HESI A2 Chemistry

Correct Answer: D

Rationale: The correct answer is D, H₂O. Water (H₂O) contains a polar covalent bond. This is because oxygen is more electronegative than hydrogen, creating an unequal sharing of electrons, resulting in a partial negative charge on oxygen and a partial positive charge on hydrogen. This polarity makes water a universal solvent and gives rise to properties like surface tension and hydrogen bonding. Choices A, B, and C (O, F, Br) do not form polar covalent bonds as they do not have significantly different electronegativities compared to the other element in the bond.

Correct Answer: B

Rationale: The correct answer is B: Beta radiation. Beta radiation consists of high-energy electrons or positrons traveling at high speeds, typically around 16,000 km/sec. This is because beta particles are emitted during the decay of a radioactive nucleus when a neutron changes into a proton and an electron (beta-minus decay) or a proton changes into a neutron and a positron (beta-plus decay). Alpha radiation (A) consists of helium nuclei, gamma radiation (C) is high-energy electromagnetic radiation, and there is no such thing as delta radiation (D) in the context of nuclear physics.

Correct Answer: B

Rationale: The correct answer is B: Decomposition. In this reaction, a single reactant (AB) breaks down into two or more products (A and B). This aligns with the definition of a decomposition reaction, where a compound is broken down into simpler substances. Choice A (Synthesis) involves combining two or more substances to form a more complex product, which is not the case here. Choices C (Single replacement) and D (Double replacement) involve the exchange of ions or elements between compounds, which is not happening in the given reaction. Thus, based on the breakdown of reactants and products, the correct classification of the reaction is decomposition.

Correct Answer: A

Rationale: The correct answer is A: Zinc sulfate. ZnSO₄ consists of zinc (Zn) and sulfate (SO₄) ions. The name of an ionic compound follows the pattern of cation name + anion name with appropriate suffixes. In this case, "sulfate" is the correct name for the polyatomic ion SO₄, so the compound is named zinc sulfate. Incorrect choices: B: Zinc sulfide - Sulfide is a different anion (S²⁻) than sulfate. C: Zinc sulfur - Incorrect as it doesn't specify the anion involved. D: Zinc oxide - Oxide is a different anion (O²⁻) than sulfate.

Correct Answer: C

Rationale: The correct answer is C: black. Red and white phosphorus are well-known allotropes. Black phosphorus is another important allotrope with a layered structure. It is stable at room temperature and has unique electronic properties. Green, gray, and silver are not commonly recognized allotropic forms of phosphorus, making them incorrect choices.