HESI A2 Chemistry Study Guide: Topics, Formulas & Practice Strategy

Chemistry is the HESI A2 section applicants most often discover late. It is optional at many programs and required at plenty of others, so the first thing worth doing is confirming whether your school scores it at all. If it does, the good news is that the content is introductory — this is first-semester general chemistry, not organic — and it is drawn from a fairly predictable list of topics. The bad news is that the section moves quickly and rewards recall of definitions and a handful of formulas rather than deep reasoning. This guide walks through what shows up, the relationships worth memorizing cold, and a study approach that fits the time most applicants actually have.

What the chemistry section looks like

The HESI A2 is modular: your nursing program chooses which sections you sit for and how heavily each one counts. Chemistry is one of the science modules, alongside Biology and Anatomy & Physiology. Before you spend a week on it, email your admissions office and ask exactly which sections are required and how they are weighted. Applicants routinely study chemistry for a program that never scores it, and skip it at a program that does.

If it is on your list, expect a short, briskly paced multiple-choice section — commonly around 25 to 30 questions with a similar number of minutes. Counts and time limits vary by exam version and by school configuration, so treat that as a planning figure and confirm it with your testing site. Many test sites also make a basic on-screen calculator available; ask rather than assume, and practice a few calculations without one so you are not dependent on it.

The practical implication of a short section is that every question carries real weight. Missing four items on a 25-question section is a much bigger score swing than missing four on a 50-question section, which is why the definition-level material below is worth locking down.

Matter, atoms, and the periodic table

This is the foundation the rest of the section is built on, and it is the cheapest material to review.

• Atomic number = the number of protons, and it is what defines the element. Mass number = protons + neutrons.
• Isotopes are atoms of the same element with different numbers of neutrons. Ions differ in electrons: losing electrons gives a positive cation, gaining them gives a negative anion.
• Protons and neutrons sit in the nucleus; electrons occupy shells around it. Protons are positive, electrons negative, neutrons neutral.
• Periods are the horizontal rows; groups (families) are the vertical columns, and elements in a group behave similarly because they share a valence-electron count.
• Know the named families: alkali metals (group 1), alkaline earth metals (group 2), halogens (group 17), and noble gases (group 18, essentially unreactive because their valence shells are full).

Two periodic trends are commonly tested, and they run opposite to each other. Moving left to right across a period, atomic radius generally decreases while ionization energy and electronegativity increase. Moving down a group, atomic radius increases while ionization energy and electronegativity decrease. If you remember only that radius shrinks across and grows down, you can reason out the rest.

Also separate the vocabulary of matter itself: an element is one kind of atom, a compound is two or more elements chemically bonded in fixed ratio, and a mixture is physically combined and separable. A homogeneous mixture (a solution) is uniform throughout; a heterogeneous one is not. And distinguish a physical change (ice melting — same substance, new form) from a chemical change (rusting, burning — a new substance forms).

Bonding and chemical reactions

Bonding questions usually ask you to classify, not to calculate.

• Ionic bonds form when electrons are transferred, typically between a metal and a nonmetal (sodium chloride is the standard example).
• Covalent bonds form when electrons are shared, typically between nonmetals. Sharing is nonpolar when it is roughly equal and polar when one atom pulls harder — water is the classic polar molecule.
• Hydrogen bonds are weak attractions between molecules, not true bonds within them. They explain water’s surface tension and high boiling point, and they matter later in nursing science when you study proteins and DNA.

Reaction types worth recognizing on sight:

1. Synthesis: A + B → AB
2. Decomposition: AB → A + B
3. Single replacement: A + BC → AC + B
4. Double replacement: AB + CD → AD + CB
5. Combustion: a hydrocarbon plus oxygen yields carbon dioxide and water

Balancing equations comes down to one principle: the law of conservation of mass means the same number of each type of atom must appear on both sides. Adjust coefficients in front of formulas — never subscripts, because changing a subscript changes the substance. Work through the most complicated compound first, save pure elements for last, and recount every atom at the end.

The mole, molarity, and the math you actually need

The calculation questions cluster around a few relationships. Memorize these and practice them until the setup is automatic:

• Avogadro’s number: one mole = 6.022 × 10²³ particles.
• Molar mass is the sum of the atomic masses in a formula, in grams per mole. moles = grams ÷ molar mass.
• Molarity (M) = moles of solute ÷ liters of solution. Note liters — converting milliliters is where most errors happen.
• Dilution: M₁V₁ = M₂V₂. This is the same algebra you will use again for IV drug calculations, so it is worth real practice.
• Percent solutions: mass/volume percent is grams of solute per 100 mL of solution.
• Temperature: K = °C + 273.15, and °F = (9/5)°C + 32.

The single highest-value skill here is dimensional analysis — writing units as fractions and cancelling until only the unit you want survives. It converts a chemistry problem into a bookkeeping exercise, it catches setup errors before you reach for the calculator, and it is the exact method used for dosage calculations in nursing school. Learn it now and you get paid for it twice.

Keep scientific notation and significant figures in range too. You do not need a rigorous treatment, but you should be able to move a decimal confidently and recognize when an answer choice is off by a factor of ten — a common distractor.

Solutions, acids and bases, and gas laws

Solutions. The solute dissolves into the solvent. Solubility generally increases with temperature for solids in liquids, and stirring, heating, or increasing surface area all speed dissolving. Be ready for the vocabulary of concentration: dilute versus concentrated, and saturated versus unsaturated.

Acids and bases. Under the Brønsted–Lowry definition, an acid donates a proton (H⁺) and a base accepts one. The pH scale runs 0–14: below 7 is acidic, 7 is neutral, above 7 is basic. Two points the exam likes: pH = −log[H⁺], so a lower pH means a higher hydrogen-ion concentration, and each whole pH unit is a tenfold change — pH 4 is ten times more acidic than pH 5, and a hundred times more acidic than pH 6. An acid plus a base yields a salt and water in a neutralization reaction, and buffers resist pH change, which is why they turn up again in blood-gas physiology.

States of matter and gas laws. Know the phase changes by name: melting, freezing, vaporization, condensation, and the two that skip the liquid phase — sublimation (solid to gas) and deposition (gas to solid). For gases, the relationships are more useful than the equations:

• Boyle’s law: at constant temperature, pressure and volume are inversely related (P₁V₁ = P₂V₂). Squeeze a gas and the pressure rises.
• Charles’s law: at constant pressure, volume and temperature are directly related. Heat a gas and it expands.
• Ideal gas law: PV = nRT, which ties the others together.

If nuclear chemistry appears, it is usually definitional: alpha, beta, and gamma radiation in order of increasing penetrating power, and half-life as the time for half of a radioactive sample to decay.

A realistic study plan and how the questions are worded

Most applicants have days, not months. Spend them in this order:

1. Confirm the section is required and find out how your program weights it.
2. Take a timed practice set first, not last. A diagnostic tells you whether you need vocabulary review or calculation drill, and those call for completely different study.
3. Front-load the definitions — atoms, bonds, reaction types, phase changes, acid/base vocabulary. This is the largest block of points per hour invested.
4. Drill the six formulas above until you can write them from memory, then work problems with units written out every time.
5. Finish with timed sets so the pace stops being a surprise.

Question patterns to expect: straightforward recall (“which bond involves the transfer of electrons?”), classification (identify the reaction type or the family an element belongs to), one-step calculation (molarity, a dilution, a unit conversion), and relationship reasoning (if temperature rises and pressure is held constant, what happens to volume?). Very little of it requires multi-step derivation.

Two habits protect your score. First, read the units in the answer choices before you calculate — they often reveal what the question actually wants and rule out two options immediately. Second, do not stall on a hard item. On a section this short, two minutes spent on one stubborn question can cost you three easy ones at the end. Flag it, move on, and come back if time allows.