TEAS Chemistry study guide

What this topic actually asks of you

Chemistry is one of the four sub-content areas of the ATI TEAS Science section. ATI publishes its size: it is 5% of the exam and 8 of the 44 scored science questions. The whole Science section is 50 items (44 scored, 6 unscored) and is timed separately — 60 minutes.

Two things follow, and they shape how to study it.

• Most of it is recall; a little of it is arithmetic. The bulk of this topic is knowing the pieces — what an atomic number tells you, how a physical change differs from a chemical one, what the pH scale means. A smaller part is a short computation: counting atoms to balance an equation, or finding a concentration or a diluted strength.
• The arithmetic is small; the setup is still the question. Every number here is small. The points are lost on the setup — which side of the equation to count, whether to multiply by the subscript, and whether a percent is the solute over the whole solution.

Our practice items are single-answer and text-only. The real ATI TEAS also uses other item types: ATI's own content outline lists multiple-choice, multiple-select, fill-in-the-blank numeric, hot spot and ordered-response items. Elk Ridge Academy's practice items are single-answer only, and they carry no pictures or diagrams — a chemical equation or a set of data is written as text — so treat the method, not any one format, as the thing you are drilling.

ATI's chemistry objectives. ATI's Content Outline names what this topic measures, and every practice item below maps to one of them: S.3.1 basic atomic structure · S.3.2 the physical properties and changes of matter · S.3.3 chemical reactions · S.3.4 how conditions affect reactions · S.3.5 the properties of solutions · S.3.6 acids and bases.

The thirteen procedures

Every practice item in this topic is one of these thirteen. Each is worked out in full below — the rule, the steps, and the trap it sets. (The ATI objective each one serves is in brackets.)

1. Atomic structure [S.3.1] — the atomic number counts protons (and electrons in a neutral atom); the mass number counts protons + neutrons, so neutrons = mass number − atomic number. → example 1
2. The periodic table [S.3.1] — a group (column) fixes the valence electrons for the main-group elements, a period (row) fixes the electron shells; metals, nonmetals and metalloids sit in their own regions. → example 2
3. Bonding [S.3.1/S.3.3] — ionic (a metal gives electrons away), covalent (two nonmetals share), metallic (a shared electron pool). → example 3
4. Physical and chemical changes [S.3.2] — a physical change alters form or state and is usually reversible; a chemical change makes a new substance and shows evidence (gas, precipitate, color change, heat or light). → example 4
5. States of matter [S.3.2] — solid, liquid and gas and the changes of state between them, named by direction. → example 5
6. Chemical reactions [S.3.3] — reactants become products; mass is conserved; reactions are classified as synthesis, decomposition, single replacement, double replacement or combustion, and as exothermic or endothermic. → example 6
7. Balancing an equation [S.3.3] — count the atoms of each element on both sides and add coefficients until the counts match. → example 7
8. Factors that affect reaction rate [S.3.4] — temperature, concentration, surface area and a catalyst change how fast a reaction goes, not how much it makes. → example 8
9. Solutions [S.3.5] — a solute dissolves in a solvent to make a solution; solubility is how much dissolves, and it rises with temperature for most solids. → example 9
10. Percent concentration [S.3.5] — solute ÷ the whole solution, × 100. → example 10
11. Dilution [S.3.5] — C₁V₁ = C₂V₂: the amount of solute stays the same when you add solvent. → example 11
12. Acids and bases [S.3.6] — acids give hydrogen ions, bases take them; litmus and other indicators tell them apart; an acid and a base neutralize each other to a salt and water. → example 12
13. The pH scale [S.3.6] — below 7 is acidic, 7 is neutral, above 7 is basic; each step of 1 is a factor of ten in hydrogen-ion concentration. → example 13

Reference tables: the facts to know cold

These are the facts the practice items draw on. Each row is a fact worth memorizing on its own. Rows do not overlap between tables.

The periodic table. Read an element's box from its numbers — the atomic number and the average atomic mass (a decimal) — and read the table's shape from its rows and columns.

Source: OpenStax Chemistry 1e and Chemistry: Atoms First 1e (CC BY 4.0).

The three bond types. Ask what the atoms are doing with their electrons.

Source: OpenStax Chemistry 1e (CC BY 4.0).

Physical vs chemical changes. A physical change keeps the substance; a chemical change makes a new one, and it usually leaves evidence.

Source: OpenStax Chemistry 1e (CC BY 4.0).

The changes of state. Each change has a direction — know which way it goes.

Source: OpenStax Chemistry 1e (CC BY 4.0).

The five reaction types. Name the pattern by what happens to the reactants.

Source: OpenStax Chemistry 1e (CC BY 4.0).

What speeds a reaction up. A rate factor changes how fast a reaction goes, not how much product it makes.

Source: OpenStax Chemistry 1e (CC BY 4.0).

What a solution is. A solute dissolved in a solvent makes a solution; solubility says how much solute can dissolve.

Source: OpenStax Chemistry 1e (CC BY 4.0).

Acids vs bases. The two opposites, by their properties.

Source: OpenStax Chemistry 1e (CC BY 4.0).

The pH scale. One number, three regions, and a factor of ten between each step.

Source: OpenStax Chemistry 1e (CC BY 4.0) — the pH scale and its direction.

A four-step method

1. Name the question mark. Before any counting, name the one thing being asked for and its unit: the number of neutrons, the number of shells, the percent, the coefficient. This step defeats the classic wrong answer on this topic: a correct count of the wrong quantity — the mass number when the neutrons were asked for, the product when the reactant was asked for, the solvent when the solution was asked for.

2. Write the formula or the equation out, with its numbers visible. Put the subscripts where you can see them (H₂O has a 2) and put every coefficient down. A number you do not write is a number you forget to multiply.

3. Do one count or one operation at a time. Count one element at a time when balancing. For a concentration, write the fraction before you divide.

4. Check the direction and the units. Is a diluted concentration lower than the one you started with? Is the mass number at least as big as the atomic number? Is a higher pH the less acidic solution? A scientist at test day checks the sense of the number, not only its arithmetic.

The thirteen worked examples

1. Atomic structure — protons from the atomic number, neutrons from the difference

An atom of sodium has an atomic number of 11 and a mass number of 23. How many protons and how many neutrons does it have?

• The atomic number is the proton count: 11 protons.
• The mass number counts protons + neutrons: 23 in all.
• Neutrons = mass number − atomic number = 23 − 11 = 12.
• So the atom has 11 protons and 12 neutrons (and, being neutral, 11 electrons).

The trap: taking the mass number (23) as the proton count. The atomic number is the small whole number; the mass number counts protons and neutrons together, so it is never smaller than it.

2. The periodic table — fill the shells, read the group and the period

A neutral atom has 12 electrons. Which period is it in, and how many valence electrons does it have?

• Fill the shells in order: the first holds 2, the second holds 8.
• 12 − 2 = 10, then 10 − 8 = 2 — two electrons are left for a third shell.
• The shells used are 2 + 8 + 2 = 12, so the atom is in period 3.
• The 2 electrons in the outer shell are the valence electrons, so it is a group-2 element.

The trap: reading a period (row) when the question asks for a group (column) — the row gives the shells, and the column gives the valence electrons in the main-group elements.

3. Bonding — balance the charges to predict a formula

Magnesium forms Mg²⁺ ions and chlorine forms Cl⁻ ions. How many chloride ions balance one magnesium ion in the compound?

• A compound's charges must sum to zero.
• Mg²⁺ carries +2; Cl⁻ carries −1.
• Two chloride ions give 2 × (−1) = −2, which balances the +2.
• So 2 chloride ions pair with each magnesium, giving MgCl₂.

The trap: treating a metal and a nonmetal as a covalent pair that shares electrons. A metal gives electrons away; the resulting ions attract — that is an ionic bond.

4. Physical and chemical changes — is a new substance made?

Classify four changes as physical or chemical, then tally.

1. Change 1 — an ice cube melts: still water, no new substance → physical.
2. Change 2 — an iron nail rusts: iron → iron oxide, a new substance → chemical.
3. Change 3 — a match burns: it gives heat and light and makes new gases → chemical.
4. Change 4 — sugar dissolves in water: the sugar is unchanged → physical.
5. Tally: 2 physical (changes 1 and 4) and 2 chemical (changes 2 and 3) out of 4.

The trap: calling any change that looks dramatic "chemical". The test is not how it looks but whether a new substance has formed. Melting and dissolving are physical; burning and rusting are chemical.

5. States of matter — name the change by its direction

Name each change of state by its direction.

1. Change 1 — solid → gas, skipping the liquid: sublimation.
2. Change 2 — gas → solid: deposition.
3. Change 3 — liquid → gas: vaporization (boiling throughout at the boiling point, or evaporation at the surface below it).
4. Change 4 — gas → liquid: condensation.
5. Each change has a reverse: melting ↔ freezing, vaporization ↔ condensation, sublimation ↔ deposition.

The trap: confusing evaporation and boiling, or calling the solid → gas jump "melting". Evaporation is a liquid turning to gas at the surface below its boiling point; boiling is the whole liquid turning to gas at the boiling point; solid → gas is sublimation.

6. Chemical reactions — name the type and the energy

Name the reaction type and the energy change.

1. A fuel reacting with oxygen to give CO₂ and water is combustion.
2. CH₄ + 2 O₂ → CO₂ + 2 H₂O balances: carbon 1 = 1, hydrogen 4 = 4, oxygen 4 = 4.
3. Heat and light leave the system, so the reaction is exothermic (energy out).
4. An endothermic reaction takes energy in instead, so its surroundings cool.

The trap: calling a combustion reaction endothermic because you had to light it. Lighting it supplies the activation energy; the reaction itself then releases far more, so it is exothermic.

7. Balancing an equation — count each element on both sides

Balance: CH₄ + O₂ → CO₂ + H₂O.

• Count carbon: 1 on the left, 1 on the right — balanced already.
• Count hydrogen: 4 on the left, but 2 in one H₂O — put a 2 in front of H₂O.
• Now count oxygen on the right: 2 (in CO₂) + 2 × 1 (one in each of the two waters) = 4. The left holds only 2 (in O₂), so put a 2 in front of O₂ to make it 2 × 2 = 4.
• Balanced: CH₄ + 2 O₂ → CO₂ + 2 H₂O — carbon 1 = 1, hydrogen 4 = 4, oxygen 4 = 4.
• The same idea handles three elements at once: Fe + O₂ → Fe₂O₃ balances as 4 Fe + 3 O₂ → 2 Fe₂O₃ — iron 4 = 4 and oxygen 3 × 2 = 2 × 3, so 6 = 6.

The trap: changing a subscript instead of adding a coefficient. H₂O is water; H₂O₂ is a different substance. Only the numbers in front of a formula may change.

8. Factors that affect reaction rate — faster, not more

A reaction between a solid metal and an acid is slow. Explain what speeds it up — and what does not.

1. Grinding the metal into a powder exposes more surface area → faster.
2. Warming the mixture makes the particles collide more often and harder → faster.
3. Adding a catalyst lowers the energy the reaction needs → faster (and it is not used up).
4. Diluting the acid lowers its concentration, so there are fewer collisions → slower.

The trap: thinking a rate factor changes how much product forms. Temperature, surface area, concentration and a catalyst change the speed; the amount of product is set by how much reactant you started with.

9. Solutions — name the solute, the solvent and the saturation

Salt is stirred into water until no more will dissolve. Name the solute, the solvent and the saturation.

1. The salt dissolves, so the salt is the solute (usually the lesser amount).
2. The water does the dissolving, so the water is the solvent (usually the greater amount).
3. No more salt will dissolve at this temperature, so the solution is saturated.
4. Warming the water raises the solubility, so a little more salt would dissolve.

The trap: swapping solute and solvent, or thinking a saturated solution is a strong one. A saturated solution is simply one that can hold no more solute at that temperature; it can still be dilute.

10. Percent concentration — solute over the whole solution

A solution is made by dissolving 5 g of salt in enough water to make 100 mL of solution. What is its percent concentration (mass of solute per volume of solution)?

• Percent = (solute ÷ solution) × 100.
• 5 ÷ 100 = 0.05.
• 0.05 × 100 = 5.
• So the solution is 5% — 5 g of salt in every 100 mL.

The trap: dividing by the solvent instead of the whole solution, or forgetting the × 100 (which leaves 0.05, a fraction, not a percent).

11. Dilution — the solute is conserved, so C₁V₁ = C₂V₂

You have 100 mL of a 10% salt solution. You add water until the total volume is 250 mL. What is the new concentration?

• Use C₁V₁ = C₂V₂: the amount of solute does not change.
• C₁V₁ = 10 × 100 = 1,000.
• C₂ = 1,000 ÷ V₂ = 1,000 ÷ 250 = 4.
• So the new concentration is 4% — weaker than 10%, as adding water must be.

The trap: multiplying by the volumes (10 × 250) or flipping the ratio (250 ÷ 100). A diluted solution is always weaker, so C₂ must land below C₁.

12. Acids and bases — litmus, hydrogen ions and neutralization

A solution turns blue litmus paper red. Classify it and say what happens when a base is added.

1. Blue litmus turning red is the test for an acid.
2. An acid gives hydrogen ions (H⁺), which puts its pH below 7.
3. A base takes up H⁺ ions, turns red litmus blue, and has a pH above 7.
4. Mixing them neutralizes: acid + base → a salt + water, and the pH moves toward 7.

The trap: reversing the litmus colors. Remember it as "acid = red": acids turn blue litmus red, and bases turn red litmus blue.

13. The pH scale — each step of one is a factor of ten

Solution A has a pH of 3. Solution B has a pH of 5. How many times more hydrogen ions does Solution A have than Solution B?

• Each step of 1 on the pH scale is a factor of ten in hydrogen-ion concentration.
• The gap is 5 − 3 = 2 steps.
• 10² = 100.
• So Solution A has 100 times as many hydrogen ions — and A, the lower pH, is the more acidic of the two.

The trap: assuming the higher pH is the more acidic solution. On the pH scale up means down — a higher pH means fewer hydrogen ions, so it is the less acidic solution.

The traps, collected

Most wrong answers in this topic's practice items are one of these.

• Atomic number vs mass number: took the mass number as the proton count (it is protons + neutrons).
• Group vs period: read the row when the column was asked for — the group fixes valence electrons (for main-group elements), the period fixes the shells.
• Ionic vs covalent: called a metal-plus-nonmetal pair covalent; a metal gives electrons away (ionic), two nonmetals share (covalent).
• Physical vs chemical: called a change of state or a dissolving "chemical"; the test is whether a new substance formed.
• Evaporation vs boiling: used them as synonyms; evaporation is at the surface below the boiling point, boiling is throughout at it.
• A reaction type misnamed: called a decomposition a synthesis, or missed that combustion is a fuel reacting with oxygen and giving off heat and light.
• Exothermic vs endothermic: called a burning endothermic because it needed lighting.
• One side balanced only: changed the coefficients on one side and left the other.
• Subscript dropped or changed: used H₂O₂ for H₂O, or one hydrogen for H₂O's two.
• A rate factor mistaken for a yield factor: thought more heat or a catalyst makes more product; it only makes it faster.
• Solute vs solvent swapped: called the water the solute.
• Percent over the wrong amount: solute over solute, or over the solvent, instead of over the whole solution.
• Dilution flipped: multiplied where the volumes divide, giving a C₂ above C₁.
• pH read backwards: treated the higher pH as the more acidic solution.

Answer-choice strategies: three moves for when the setup will not come

1. Write the units and see what they build. A concentration is a mass over a volume. If the units do not come out to the unit the answer needs, the setup is upside-down.

8 g ÷ 200 mL × 100: the division gives grams per mL, and × 100 turns that into grams per 100 mL — what a mass percent (m/v) means — 4%.

2. Count on both sides with a pencil. In a balancing item, write the atom counts under each side as you go. A side that reads 4 = 2 is the one that still needs a coefficient.

3. Check the direction before you choose. A dilute solution is weaker (C₂ < C₁); a higher pH is the less acidic solution; a mass number is never smaller than its atomic number; a reaction that gives off heat is exothermic. A choice that violates the direction is gone before you finish the arithmetic.

How to practice this

• Work the free Chemistry practice set with a pencil, writing the formula or the equation out each time. The setup is where these items set their traps.
• Drill the procedures you get wrong, not the ones you already know. Sorting your errors by procedure beats counting them.
• Read the explanation on every practice item, including the ones you got right. A right answer reached with the wrong count comes back on test day.
• Re-attempt what you missed a day later, not immediately: a day later measures whether the method stuck.

Where to go next

• Next topic — Scientific reasoning
• The rest of the section — Human anatomy and physiology, Biology, and Scientific reasoning.

This topic sits inside the Science section and carries ATI's published 5% of the exam and 8 of the section's 44 scored items — the blueprint's own split.

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