the edge of the fire ring.

Farmers who plant corn engineered to produce the Bt insecticidal protein are required to plant nearby refuges of non-Bt corn. What is the primary purpose of these refuges?

They slow the evolution of resistance by preserving susceptible pests that can mate with resistant ones.

They provide a backup harvest in case the Bt corn fails to produce the insecticidal protein in time.

They give beneficial insects a place to live because Bt corn is toxic to all insect species present.

They prevent pollen from Bt corn from spreading the insecticide gene into wild plant species nearby.

Cyanide binds to cytochrome c oxidase, the final protein complex of the mitochondrial electron transport chain. Which of the following is the MOST direct effect of cyanide poisoning on a cell?

Electrons cannot be passed to oxygen, so oxidative phosphorylation stops

Lactic acid cannot be produced, so fermentation stops completely

Glucose cannot be split into pyruvate, so glycolysis stops completely

Carbon dioxide cannot be released, so the Krebs cycle speeds up

The volcanic islands and seamounts of the Hawaiian chain become progressively older toward the northwest, and only the southeasternmost island has active volcanoes. Which explanation BEST accounts for this pattern?

The Pacific Plate is moving northwest over a stationary hot spot.

A hot spot is moving northwest beneath a stationary plate.

The Pacific Plate is moving southeast over a stationary hot spot.

The islands formed above a subduction zone along the chain.

Praxis Pennsylvania Grades 4–8 Subject Concentration: Science (5159)

Free Praxis Pennsylvania Grades Practice Questions

10 sample questions for the Praxis Pennsylvania Grades 4–8 Subject Concentration: Science (5159), 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: Basic Principles and Processes, Physical Sciences, Life Sciences, Earth and Space Sciences.

Question 1 · Basic Principles and Processes
A student argues that the kinetic molecular theory of gases will eventually be promoted to a law once enough supporting experiments have been performed. Which statement BEST corrects the student's reasoning?
- A.Theories and laws are interchangeable terms, so the label depends only on which scientist named the idea.
- B.Laws are tentative hypotheses that are later upgraded to theories once a mechanism has been identified.
- C.Theories explain why phenomena occur, while laws describe patterns, so a theory does not become a law.
- D.Theories become laws only after they are confirmed by experiments repeated in many different countries.
Show answer and explanation
Answer: C. Theories explain why phenomena occur, while laws describe patterns, so a theory does not become a law.
A scientific law summarizes an observed regularity (often mathematically), whereas a theory is a well-supported explanation of why that regularity occurs; they are different kinds of knowledge, so a theory does not become a law. The idea that laws are upgraded hypotheses reverses the relationship, and the claim that the terms are interchangeable ignores this real distinction.
Question 2 · Basic Principles and Processes
Chlorofluorocarbons (CFCs) were once widely used as refrigerants because they are nontoxic and nonflammable, but they were found to destroy stratospheric ozone. Their common replacements, hydrofluorocarbons (HFCs), do not harm ozone. Which issue BEST illustrates a continuing trade-off with HFCs?
- A.HFCs are highly flammable, so refrigerators that use them frequently catch on fire.
- B.HFCs break down quickly into chlorine atoms that destroy ozone in the upper atmosphere.
- C.HFCs are potent greenhouse gases that trap far more heat per molecule than carbon dioxide.
- D.HFCs are radioactive, so their leaks expose households to increased levels of radiation.
Show answer and explanation
Answer: C. HFCs are potent greenhouse gases that trap far more heat per molecule than carbon dioxide.
Because HFCs contain no chlorine they do not deplete ozone, but many are powerful greenhouse gases with global warming potentials hundreds to thousands of times that of carbon dioxide, which is why their use is now being phased down. The chlorine claim describes CFCs, not HFCs, and HFCs are neither radioactive nor characteristically highly flammable.
Question 3 · Physical Sciences
Calcium phosphate is the main mineral in bone and tooth enamel. What is its correct chemical formula?
- A.CaPO₄
- B.Ca(PO₄)₂
- C.Ca₂(PO₄)₃
- D.Ca₃(PO₄)₂
Show answer and explanation
Answer: D. Ca₃(PO₄)₂
Calcium forms Ca²⁺ ions and phosphate is the polyatomic ion PO₄³⁻. Balancing charge requires three calcium ions (+6) for every two phosphate ions (−6), giving Ca₃(PO₄)₂. The formula CaPO₄ ignores the unequal charges, and Ca₂(PO₄)₃ swaps the subscripts, giving +4 and −9.
Question 4 · Life Sciences
A plant is supplied with water in which the oxygen atoms are the heavy isotope oxygen-18, while the carbon dioxide it absorbs contains only ordinary oxygen-16. During photosynthesis, in which product will the oxygen-18 label first appear?
- A.Oxygen gas released from the thylakoids
- B.Carbon dioxide fixed by the enzyme rubisco
- C.Three-carbon sugar made in the Calvin cycle
- D.Glucose assembled from Calvin cycle products
Show answer and explanation
Answer: A. Oxygen gas released from the thylakoids
In the light reactions, photosystem II splits water to replace electrons, releasing the oxygen atoms of water as O2 gas. The oxygen atoms incorporated into sugars produced in the Calvin cycle come from carbon dioxide, not from water.
Question 5 · Earth and Space Sciences
Along a coastline, a deep ocean trench lies offshore, a chain of explosive andesitic volcanoes rises on the continent inland, and earthquake foci become progressively deeper with distance inland from the trench. Which type of plate boundary BEST explains these features?
- A.Continental–continental convergent
- B.Oceanic–continental convergent
- C.Continental transform
- D.Oceanic–oceanic convergent
Show answer and explanation
Answer: B. Oceanic–continental convergent
Dense oceanic lithosphere subducting beneath a continent produces a trench, a dipping zone of deepening earthquakes, and a continental volcanic arc fed by melting above the slab, as in the Andes. Oceanic–oceanic convergence produces a volcanic island arc rather than volcanoes on a continent. Continental collisions build high mountains with little volcanism, and transform boundaries lack trenches and deep earthquakes.
Question 6 · Basic Principles and Processes
A student records the following statements about a burnt-out campfire site. Which statement is an inference rather than an observation?
- A.The campers left the site early this morning before the fire had cooled.
- B.Three partly burned logs lie in the center of the ring of stones.
- C.The ashes in the fire ring are gray and still feel warm to the touch.
- D.A faint smell of smoke is present near the edge of the fire ring.
Show answer and explanation
Answer: A. The campers left the site early this morning before the fire had cooled.
An observation is information gathered directly through the senses, such as the color and warmth of ashes, the number of logs, or a smell. Concluding when the campers left is an inference: a reasoned interpretation of those observations that could turn out to be wrong.
Question 7 · Basic Principles and Processes
Farmers who plant corn engineered to produce the Bt insecticidal protein are required to plant nearby refuges of non-Bt corn. What is the primary purpose of these refuges?
- A.They slow the evolution of resistance by preserving susceptible pests that can mate with resistant ones.
- B.They provide a backup harvest in case the Bt corn fails to produce the insecticidal protein in time.
- C.They give beneficial insects a place to live because Bt corn is toxic to all insect species present.
- D.They prevent pollen from Bt corn from spreading the insecticide gene into wild plant species nearby.
Show answer and explanation
Answer: A. They slow the evolution of resistance by preserving susceptible pests that can mate with resistant ones.
Bt crops impose strong selection for resistant pests; refuges maintain a large population of susceptible insects whose matings with rare resistant insects dilute resistance genes in the next generation. The Bt protein is toxic mainly to specific insect groups, not all insects, and refuges are not intended as backup harvests.
Question 8 · Physical Sciences
A compound contains iron ions with a 3+ charge combined with chloride ions. Which choice gives the correct formula and systematic name for this compound?
- A.FeCl₂, iron(III) chloride
- B.Fe₃Cl, iron(III) chloride
- C.FeCl₃, iron(II) chloride
- D.FeCl₃, iron(III) chloride
Show answer and explanation
Answer: D. FeCl₃, iron(III) chloride
Each Fe³⁺ ion requires three Cl⁻ ions to balance its charge, giving FeCl₃, and the Stock system shows the metal's charge with a Roman numeral, so it is iron(III) chloride. FeCl₂ contains Fe²⁺ and is iron(II) chloride, and Fe₃Cl reverses the ratio of ions.
Question 9 · Life Sciences
Cyanide binds to cytochrome c oxidase, the final protein complex of the mitochondrial electron transport chain. Which of the following is the MOST direct effect of cyanide poisoning on a cell?
- A.Electrons cannot be passed to oxygen, so oxidative phosphorylation stops
- B.Lactic acid cannot be produced, so fermentation stops completely
- C.Glucose cannot be split into pyruvate, so glycolysis stops completely
- D.Carbon dioxide cannot be released, so the Krebs cycle speeds up
Show answer and explanation
Answer: A. Electrons cannot be passed to oxygen, so oxidative phosphorylation stops
Cytochrome c oxidase transfers electrons to O2, the final electron acceptor; blocking it halts electron flow, the proton gradient collapses, and ATP synthase stops making ATP. Glycolysis and fermentation occur in the cytosol and can continue, which is why cells shift to anaerobic metabolism and accumulate lactate.
Question 10 · Earth and Space Sciences
The volcanic islands and seamounts of the Hawaiian chain become progressively older toward the northwest, and only the southeasternmost island has active volcanoes. Which explanation BEST accounts for this pattern?
- A.A hot spot is moving northwest beneath a stationary plate.
- B.The Pacific Plate is moving southeast over a stationary hot spot.
- C.The islands formed above a subduction zone along the chain.
- D.The Pacific Plate is moving northwest over a stationary hot spot.
Show answer and explanation
Answer: D. The Pacific Plate is moving northwest over a stationary hot spot.
A mantle plume that stays roughly fixed builds a volcano on the plate above it, and as the plate moves northwest, each volcano is carried away and goes extinct while a new one forms over the plume. That leaves the youngest, active volcanoes at the southeast end. A plate moving southeast would leave older islands to the southeast, and the chain lies in the middle of a plate rather than along a subduction zone.

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