HESI A2 Chemistry Practice Questions: 30 Questions on Atoms, Reactions & Solutions [2026]

Why Chemistry Matters on the HESI A2

The Chemistry section of the HESI A2 exam typically contains 30 questions and tests your understanding of fundamental chemistry concepts that are directly relevant to nursing practice. From understanding how medications dissolve in the body to interpreting lab values, chemistry knowledge is essential for safe patient care.

Many nursing students find chemistry intimidating, but the HESI A2 focuses on foundational concepts rather than advanced organic chemistry. This practice test mirrors the actual exam format and difficulty level, giving you the targeted preparation you need to score well.

How to Use This Practice Test

For the best results, follow these steps:

• Simulate real test conditions — Give yourself 25 minutes to complete all 30 questions without notes
• Mark uncertain answers — Flag questions you're unsure about before checking explanations
• Study the explanations — Read every rationale, even for questions you answered correctly
• Track your weak areas — Note which subcategories need more review

Section 1: Atomic Structure & The Periodic Table (Questions 1–10)

Question 1

What subatomic particle determines the identity of an element?

• A) Electron
• B) Neutron
• C) Proton
• D) Photon

Answer: C) Proton

Explanation: The number of protons (atomic number) defines what element an atom is. For example, every atom with 6 protons is carbon, regardless of how many neutrons or electrons it has. Neutrons affect the mass number and isotope identity, while electrons determine charge and chemical reactivity.

Question 2

An atom has 11 protons, 12 neutrons, and 11 electrons. What is its mass number?

• A) 11
• B) 12
• C) 22
• D) 23

Answer: D) 23

Explanation: Mass number = protons + neutrons = 11 + 12 = 23. This atom is sodium (Na), which has an atomic number of 11. Electrons are not included in the mass number calculation because their mass is negligible compared to protons and neutrons.

Question 3

Which of the following elements is classified as a noble gas?

• A) Chlorine (Cl)
• B) Neon (Ne)
• C) Sodium (Na)
• D) Oxygen (O)

Answer: B) Neon (Ne)

Explanation: Noble gases are found in Group 18 (far right column) of the periodic table. They include helium, neon, argon, krypton, xenon, and radon. Noble gases have a full outer electron shell, making them extremely stable and largely unreactive. This is why they are sometimes called "inert gases."

Question 4

What does the atomic number of an element represent?

• A) The number of neutrons in the nucleus
• B) The total number of protons and neutrons
• C) The number of protons in the nucleus
• D) The number of electron shells

Answer: C) The number of protons in the nucleus

Explanation: The atomic number is the defining characteristic of an element and equals the number of protons in the nucleus. Elements on the periodic table are arranged by increasing atomic number. In a neutral atom, the number of protons also equals the number of electrons.

Question 5

Isotopes of the same element differ in their number of:

• A) Protons
• B) Electrons
• C) Neutrons
• D) Electron shells

Answer: C) Neutrons

Explanation: Isotopes are atoms of the same element (same number of protons) that have different numbers of neutrons. For example, Carbon-12 has 6 neutrons and Carbon-14 has 8 neutrons, but both have 6 protons. Some isotopes are radioactive and are used in medical imaging and treatment.

Question 6

Which electron shell is closest to the nucleus?

• A) K shell (n=1)
• B) L shell (n=2)
• C) M shell (n=3)
• D) N shell (n=4)

Answer: A) K shell (n=1)

Explanation: Electron shells are numbered starting from the nucleus outward. The K shell (n=1) is closest to the nucleus and holds a maximum of 2 electrons. The L shell (n=2) holds up to 8, and the M shell (n=3) holds up to 18. Electrons fill the innermost available shells first (Aufbau principle).

Question 7

Elements in the same group (column) of the periodic table share similar:

• A) Mass numbers
• B) Numbers of neutrons
• C) Chemical properties
• D) Numbers of electron shells

Answer: C) Chemical properties

Explanation: Elements in the same group have the same number of valence electrons, which determines their chemical behavior. For example, Group 1 elements (lithium, sodium, potassium) all have 1 valence electron and react vigorously with water. Elements in the same period (row) share the same number of electron shells.

Question 8

What is the maximum number of electrons that can occupy the second energy level (n=2)?

• A) 2
• B) 6
• C) 8
• D) 18

Answer: C) 8

Explanation: The maximum number of electrons per energy level is calculated using the formula 2n², where n is the shell number. For the second energy level: 2(2²) = 2(4) = 8 electrons. This shell contains one s orbital (2 electrons) and three p orbitals (6 electrons).

Question 9

Which of the following is an alkaline earth metal?

• A) Potassium (K)
• B) Calcium (Ca)
• C) Iron (Fe)
• D) Aluminum (Al)

Answer: B) Calcium (Ca)

Explanation: Alkaline earth metals are in Group 2 of the periodic table and include beryllium, magnesium, calcium, strontium, barium, and radium. Calcium is especially important in nursing because it plays crucial roles in bone health, muscle contraction, blood clotting, and nerve function. Potassium is in Group 1 (alkali metals).

Question 10

An ion with a charge of +2 has:

• A) Gained 2 protons
• B) Lost 2 electrons
• C) Gained 2 electrons
• D) Lost 2 protons

Answer: B) Lost 2 electrons

Explanation: When an atom loses electrons, it becomes a positively charged ion (cation). Losing 2 electrons means there are 2 more protons than electrons, giving a +2 charge. For example, Ca²⁺ is a calcium ion that has lost 2 electrons. This is clinically significant because electrolyte imbalances involve these charged ions.

Section 2: Chemical Bonds & Reactions (Questions 11–20)

Question 11

What type of bond forms when electrons are shared between two atoms?

• A) Ionic bond
• B) Covalent bond
• C) Hydrogen bond
• D) Metallic bond

Answer: B) Covalent bond

Explanation: Covalent bonds form when two atoms share one or more pairs of electrons. This typically occurs between nonmetal atoms. For example, water (H₂O) has covalent bonds between oxygen and hydrogen. Ionic bonds involve the transfer of electrons, while hydrogen bonds are weak attractions between molecules.

Question 12

In the reaction 2H₂ + O₂ → 2H₂O, what are the products?

• A) H₂ and O₂
• B) 2H₂O
• C) H₂O₂
• D) 2H₂ and O₂

Answer: B) 2H₂O

Explanation: Products are the substances formed on the right side of a chemical equation (after the arrow). In this synthesis reaction, hydrogen gas and oxygen gas combine to form water. The reactants (starting materials) are on the left side: 2H₂ and O₂.

Question 13

Which type of reaction involves a substance breaking down into simpler components?

• A) Synthesis
• B) Single replacement
• C) Decomposition
• D) Double replacement

Answer: C) Decomposition

Explanation: In a decomposition reaction, a single compound breaks down into two or more simpler substances (AB → A + B). For example, hydrogen peroxide decomposes into water and oxygen: 2H₂O₂ → 2H₂O + O₂. Synthesis reactions are the opposite, combining simpler substances into a more complex one.

Question 14

What is the correct way to balance the equation: Fe + O₂ → Fe₂O₃?

• A) Fe + O₂ → Fe₂O₃
• B) 2Fe + O₂ → Fe₂O₃
• C) 4Fe + 3O₂ → 2Fe₂O₃
• D) 3Fe + 2O₂ → Fe₂O₃

Answer: C) 4Fe + 3O₂ → 2Fe₂O₃

Explanation: A balanced equation has equal numbers of each type of atom on both sides. Checking: Left side has 4 Fe and 6 O atoms; right side has 4 Fe (2×2) and 6 O atoms (2×3). The law of conservation of mass requires that matter is neither created nor destroyed in a chemical reaction.

Question 15

Which of the following is an example of a chemical change?

• A) Ice melting into water
• B) Sugar dissolving in tea
• C) Iron rusting
• D) Cutting paper into pieces

Answer: C) Iron rusting

Explanation: Rusting is a chemical change because iron reacts with oxygen and water to form iron oxide (rust), creating a new substance with different properties. Melting, dissolving, and cutting are all physical changes—the substance's chemical identity remains the same even though its form changes.

Question 16

A catalyst in a chemical reaction:

• A) Increases the amount of product formed
• B) Decreases the activation energy needed
• C) Is consumed during the reaction
• D) Changes the products of the reaction

Answer: B) Decreases the activation energy needed

Explanation: A catalyst speeds up a chemical reaction by lowering the activation energy (the minimum energy needed to start the reaction). Importantly, the catalyst is not consumed—it can be used repeatedly. In the body, enzymes act as biological catalysts, speeding up metabolic reactions essential for life.

Question 17

In an exothermic reaction, energy is:

• A) Absorbed from the surroundings
• B) Released to the surroundings
• C) Neither absorbed nor released
• D) Converted into matter

Answer: B) Released to the surroundings

Explanation: Exothermic reactions release energy, usually as heat, making the surroundings warmer. The prefix "exo-" means "out." Examples include combustion and cellular respiration. Endothermic reactions absorb energy from the surroundings. In nursing, understanding these concepts helps explain body temperature regulation and metabolic processes.

Question 18

What type of bond forms between sodium (Na) and chlorine (Cl)?

• A) Covalent bond
• B) Hydrogen bond
• C) Ionic bond
• D) Van der Waals force

Answer: C) Ionic bond

Explanation: Sodium (a metal) transfers its one valence electron to chlorine (a nonmetal), forming Na⁺ and Cl⁻ ions. The electrostatic attraction between these oppositely charged ions creates an ionic bond, producing sodium chloride (NaCl, table salt). Ionic bonds typically form between metals and nonmetals with large electronegativity differences.

Question 19

Which factor will NOT increase the rate of a chemical reaction?

• A) Increasing the temperature
• B) Adding a catalyst
• C) Increasing the concentration of reactants
• D) Increasing the volume of the container

Answer: D) Increasing the volume of the container

Explanation: Increasing the container volume actually decreases the concentration of reactants (same amount of substance in more space), which slows the reaction. Higher temperature, catalysts, and increased concentration all increase reaction rates by increasing the frequency and energy of molecular collisions.

Question 20

The law of conservation of mass states that:

• A) Energy cannot be created or destroyed
• B) Mass is always gained in chemical reactions
• C) Matter cannot be created or destroyed in a chemical reaction
• D) Products always weigh more than reactants

Answer: C) Matter cannot be created or destroyed in a chemical reaction

Explanation: The law of conservation of mass means the total mass of reactants must equal the total mass of products. This is why chemical equations must be balanced—the same number and type of atoms must appear on both sides. Choice A describes the law of conservation of energy, which is a separate principle.

Section 3: Solutions, Acids, Bases & Biochemistry (Questions 21–30)

Question 21

A solution with a pH of 3 is:

• A) Strongly basic
• B) Neutral
• C) Weakly acidic
• D) Strongly acidic

Answer: D) Strongly acidic

Explanation: The pH scale ranges from 0 to 14. Values below 7 are acidic, 7 is neutral, and above 7 is basic (alkaline). A pH of 3 is strongly acidic. Stomach acid (gastric juice) has a pH of about 1.5–3.5. In nursing, understanding pH is critical for interpreting blood gas values and recognizing acidosis or alkalosis.

Question 22

What is the solvent in a saline (salt water) solution?

• A) Sodium chloride
• B) Water
• C) Salt crystals
• D) Chlorine gas

Answer: B) Water

Explanation: In any solution, the solvent is the substance that does the dissolving (present in greater quantity), and the solute is the substance being dissolved. In saline solution, water is the solvent and sodium chloride (salt) is the solute. Water is often called the "universal solvent" because it dissolves more substances than any other liquid.

Question 23

Which of the following is a property of acids?

• A) They feel slippery
• B) They taste bitter
• C) They turn blue litmus paper red
• D) They have a pH above 7

Answer: C) They turn blue litmus paper red

Explanation: Acids turn blue litmus paper red, taste sour (though you should never taste chemicals), and have a pH below 7. Bases feel slippery, taste bitter, and turn red litmus paper blue. This is a commonly tested concept on the HESI A2 because nurses work with acidic and basic solutions regularly.

Question 24

Normal blood pH in the human body is approximately:

• A) 6.8–7.0
• B) 7.35–7.45
• C) 7.5–8.0
• D) 8.0–8.5

Answer: B) 7.35–7.45

Explanation: Normal arterial blood pH ranges from 7.35 to 7.45, making it slightly alkaline. A pH below 7.35 indicates acidosis, while above 7.45 indicates alkalosis. Both conditions can be life-threatening. The body uses buffer systems, the lungs, and the kidneys to maintain this narrow pH range—a concept you'll study extensively in nursing school.

Question 25

When an acid and a base react together, this is called a:

• A) Combustion reaction
• B) Decomposition reaction
• C) Neutralization reaction
• D) Oxidation reaction

Answer: C) Neutralization reaction

Explanation: A neutralization reaction occurs when an acid reacts with a base to produce water and a salt. For example: HCl + NaOH → NaCl + H₂O. The products are generally closer to neutral pH than the reactants. Antacids work through neutralization, using a base (like calcium carbonate) to neutralize excess stomach acid.

Question 26

What happens when you increase the concentration of a solute in a solution?

• A) The boiling point decreases
• B) The freezing point increases
• C) The boiling point increases
• D) The solution becomes less dense

Answer: C) The boiling point increases

Explanation: Adding more solute to a solution raises its boiling point (boiling point elevation) and lowers its freezing point (freezing point depression). These are colligative properties—they depend on the number of solute particles, not their identity. This principle is why salt is added to icy roads and why saline solutions have slightly different properties than pure water.

Question 27

A buffer solution is important in the human body because it:

• A) Increases the pH of blood
• B) Decreases the pH of blood
• C) Resists changes in pH when acids or bases are added
• D) Converts acids into bases

Answer: C) Resists changes in pH when acids or bases are added

Explanation: Buffer solutions resist dramatic pH changes by neutralizing small amounts of added acid or base. The bicarbonate buffer system (H₂CO₃/HCO₃⁻) is the most important buffer in human blood, maintaining the critical pH range of 7.35–7.45. Without buffers, even small metabolic changes could cause fatal shifts in blood pH.

Question 28

Which of the following correctly describes a saturated solution?

• A) It contains no dissolved solute
• B) It contains the maximum amount of dissolved solute at a given temperature
• C) It can dissolve more solute
• D) It always appears cloudy

Answer: B) It contains the maximum amount of dissolved solute at a given temperature

Explanation: A saturated solution has dissolved the maximum amount of solute possible at a specific temperature. Any additional solute will remain undissolved. An unsaturated solution can dissolve more solute, while a supersaturated solution temporarily holds more solute than normal (unstable). Increasing temperature usually increases solubility for solid solutes.

Question 29

Which organic molecule stores and transmits genetic information?

• A) Lipids
• B) Carbohydrates
• C) Proteins
• D) Nucleic acids

Answer: D) Nucleic acids

Explanation: Nucleic acids (DNA and RNA) store and transmit genetic information. DNA holds the genetic blueprint in the nucleus, while RNA helps translate that information into proteins. Proteins perform most cellular functions, carbohydrates provide energy, and lipids store energy and form cell membranes. These four macromolecules are the biochemical basis of life.

Question 30

Water is considered a polar molecule because:

• A) It has an equal distribution of charge
• B) It has an unequal distribution of charge, with partial positive and negative ends
• C) It is made up of ions
• D) It cannot dissolve other substances

Answer: B) It has an unequal distribution of charge, with partial positive and negative ends

Explanation: Water (H₂O) is polar because oxygen is more electronegative than hydrogen, pulling shared electrons closer to itself. This creates a partial negative charge on the oxygen end and partial positive charges on the hydrogen ends. Water's polarity is responsible for many of its unique properties, including its ability to dissolve ionic and polar substances, high specific heat, and cohesion—all critical for biological processes.

Score Interpretation

After completing all 30 questions, use this guide to assess your readiness:

• 27–30 correct (90–100%) — Excellent! You're well-prepared for the chemistry section
• 24–26 correct (80–89%) — Strong performance; review any missed topics for reinforcement
• 21–23 correct (70–79%) — Good foundation; dedicate additional study time to weak areas
• Below 21 correct (<70%) — Needs improvement; focus on building core concepts before retesting

Key Concepts to Review

Based on common HESI A2 chemistry question patterns, prioritize these topics:

• Periodic table organization — Groups, periods, metals vs. nonmetals
• Atomic structure — Protons, neutrons, electrons, electron configuration
• Chemical bonds — Ionic, covalent, hydrogen bonds
• Balancing equations — Conservation of mass, counting atoms
• Acids and bases — pH scale, properties, neutralization
• Solutions — Solute, solvent, concentration, saturation
• Reaction types — Synthesis, decomposition, single/double replacement

Next Steps for Your Chemistry Preparation

These practice questions cover the core chemistry topics tested on the HESI A2. For comprehensive preparation with hundreds more practice questions across all exam sections, explore our complete study program with detailed explanations and progress tracking to ensure you're fully prepared on test day.