Chemistry: Matter and Reactions

⚗️ Chemistry: Matter and Reactions

Part 1 of 7 — Chemistry on the ASVAB

Topic Overview

Where Chemistry Fits

General Science (GS) is the first subtest you see on the CAT-ASVAB. It has 15 scored questions and a 12-minute limit, which works out to about 48 seconds per question. GS mixes life science, earth and space science, and physical science, and the official content outline puts atomic structure and chemistry inside the physical-science share.

Two rules about the computer test matter right now:

• You can't go back. Once you answer a question, it is locked in.
• Some questions may be unscored tryout items, and you can't tell which ones, so treat every question as if it counts.

Key Fact: GS chemistry is high-school level. No question asks you to memorize the whole periodic table or do long calculations. It asks for one fact or one short step, and you do any arithmetic on scratch paper (no calculator).

What the Questions Look Like

Chemistry questions on GS come in a few familiar shapes:

The facts that show up again and again

You will learn each of these in Parts 2–5, with a memory hook for each one.

Warm-up. 🎯 Answer from what you already know. Nobody expects a perfect score yet: this shows you what the lesson will cover.

How to Use This Lesson

• Parts 2–5 each teach one cluster of facts with a table or memory hook, then let you practice right away.
• Part 6 shows the traps the wrong answers are built from, and how to spend your 48 seconds.
• Part 7 is a mixed set at ASVAB difficulty, so you can check you are ready.

Study tip

Chemistry questions reward recognition. If you can picture the table for a fact (the pH scale, the three particles of an atom, the six phase changes), you can answer in well under 48 seconds and bank time for the harder science questions.

Key Takeaways — Part 1

• GS is the first CAT-ASVAB subtest: 15 scored questions in 12 minutes, about 48 seconds each
• You can't go back, and some questions may be unscored tryouts you can't spot, so treat every question as scored
• Chemistry questions are high-school level: one fact or one short step, no calculator
• The big repeat facts: density, atomic and mass number, ions, pH, chemical vs physical change, catalysts, salt on ice

Up Next: Part 2 — matter, the states of matter, and the difference between a physical and a chemical change.

⚗️ Chemistry: Matter and Reactions

Part 2 of 7 — Matter, States and Changes

What Matter Is

Matter is anything that has mass and takes up space (has volume). Air, water, steel and your own body are all matter. Light, heat and sound are not matter: they are forms of energy.

One cubic centimeter and one milliliter are the same volume: $1 \text{ cm}^3 = 1 \text{ mL}$.

Density

$\text{density} = \frac{\text{mass}}{\text{volume}}$

Water's density is about 1 g/cm³ (1 g/mL). That single number answers most floating questions:

Density belongs to the substance, not the size of the piece. Cut a steel bar in half and each half has half the mass and half the volume, so the density stays the same.

Density and particle spacing

Density depends on how tightly the particles are packed. When the same mass is squeezed into less space (particles closer together), density goes up. When particles spread out, density goes down. That is why a gas is far less dense than the same substance as a liquid.

Key Fact: Water is the famous exception. In ice, water molecules lock into an open, spread-out pattern, so ice is less dense than liquid water and floats. That is why lakes freeze from the top down.

The States of Matter

A fourth state, plasma, is a very hot gas whose atoms have lost electrons. Lightning, neon signs and the Sun are plasma.

Memory hook: going from solid → liquid → gas, the particles get more energy, move faster and spread farther apart.

Phase Changes: Six Names to Know

Vaporization comes in two kinds:

• Boiling happens throughout the liquid, at the boiling point.
• Evaporation happens only at the surface, at temperatures below the boiling point (wet clothes drying on a line).

Water's landmarks (at sea level): freezes at 0 °C (32 °F), boils at 100 °C (212 °F). While a substance is melting or boiling, its temperature stays constant until the change is complete: the added energy goes into changing the state.

Physical vs Chemical Changes

Signs that a chemical change happened (memory hook: "Gas, Glow, Color, Solid, Smell"):

1. Gas bubbles form (when the liquid is not just boiling)
2. Glow: light or heat is given off
3. Color changes unexpectedly
4. A solid (a precipitate) forms when two liquids are mixed
5. A new smell appears

Physical vs chemical properties

• A physical property can be observed without changing the substance: color, density, hardness, melting point, boiling point.
• A chemical property describes how a substance reacts: flammability, ability to rust, reaction with acids.

Worked Examples

1. Density. An aluminum block has a mass of 54 g and a volume of 20 cm³.

$\text{density} = \frac{54}{20} = 2.7 \text{ g/cm}^3$

That is more than 1 g/cm³, so the block sinks in water.

2. Floating. A piece of wood has a mass of 30 g and a volume of 50 cm³: $\frac{30}{50} = 0.6$ g/cm³, less than 1, so it floats.

3. Classifying. Burning gasoline makes carbon dioxide and water, new substances, so it is a chemical change. Gasoline evaporating from a spill is still gasoline, now as a vapor, so it is a physical change.

Read the particles. 🎯 Use Figure 1.

States and changes. 🎯

Name the phase change. 🔽 Pick the name for each everyday event.

Density on scratch paper. ✍️

1) A rock has a mass of 60 g and a volume of 20 cm³. Its density, in g/cm³:

2) A plastic toy has a mass of 45 g and a volume of 50 cm³. Its density, in g/cm³:

3) Of the rock and the toy, how many would float in water? (Enter a number from 0 to 2.)

Key Takeaways — Part 2

• Matter has mass and volume; $1 \text{ cm}^3 = 1 \text{ mL}$
• Density = mass ÷ volume; water is about 1 g/cm³; less dense floats, more dense sinks
• Closer particles mean higher density; ice is the exception: less dense than liquid water, so it floats
• Solid (fixed shape and volume), liquid (fixed volume), gas (fills its container)
• Six phase changes: melting, freezing, vaporization, condensation, sublimation (solid → gas), deposition (gas → solid)
• Water freezes at 0 °C / 32 °F and boils at 100 °C / 212 °F
• Chemical change = new substance (gas, glow, color, solid, smell); melting, dissolving and cutting are physical

Up Next: Part 3 — what atoms are made of and how the periodic table is organized.

⚗️ Chemistry: Matter and Reactions

Part 3 of 7 — Atoms and the Periodic Table

Element, Atom, Molecule

Every compound is made of molecules or ions, but not every molecule is a compound: $O_2$ is a molecule of a single element.

Inside the Atom

Memory hook: P-E-N. Protons are Positive, Electrons carry the nEgative charge, Neutrons are Neutral.

The nucleus holds almost all of the atom's mass but takes up a tiny fraction of its space. Most of an atom is empty space where the electrons move.

The two numbers

So: $\text{neutrons} = \text{mass number} - \text{atomic number}$.

In a neutral atom, electrons = protons, so the charges cancel.

Ions and Isotopes

Ions form when an atom gains or loses electrons. The protons never change.

Count it: charge = protons − electrons.

Isotopes are atoms of the same element (same number of protons) with different numbers of neutrons, so their mass numbers differ. Carbon-12 has 6 protons and 6 neutrons; carbon-14 has 6 protons and 8 neutrons. Both are carbon.

Worked Examples

1. Sodium-23. Sodium's atomic number is 11.

• Protons: 11 (the atomic number)
• Neutrons: $23 - 11 =$ 12
• Electrons (neutral atom): 11

2. A magnesium ion, $Mg^{2+}$. Magnesium has 12 protons. A charge of $+2$ means two more protons than electrons, so it has $12 - 2 =$ 10 electrons.

3. The decimal on the table. The periodic table lists carbon's atomic mass as about 12.01, not exactly 12, because it is an average over carbon's natural isotopes. Round it to get the most common mass number.

The Periodic Table

The table lists elements in order of atomic number (number of protons).

Families worth knowing

Key Fact: The outer electrons decide how an element reacts. Same group → same number of outer electrons → similar chemistry.

Symbols that don't match the English name

Most symbols are easy (H hydrogen, C carbon, O oxygen, N nitrogen, Ca calcium). These come from Latin and trip people up:

Memory hook: gold is Au ("Au-some gold"), and silver is Ag (picture a federal agent's silver badge). Don't swap them: that swap is one of the most common wrong answers.

Read the atom. ✍️ Use Figure 1.

1) The atomic number of this atom:

2) The mass number of this atom:

Change the atom. 🎯 Use Figure 1 again.

Atoms and the table. 🎯

Map the table. 🔽

Key Takeaways — Part 3

• P-E-N: protons positive, electrons negative, neutrons neutral; protons and neutrons sit in the nucleus
• Atomic number = protons (decides the element); mass number = protons + neutrons
• Neutral atom: electrons = protons. Lose electrons → positive ion; gain electrons → negative ion
• Isotopes: same element, same protons, different neutrons
• Rows are periods, columns are groups; metals left and center, nonmetals upper right, metalloids on the staircase
• Group 1 alkali metals and group 17 halogens are very reactive; group 18 noble gases barely react
• Tricky symbols: Na, K, Fe, Cu, Ag, Au, Pb, Sn, Hg

Up Next: Part 4 — how atoms combine into compounds, and how compounds differ from mixtures and solutions.

⚗️ Chemistry: Matter and Reactions

Part 4 of 7 — Compounds, Mixtures and Solutions

Sorting All Matter

Every sample of matter is either a pure substance or a mixture:

Compound vs mixture: the four tests

The classic example: sodium is a soft metal that reacts violently with water, and chlorine is a poisonous green gas. Bonded together as sodium chloride, they make table salt, which you sprinkle on food. New substance, new properties: a compound.

Solutions

A solution is a homogeneous mixture.

Water is called the "universal solvent" because it dissolves more substances than any other common liquid. It does not dissolve everything: oil and water stay separate ("like dissolves like").

Temperature and solubility:

• Most solids dissolve better in hot water (sugar in hot tea vs iced tea).
• Gases dissolve better in cold water. A warm soda loses its fizz faster, and warm water holds less dissolved oxygen.

How Atoms Bond

Memory hook: ionic = "I give," covalent = "co-operate" (share).

Reading a Chemical Formula

• Symbols name the elements: $CO_2$ contains carbon and oxygen.
• A subscript (small number below) counts the atom just before it: $H_2O$ has 2 H and 1 O.
• No subscript means 1.
• Parentheses multiply everything inside: $Ca(OH)_2$ has 1 Ca, 2 O and 2 H, so 5 atoms.
• A coefficient (big number in front) multiplies the whole formula: $2H_2O$ means 2 water molecules, so 4 H and 2 O.

Separating Mixtures

Freezing-Point Depression: Why Salt Melts Ice

Dissolved particles get in the way of water molecules trying to lock into ice crystals. So a solution freezes at a lower temperature than pure water, and it also boils at a higher temperature.

Key Fact: Salt does not heat the ice. It lowers the freezing point of the water, so the ice melts at a temperature where pure water would stay frozen.

Classify the matter. 🔽 Choose the category for each sample.

Count the atoms. ✍️

1) How many hydrogen atoms are in one molecule of glucose, $C_6H_{12}O_6$?

2) How many atoms in total are in one molecule of sulfuric acid, $H_2SO_4$?

3) How many oxygen atoms are in $3CO_2$?

Mixtures, bonds and solutions. 🎯

Key Takeaways — Part 4

• Pure substances: elements (one kind of atom) and compounds (elements bonded in a fixed ratio, with new properties)
• Mixtures: homogeneous (solutions: salt water, air, brass) or heterogeneous (sand and gravel, granite)
• Mixtures separate physically: filtration, evaporation, distillation (boil and condense), magnets
• Solute dissolves in a solvent; water is the "universal solvent"
• Solids dissolve better when hot; gases dissolve better when cold
• Ionic: metal gives electrons to nonmetal. Covalent: nonmetals share electrons
• Subscripts count the atom before them; coefficients multiply the whole formula
• Dissolved substances lower the freezing point and raise the boiling point (salt on roads, antifreeze)

Up Next: Part 5 — acids, bases, pH, and what happens during a chemical reaction.

⚗️ Chemistry: Matter and Reactions

Part 5 of 7 — Acids, Bases and Chemical Reactions

The pH Scale

pH measures how acidic or basic (alkaline) a solution is, on a scale from 0 to 14:

Key Fact: Each step on the pH scale is a factor of 10. A pH 3 solution is 10 times as acidic as pH 4 and 100 times as acidic as pH 5. Lower pH = more acidic.

Acids vs bases

Memory hook: "BRA" = Blue turns Red in Acid. (Never taste or touch lab chemicals to test them: indicators exist for a reason.)

Neutralization

An acid and a base react to cancel each other out:

$\text{acid} + \text{base} \rightarrow \text{salt} + \text{water}$

For example, hydrochloric acid plus sodium hydroxide gives table salt and water: $HCl + NaOH \rightarrow NaCl + H_2O$. An antacid tablet (a mild base) neutralizes extra stomach acid, and baking soda is used to neutralize acid spills.

Read the pH scale. 🎯 Use Figure 1.

Chemical Reactions

In a reaction, reactants (on the left) turn into products (on the right):

$\text{reactants} \rightarrow \text{products}$

Conservation of mass

Atoms are rearranged, never created or destroyed. So the total mass of the products equals the total mass of the reactants in a closed container. If a burning log seems to "lose" mass, it is because gases escaped into the air.

Balancing: count the atoms

A balanced equation has the same number of each kind of atom on both sides. You balance by changing coefficients (big numbers in front), never the subscripts.

Worked example: $H_2 + O_2 \rightarrow H_2O$ has 2 O atoms on the left but 1 on the right.

1. Put a 2 in front of water: $H_2 + O_2 \rightarrow 2H_2O$. Now O: 2 = 2, but H: 2 on the left, 4 on the right.
2. Put a 2 in front of hydrogen: $2H_2 + O_2 \rightarrow 2H_2O$. H: 4 = 4, O: 2 = 2. Balanced.

Five reaction types

Speed, energy and catalysts

Reactions go faster with: higher temperature, higher concentration, more surface area (powder reacts faster than a lump), or a catalyst.

A catalyst speeds up a reaction without being used up; it lowers the energy needed to get the reaction started. Enzymes are the catalysts in living things, and a car's catalytic converter speeds up reactions that clean exhaust.

Oxidation: rusting is iron combining with oxygen (helped by water) to make iron oxide. Burning is a very fast oxidation.

Acids, bases and reactions. 🎯

Name the reaction type. 🔽

Conservation and balancing. ✍️

1) Fill in the blank to balance the equation: $\_\_\, Mg + O_2 \rightarrow 2MgO$. The coefficient in front of $Mg$ is:

2) In a sealed jar, 12 g of iron powder reacts completely with 5 g of sulfur. The mass of the iron sulfide produced, in grams:

3) In the reactants $CH_4 + 2O_2$, how many oxygen atoms are there in total?

Key Takeaways — Part 5

• pH runs 0–14: 7 neutral, below 7 acid, above 7 base; each step is 10 times
• Acids: sour, release $H^{+}$, turn blue litmus red (BRA). Bases: bitter, slippery, release $OH^{-}$, turn red litmus blue
• Acid + base → salt + water (neutralization: antacids, baking soda on spills)
• Mass is conserved: atoms are rearranged, not created or destroyed; balance with coefficients
• Five types: synthesis, decomposition, single replacement, double replacement, combustion (fuel + $O_2$ → $CO_2$ + $H_2O$)
• Faster reactions: heat, concentration, surface area, catalysts (speed up, not used up; enzymes are biological catalysts)
• Exothermic releases heat, endothermic absorbs heat; rusting is slow oxidation

Up Next: Part 6 — the traps GS chemistry questions are built from, and how to use your 48 seconds.

⚗️ Chemistry: Matter and Reactions

Part 6 of 7 — Test Strategy and Common Traps

Your Clock on GS

On the CAT-ASVAB, remaining time and remaining questions show in the lower-right corner of the screen.

How the computer test treats you

• No going back. Answer each question for real before you move on: there is no "flag and return."
• Adaptive. The first question is of average difficulty; a correct answer usually brings a harder question and a wrong one an easier question. A hard question means you are doing well, so don't panic.
• Tryout questions may be mixed in, and you can't tell which, so treat every question as scored.
• Don't speed-guess at the end. Unfinished questions are scored as if you had guessed at random, and the official guidance says the CAT applies a relatively large penalty when several wrong answers come in a row near the end of a subtest. If time is short, keep reading and genuinely answering.
• Paper test: answer every question; there is no penalty for guessing.

Key Fact: Most chemistry questions take one fact. If you know the fact, answer in 15–20 seconds and bank the time for the questions that need a step of reasoning.

The Traps Wrong Answers Are Built From

Three Moves for Every Question

1. Spot the keyword. "Protons" means atomic number; "neutral" means pH 7 or no charge; "new substance" means chemical change; "not used up" means catalyst.
2. Kill the opposite. Many wrong answers flip the right one (gain/lose, acid/base, raise/lower). Decide the direction first, then choose.
3. Distrust extreme words. Options with "always," "never" or "all" are usually wrong in science unless the rule truly has no exceptions.

Trap check. 🎯 Each question is built around one of the traps above.

Decide the direction first. 🔽

Key Takeaways — Part 6

• About 48 seconds per GS question on the CAT (15 in 12 minutes); most chemistry facts take far less
• No going back, and don't speed-guess at the end: a run of wrong answers late in a subtest is penalized
• Common traps: mass number vs atomic number, ion charge direction, pH direction and 10× steps, dissolving is physical, Latin symbols, catalysts not used up, mixture vs compound
• Spot the keyword, decide the direction, then distrust "always" and "never"

Up Next: Part 7 — a mixed mastery set at ASVAB difficulty.

⚗️ Chemistry: Matter and Reactions

Part 7 of 7 — Mixed Mastery Set

How to Use This Set

These 11 questions mix everything from Parts 2–5, in the same style and difficulty range as GS chemistry on the ASVAB: a few quick recall questions, several one-step reasoning questions, and a few that need careful thought.

• Give yourself about 48 seconds per question, the CAT-ASVAB pace.
• Do the arithmetic on scratch paper; no calculator.
• When you miss one, go back to the part it came from:

Mastery set, questions 1–4. 🎯

Mastery set, questions 5–8. 🎯 These take a step of reasoning.

Mastery set, questions 9–11. ✍️

An atom has 19 protons and 20 neutrons.

9) Its mass number:

10) The number of electrons in the ion it forms when it has a charge of $+1$:

11) Balance the equation $N_2 + \_\_\, H_2 \rightarrow 2NH_3$. The coefficient in front of $H_2$ is:

Key Takeaways — Part 7

• Density decides floating (water ≈ 1 g/cm³), and squeezing the same mass into less space raises it
• Atomic number = protons, mass number = protons + neutrons, charge = protons − electrons
• Same group of the periodic table → similar reactions
• Dissolved substances raise the boiling point and lower the freezing point
• Know the five reaction types, and balance with coefficients
• Rusting = iron + oxygen (oxidation), a chemical change

You finished Chemistry: Matter and Reactions. Next, take the exit quiz for this topic, then move on to the next General Science topic.