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HESI A2 Chemistry study materials
HESI A2 Chemistry study resources

HESI A2 Chemistry practice questions

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Question 1Matter, Measurement, and Chemical Foundations

Which statement correctly describes a physical change rather than a chemical change?

  1. Ice melts into liquid water when warmed. Correct answer.
  2. Milk sours after standing at room temperature.
  3. A match burns and releases heat.
  4. Iron rusts when exposed to moist air.

Explanation

A physical change alters form or state without producing a new substance, so melting ice remains H2O and is reversible by cooling. Chemical changes create new substances with different properties, as in rusting, burning, and souring.

Question 2Matter, Measurement, and Chemical Foundations

A pure substance is heated and gives off two different gases that cannot be recombined by simple physical means. This observation is most consistent with which classification?

  1. An element
  2. A compound Correct answer.
  3. A heterogeneous mixture
  4. A homogeneous mixture

Explanation

A compound is a pure substance that can be broken down chemically into two or more simpler substances. Producing two distinct gases by heating indicates chemical decomposition, which mixtures and elements do not show in this way.

Question 3Matter, Measurement, and Chemical Foundations

Which set contains only homogeneous mixtures?

  1. Brass, sand and water, and air
  2. Salt water, granite, and air
  3. Sand and water, oil and water, and granite
  4. Salt water, air, and brass Correct answer.

Explanation

Homogeneous mixtures have the same composition throughout a sample, so dissolved salt in water, well-mixed air, and a brass alloy all qualify. Sand with water, oil with water, and granite are visibly non-uniform and count as heterogeneous.

Question 4Matter, Measurement, and Chemical Foundations

Which measurement has four significant figures?

  1. 2.500 g Correct answer.
  2. 0.006 g
  3. 1200 g
  4. 0.0450 g

Explanation

In 2.500 g the trailing zeros after the decimal point are significant, giving four significant figures. Leading zeros after a decimal point are never significant, and trailing zeros in a whole number without a decimal point are ambiguous.

Question 5Matter, Measurement, and Chemical Foundations

A sample of metal has a mass of 54.0 g and a volume of 20.0 mL. What is its density?

  1. 34.0 g/mL
  2. 0.370 g/mL
  3. 1080 g/mL
  4. 2.70 g/mL Correct answer.

Explanation

Density equals mass divided by volume, so 54.0 g divided by 20.0 mL gives 2.70 g/mL. The result carries three significant figures because both given values have three.

Question 6Matter, Measurement, and Chemical Foundations

Which statement correctly distinguishes mass from weight?

  1. Mass depends on local gravity, while weight does not.
  2. Mass and weight are both measured in newtons.
  3. Mass changes when an object is taken to the Moon, but weight stays the same.
  4. Mass is an amount of matter, while weight is the gravitational force on that matter. Correct answer.

Explanation

Mass measures the quantity of matter and is independent of location, while weight is the force of gravity acting on that mass, equal to mass times gravitational acceleration. Because gravity differs between Earth and the Moon, weight changes while mass does not.

Question 7Matter, Measurement, and Chemical Foundations

Which of the following is an intensive property of a sample of pure water?

  1. Its volume
  2. Its mass
  3. Its heat capacity in joules per kelvin
  4. Its density Correct answer.

Explanation

Intensive properties do not depend on the amount of sample, and density of pure water is the same for a drop or a liter at a given temperature. Volume, mass, and total heat capacity all scale with the quantity of material present.

Question 8Matter, Measurement, and Chemical Foundations

A sealed container holds a liquid in equilibrium with its vapor at a fixed temperature. Which change increases the vapor pressure of the liquid?

  1. Increasing the surface area of the liquid
  2. Increasing the volume of the container
  3. Adding more of the same liquid
  4. Increasing the temperature of the container Correct answer.

Explanation

Vapor pressure depends only on temperature and the identity of the liquid, because raising temperature increases the fraction of molecules with enough energy to escape. Volume, amount of liquid, and surface area affect how fast equilibrium is reached but not the equilibrium pressure.

Question 9Matter, Measurement, and Chemical Foundations

Which description matches a solid that has a definite shape and a definite volume?

  1. Particles are ionized and move independently at high speed.
  2. Particles are widely separated and move freely past one another.
  3. Particles are closely packed but slide past one another freely.
  4. Particles are closely packed in fixed positions and vibrate about those positions. Correct answer.

Explanation

In a solid, particles are held close together in an ordered arrangement and can only vibrate about fixed positions, which gives the sample a definite shape and volume. Freely sliding or widely separated particles describe liquids and gases.

Question 10Matter, Measurement, and Chemical Foundations

Which process changes a gas directly into a solid without passing through the liquid state?

  1. Condensation
  2. Deposition Correct answer.
  3. Fusion
  4. Sublimation

Explanation

Deposition is the phase change in which a gas becomes a solid directly, such as frost forming from water vapor. Sublimation is the reverse, and condensation and fusion involve the liquid state.

Question 11Matter, Measurement, and Chemical Foundations

During the boiling of pure water at constant pressure, which statement is true?

  1. The temperature falls as vapor forms.
  2. The temperature remains constant while energy is used to separate particles. Correct answer.
  3. The temperature rises steadily while liquid remains.
  4. No energy is absorbed once boiling begins.

Explanation

At the boiling point, added energy goes into overcoming intermolecular attractions rather than raising kinetic energy, so the temperature stays constant until all liquid has vaporized. This plateau is a characteristic of a phase change at constant pressure.

Question 12Matter, Measurement, and Chemical Foundations

Which is an example of a chemical property of a substance?

  1. Its density
  2. Its melting point
  3. Its color
  4. Its flammability Correct answer.

Explanation

A chemical property describes how a substance reacts or transforms into a new substance, as flammability does during combustion. Melting point, color, and density are observed without changing the substance's identity.

Question 13Matter, Measurement, and Chemical Foundations

Which technique is best suited to separate a dissolved salt from its aqueous solution while recovering the water?

  1. Filtration
  2. Simple distillation Correct answer.
  3. Magnetic separation
  4. Decantation

Explanation

Simple distillation boils off the water and condenses it in a separate vessel, leaving the dissolved salt behind and recovering the water. Filtration and decantation separate undissolved solids or immiscible liquids, not dissolved ions.

Question 14Matter, Measurement, and Chemical Foundations

A liquid is described as having a high viscosity. What does this indicate about the liquid?

  1. It evaporates very quickly at room temperature.
  2. It flows relatively slowly because of strong internal resistance to flow. Correct answer.
  3. It conducts electricity well in the pure state.
  4. It has a very low boiling point.

Explanation

Viscosity measures resistance to flow, so a high-viscosity liquid such as honey pours slowly because its particles interact strongly. Viscosity is not directly tied to evaporation rate, boiling point, or electrical conductivity.

Question 15Matter, Measurement, and Chemical Foundations

Which statement describes the law of conservation of mass as applied to a chemical reaction in a closed system?

  1. The total volume of the products equals the total volume of the reactants.
  2. The number of molecules is the same before and after the reaction.
  3. The total mass of the products equals the total mass of the reactants. Correct answer.
  4. The total energy of the system increases as products form.

Explanation

In a closed system, atoms are rearranged but not created or destroyed, so the total mass before and after a reaction is unchanged. Molecule counts and volumes can change because atoms recombine into different substances.

Question 16Matter, Measurement, and Chemical Foundations

A mixture of iron filings and sulfur powder is heated strongly until a new black solid forms that is no longer attracted to a magnet. What does this demonstrate?

  1. The iron sublimed and left the sulfur unchanged.
  2. A physical mixture was formed, since no color change occurred.
  3. The sulfur melted and simply coated the iron filings.
  4. A compound formed through a chemical reaction, with properties different from the starting materials. Correct answer.

Explanation

Heating iron with sulfur produces iron sulfide, a compound whose lack of magnetic attraction and new color show that a chemical reaction occurred. A physical mixture would retain the properties of both components and could be separated by a magnet.

Question 17Matter, Measurement, and Chemical Foundations

A graduated cylinder contains 25.0 mL of water. A metal object is lowered in and the level rises to 31.5 mL. What is the volume of the object?

  1. 6.5 mL Correct answer.
  2. 0.25 mL
  3. 31.5 mL
  4. 56.5 mL

Explanation

By displacement, the object's volume equals the increase in the water level, so 31.5 mL minus 25.0 mL equals 6.5 mL. The subtraction preserves one decimal place, matching the precision of the readings.

Question 18Matter, Measurement, and Chemical Foundations

A thermometer reads 303 K. What is this temperature in degrees Celsius?

  1. 273 degrees Celsius
  2. 576 degrees Celsius
  3. 3 degrees Celsius
  4. 30 degrees Celsius Correct answer.

Explanation

Celsius temperature equals Kelvin temperature minus 273, so 303 K minus 273 gives 30 degrees Celsius. This conversion follows from the definition of the Kelvin scale relative to the Celsius scale.

Question 19Matter, Measurement, and Chemical Foundations

Which of the following is a correct statement about the precision of a measurement?

  1. Precision is improved only by using a larger sample.
  2. Precision refers to how close a result is to the accepted value.
  3. Precision describes how closely repeated measurements agree with one another. Correct answer.
  4. Precision is the same as accuracy for a single trial.

Explanation

Precision describes the reproducibility of repeated measurements, while accuracy describes closeness to the true value. A set of readings can be precise but inaccurate if the instrument is miscalibrated.

Question 20Matter, Measurement, and Chemical Foundations

Which value is expressed with the greatest number of significant figures?

  1. 900
  2. 0.0042
  3. 5.607 Correct answer.
  4. 3.1 x 10^2

Explanation

The value 5.607 contains four significant figures because every nonzero digit and the zero between them counts. Scientific notation, leading zeros, and ambiguous trailing zeros reduce the count in the other choices.

Question 21Matter, Measurement, and Chemical Foundations

Which observation best supports the conclusion that a sample is a heterogeneous mixture rather than a compound?

  1. Different regions of the sample show visibly different colors and can be separated mechanically. Correct answer.
  2. The sample reacts with acid to produce a gas.
  3. The sample melts sharply at a single temperature.
  4. The sample has a constant density throughout.

Explanation

Heterogeneous mixtures are non-uniform, so different regions have different composition and the parts can be separated by physical means. A compound is uniform and melts at a sharp characteristic temperature.

Question 22Matter, Measurement, and Chemical Foundations

Which change of state requires the absorption of energy from the surroundings?

  1. Condensation of steam into liquid water
  2. Evaporation of water into vapor Correct answer.
  3. Freezing of water into ice
  4. Deposition of water vapor into frost

Explanation

Evaporation is endothermic because particles must gain energy to overcome intermolecular attractions and escape the liquid. Freezing, condensation, and deposition are exothermic, releasing energy as attractions form.

Question 23Matter, Measurement, and Chemical Foundations

Two samples of the same pure liquid are placed in identical open dishes, one with a large surface area and one with a small surface area, at the same temperature. Which statement is correct?

  1. The sample with the smaller surface area evaporates faster.
  2. Neither sample evaporates because the liquid is pure.
  3. The sample with the larger surface area has a higher vapor pressure.
  4. Both samples have the same vapor pressure, but the larger surface area evaporates faster. Correct answer.

Explanation

Vapor pressure is an intensive property fixed by temperature and the identity of the liquid, so both samples have the same value. A larger surface area exposes more molecules to the gas phase, increasing the rate of evaporation without changing the equilibrium pressure.

Question 24Matter, Measurement, and Chemical Foundations

Which statement about the arrangement of particles in a gas is accurate?

  1. Particles are close together but held in fixed positions.
  2. Particles are far apart relative to their size and fill the container uniformly. Correct answer.
  3. Particles are fixed in a rigid lattice.
  4. Particles are ordered in layers that resist compression.

Explanation

Gas particles are separated by large distances compared with their own size and move randomly, so a gas expands to fill its container and is highly compressible. Fixed or ordered arrangements describe solids and liquids instead.

Question 25Matter, Measurement, and Chemical Foundations

A student reports the density of a metal as 7.85 g/mL, but the accepted value is 7.87 g/mL. Which statement best characterizes this result?

  1. The result is precise but not accurate.
  2. The result is both accurate and precise.
  3. The result is accurate but not necessarily precise. Correct answer.
  4. The result is neither accurate nor precise.

Explanation

Accuracy measures closeness to the accepted value, and 7.85 g/mL is very close to 7.87 g/mL, so the single result is accurate. Precision concerns agreement among repeated measurements, which a single reported value cannot establish.

Question 26Matter, Measurement, and Chemical Foundations

Which SI base unit is used to express the amount of a substance in chemistry calculations?

  1. The mole Correct answer.
  2. The kelvin
  3. The candela
  4. The ampere

Explanation

The mole is the SI base unit for amount of substance and represents an Avogadro number of elementary entities. The kelvin measures thermodynamic temperature, the candela measures luminous intensity, and the ampere measures electric current, so none of those quantify how many particles a sample contains.

Question 27Matter, Measurement, and Chemical Foundations

A sample of pure table salt is described as a compound. Which statement explains why salt is classified this way rather than as a mixture?

  1. Its composition varies depending on the source from which it was mined
  2. Its particles settle out when the sample is left standing undisturbed
  3. Its elements are combined in a fixed mass ratio and cannot be separated by physical means Correct answer.
  4. Its components can be separated by dissolving it in water and filtering

Explanation

A compound consists of two or more elements chemically bonded in a definite, fixed proportion by mass, and separating those elements requires a chemical change. Dissolving and filtering are physical operations that cannot break the sodium-chloride bonds, so salt does not behave like a mixture.

Question 28Matter, Measurement, and Chemical Foundations

Which term describes a measurement result that agrees closely with the true or accepted value?

  1. Random
  2. Systematic
  3. Accurate Correct answer.
  4. Precise

Explanation

Accuracy refers to how close a measured value lies to the accepted or true value. Precision instead describes how closely repeated measurements agree with one another, which can be high even when every reading is shifted away from the true value.

Question 29Matter, Measurement, and Chemical Foundations

Which statement correctly describes the particles in a crystalline solid?

  1. They are separated by distances much larger than their own diameters
  2. They are ionized into freely moving electrons and cations throughout
  3. They vibrate about fixed positions arranged in a repeating geometric pattern Correct answer.
  4. They move freely past one another while staying in contact

Explanation

In a crystalline solid, particles occupy fixed average positions in an orderly, repeating lattice and can only vibrate about those positions. This rigid, ordered arrangement gives crystals definite shape and definite volume.

Question 30Matter, Measurement, and Chemical Foundations

A beaker of water is heated until it boils, and heating continues at constant pressure. Which quantity remains constant while the water is boiling?

  1. The mass of liquid water remaining
  2. The average kinetic energy of the escaping vapor
  3. The temperature of the boiling liquid Correct answer.
  4. The total volume of liquid present

Explanation

At constant pressure, a pure liquid boils at a single fixed temperature, and added energy goes into overcoming intermolecular attractions rather than raising the temperature. The liquid volume and mass both decrease as molecules escape into the vapor phase.

Question 31Matter, Measurement, and Chemical Foundations

Which property is an example of an extensive property of a sample?

  1. Density
  2. Melting point
  3. Volume Correct answer.
  4. Boiling point

Explanation

Extensive properties depend on the size of the sample, so doubling the amount of substance doubles the volume. Density, boiling point, and melting point are intensive because they are unchanged when the sample size changes.

Question 32Matter, Measurement, and Chemical Foundations

A student records the mass of a sample as 12.40 g using a balance that reads to the nearest 0.01 g. How many significant figures are in this recorded mass?

  1. Two
  2. Three
  3. Four Correct answer.
  4. Five

Explanation

All nonzero digits are significant, and a trailing zero after a decimal point is significant because it reflects the balance's precision. The digits 1, 2, 4, and the final 0 therefore give four significant figures.

Question 33Matter, Measurement, and Chemical Foundations

Which description correctly characterizes a homogeneous mixture?

  1. Its different phases are visible as distinct layers
  2. Its components are chemically bonded in fixed proportions
  3. It can contain only a single element in one phase
  4. Its composition is uniform throughout, though it contains two or more substances Correct answer.

Explanation

A homogeneous mixture, such as salt water or air, has uniform composition throughout even though it contains more than one substance that is not chemically bonded. Its components can be separated by physical means.

Question 34Matter, Measurement, and Chemical Foundations

A block has a mass of 54.0 g and a volume of 20.0 cm³. What is its density?

  1. 2.70 g/cm³ Correct answer.
  2. 1080 g/cm³
  3. 0.370 g/cm³
  4. 74.0 g/cm³

Explanation

Density is mass divided by volume, so 54.0 g ÷ 20.0 cm³ = 2.70 g/cm³. The value agrees with the density of aluminum and is reported to three significant figures.

Question 35Matter, Measurement, and Chemical Foundations

A piece of metal at 95 °C is dropped into cooler water, and the two reach a common final temperature. Which statement correctly describes the energy transfer?

  1. No energy is transferred because the metal and water never touch
  2. Energy flows in both directions equally so the final temperature equals the metal's initial temperature
  3. Energy flows from the metal to the water until both reach the same temperature Correct answer.
  4. Energy flows from the water to the metal because water has a higher specific heat

Explanation

Heat transfers spontaneously from the hotter object to the cooler one until thermal equilibrium is reached, at which point both have the same temperature. The direction of flow depends on temperature difference, not on specific heat capacity.

Question 36Matter, Measurement, and Chemical Foundations

A sample of gas is held at constant temperature while its volume is reduced to one-third of its original value. What happens to its pressure?

  1. It stays the same
  2. It triples Correct answer.
  3. It drops to one-third
  4. It increases by a factor of nine

Explanation

Boyle's law states that at constant temperature and amount, pressure and volume are inversely proportional, so PV is constant. Reducing the volume to one-third requires the pressure to rise to three times its original value.

Question 37Matter, Measurement, and Chemical Foundations

A compound is found to contain 40.0% carbon, 6.7% hydrogen, and 53.3% oxygen by mass. What is its empirical formula?

  1. C₂H₆O
  2. CH₂O Correct answer.
  3. CHO
  4. C₂H₄O₂

Explanation

Dividing each percentage by the atomic mass gives 3.33 mol C, 6.7 mol H, and 3.33 mol O, and dividing by the smallest value gives a 1:2:1 ratio. The empirical formula is therefore CH₂O, which is the simplest whole-number ratio regardless of molecular size.

Question 38Matter, Measurement, and Chemical Foundations

Which observation would indicate that a chemical change has occurred rather than a physical change?

  1. A mixture of two liquids is separated by distillation
  2. A new gas is produced that was not present before, along with a temperature rise Correct answer.
  3. A solid is broken into smaller pieces without any other change
  4. A liquid is allowed to evaporate in an open dish

Explanation

Formation of a new substance, such as a gas not previously present, together with an unexpected temperature change, indicates that chemical bonds were broken and formed. Physical changes alter only the form or state of a substance without producing new substances.

Question 39Matter, Measurement, and Chemical Foundations

A sealed rigid container holds a fixed amount of gas. If the gas is warmed from 300 K to 600 K, what happens to the average kinetic energy of its molecules?

  1. It doubles Correct answer.
  2. It quadruples
  3. It halves
  4. It remains unchanged

Explanation

The average kinetic energy of gas molecules is directly proportional to absolute temperature. Doubling the temperature from 300 K to 600 K therefore doubles the average kinetic energy, which also raises the pressure in a rigid container.

Question 40Matter, Measurement, and Chemical Foundations

How many significant figures should be reported in the result of the calculation 4.56 × 1.2?

  1. Four
  2. Three
  3. Two Correct answer.
  4. One

Explanation

In multiplication, the answer carries the same number of significant figures as the factor with the fewest, which here is 1.2 with two. The product 5.472 is therefore rounded to 5.5, reported with two significant figures.

Question 41Matter, Measurement, and Chemical Foundations

A student measures the density of a metal three times and obtains 8.92, 8.94, and 8.91 g/mL, while the accepted value is 8.96 g/mL. Which statement best describes these results?

  1. They are accurate but imprecise
  2. They are neither precise nor accurate
  3. They are precise but slightly inaccurate Correct answer.
  4. They are both accurate and imprecise

Explanation

The three readings cluster tightly together, showing high precision, but all fall slightly below the accepted value, showing a small systematic inaccuracy. Precision and accuracy are independent qualities of a data set.

Question 42Matter, Measurement, and Chemical Foundations

Which change in conditions would increase the solubility of a solid ionic compound in water?

  1. Lowering the temperature of the water
  2. Increasing the pressure above the solution
  3. Decreasing the surface area of the solid
  4. Raising the temperature of the water Correct answer.

Explanation

Dissolving most solid ionic compounds in water is endothermic, so adding heat shifts the equilibrium toward dissolution and increases solubility. Pressure has essentially no effect on solids, and larger particles dissolve more slowly.

Question 43Matter, Measurement, and Chemical Foundations

A sample contains only one type of atom but the atoms have different numbers of neutrons. How is this sample classified?

  1. A homogeneous mixture of two compounds
  2. A compound of two elements
  3. A solution of one element in another
  4. A mixture of isotopes of one element Correct answer.

Explanation

Atoms of the same element share the same number of protons but can differ in neutron number, making them isotopes. A sample containing only one element's isotopes is still a single element, not a mixture of different substances.

Question 44Matter, Measurement, and Chemical Foundations

A 25.0 mL sample of an unknown liquid has a mass of 19.75 g. Based on density, which liquid is most likely present?

  1. Ethanol, density 0.789 g/mL Correct answer.
  2. Mercury, density 13.6 g/mL
  3. Glycerol, density 1.26 g/mL
  4. Water, density 1.00 g/mL

Explanation

Dividing 19.75 g by 25.0 mL gives 0.790 g/mL, which matches the density of ethanol within rounding. Matching an unknown's density to tabulated values is a standard way to help identify a substance.

Question 45Matter, Measurement, and Chemical Foundations

A gas is collected in a cylinder at 27 °C. What is this temperature on the Kelvin scale?

  1. 300 K Correct answer.
  2. 273 K
  3. 246 K
  4. 327 K

Explanation

Kelvin temperature is found by adding 273 to the Celsius value, so 27 + 273 = 300 K. Kelvin values must be used in gas-law calculations because they are proportional to absolute temperature.

Question 46Matter, Measurement, and Chemical Foundations

A sample of an element conducts electricity as a solid, is malleable, and has a high melting point. Which type of bonding best explains these properties?

  1. Covalent bonding in discrete molecules
  2. Hydrogen bonding between polar molecules
  3. Ionic bonding between fixed ions
  4. Metallic bonding with delocalized electrons Correct answer.

Explanation

In metallic bonding, a lattice of cations is surrounded by delocalized electrons that can move freely, which explains electrical conductivity, malleability, and high melting points. Ionic solids conduct only when molten or dissolved, and molecular solids are generally soft and low melting.

Question 47Matter, Measurement, and Chemical Foundations

A hydrate of copper(II) sulfate has a mass of 25.0 g before heating and 16.0 g after all the water of hydration is driven off. What is the mass percent of water in the hydrate?

  1. 56.0%
  2. 36.0% Correct answer.
  3. 64.0%
  4. 9.0%

Explanation

The mass lost on heating is 25.0 − 16.0 = 9.0 g, which is the water driven off. Dividing 9.0 g by the original 25.0 g and multiplying by 100 gives 36.0% water by mass.

Question 48Matter, Measurement, and Chemical Foundations

Two samples of gas at the same temperature and pressure occupy equal volumes. Which statement must be true?

  1. They contain equal numbers of molecules Correct answer.
  2. They have equal molar masses
  3. They have equal densities in grams per liter
  4. They contain equal masses of gas

Explanation

Avogadro's law states that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules. Since the molecules may differ in mass, the samples can have different molar masses, densities, and total masses.

Question 49Matter, Measurement, and Chemical Foundations

A 0.500 mol sample of a compound has a mass of 30.0 g. What is the molar mass of the compound?

  1. 30.5 g/mol
  2. 60.0 g/mol Correct answer.
  3. 15.0 g/mol
  4. 0.0167 g/mol

Explanation

Molar mass equals mass divided by amount in moles, so 30.0 g ÷ 0.500 mol = 60.0 g/mol. This value could correspond to a compound such as urea or acetic acid.

Question 50Matter, Measurement, and Chemical Foundations

A mixture of two liquids with different boiling points is separated by fractional distillation. Which principle explains why the component with the lower boiling point is collected first?

  1. It has stronger intermolecular forces and evaporates more easily
  2. It dissolves the other component and carries it along
  3. It has a greater molar mass and therefore condenses more readily
  4. It has a higher vapor pressure at any given temperature and enriches the vapor phase Correct answer.

Explanation

The component with the lower boiling point has weaker intermolecular forces and a higher vapor pressure, so its vapor fraction is enriched and it distills first. Fractional distillation exploits repeated vaporization and condensation along a column.