Chemistry is the study of matter, its properties, and how substances interact, combine, and change. Mastering these fundamental concepts is key to passing the ASVAB chemistry section.
1. Matter: The Building Blocks of Chemistry 🌍
Everything is made of matter, which consists of atoms, molecules, and compounds.
| Concept | Description | Example |
| Atom | Smallest unit of an element, made of protons, neutrons, and electrons | Hydrogen (H) atom |
| Element | Pure substance with only one type of atom; defined by its atomic number | Oxygen (O), Carbon (C) |
| Molecule | Two or more atoms chemically bonded together | H₂ (Hydrogen gas) |
| Compound | A molecule containing two or more different elements | NaCl (Table salt) |
🔹 Fun Fact: There are 118 elements on the Periodic Table, with oxygen, hydrogen, carbon, and nitrogen making up 96% of the human body!
2. States of Matter: Solid, Liquid, or Gas? 🌡️
Matter exists in different states depending on temperature and energy levels.
| State of Matter | Particle Movement | Characteristics | Example |
| Solid | Particles vibrate in place | Definite shape & volume | Ice 🧊 |
| Liquid | Particles move freely | Takes shape of container, definite volume | Water 💧 |
| Gas | Particles move rapidly, spread apart | No fixed shape or volume | Oxygen in air 🌬️ |
Phase Changes 🔄
Matter can change from one state to another through reversible processes:
| Phase Change | From → To | Example |
| Freezing | Liquid → Solid | Water to ice |
| Melting | Solid → Liquid | Ice to water |
| Condensation | Gas → Liquid | Water vapor to dew 🌫️ |
| Vaporization | Liquid → Gas | Boiling water to steam |
| Sublimation | Solid → Gas (no liquid) | Dry ice turning into CO₂ gas |
| Deposition | Gas → Solid (no liquid) | Frost forming on a cold surface ❄️ |
🔹 Fun Fact: Water is one of the few substances that naturally exists in all three states on Earth!
3. Atomic Structure: Inside the Atom ⚛️
Atoms are made of subatomic particles, each with a specific charge.
| Particle | Charge | Location | Relative Mass |
| Proton (p⁺) | +1 | Nucleus | Heavy |
| Neutron (n⁰) | 0 (neutral) | Nucleus | Heavy |
| Electron (e⁻) | -1 | Orbiting nucleus | Very light |
Ions: Charged Atoms ⚡
Atoms become ions when they gain or lose electrons:
- Cation (+): Atom loses electrons (e.g., Na⁺)
- Anion (-): Atom gains electrons (e.g., Cl⁻)
🔹 Example: Table salt (NaCl) forms when sodium (Na⁺) donates an electron to chlorine (Cl⁻)!
4. Atomic Number & Mass Number 🔢
- Atomic Number = Number of protons in an atom (defines the element).
- Mass Number = Protons + Neutrons (total nucleus mass).
- Number of Neutrons = Mass Number – Atomic Number
| Element | Protons | Neutrons | Atomic Number | Mass Number |
| Hydrogen (H) | 1 | 0 | 1 | 1 |
| Carbon (C) | 6 | 6 | 6 | 12 |
| Oxygen (O) | 8 | 8 | 8 | 16 |
| Silicon (Si) | 14 | 14 | 14 | 28 |
🔹 Example: Silicon (Si) has 14 protons, 14 neutrons, and an atomic number of 14.
5. Chemical Equations: Describing Reactions 🧪
A chemical equation shows how substances react and form new products.
| Term | Description | Example |
| Reactants | Substances before the reaction (left side of equation) | Carbon + Oxygen |
| Products | Substances after the reaction (right side) | Carbon dioxide |
| Equation | Chemical shorthand for the reaction | C + O₂ → CO₂ |
🔹 Example:
Burning methane gas (CH₄) in oxygen:
CH₄ + 2O₂ → CO₂ + 2H₂O
6. Reduction & Oxidation Reactions (Redox) 🔥
Redox reactions involve electron transfer between substances.
| Process | What Happens? | Example |
| Oxidation | Loses electrons | Iron rusting (Fe → Fe²⁺) |
| Reduction | Gains electrons | Oxygen gaining electrons in rust (O₂ → O²⁻) |
🔹 Memory Trick:
- OIL = Oxidation Is Loss (of electrons)
- RIG = Reduction Is Gain (of electrons)
7. The pH Scale: Acids & Bases 🧪
The pH scale (0-14) measures how acidic or basic a solution is.
| Type | pH Range | Example |
| Acidic | 0-6 (More H⁺ ions) | Lemon juice (pH 2), Vinegar (pH 3) 🍋 |
| Neutral | 7 | Pure Water 💧 |
| Basic (Alkaline) | 8-14 (More OH⁻ ions) | Soap (pH 9), Bleach (pH 12) 🧼 |
🔹 Example: Stomach acid (pH 1-2) helps digest food, while baking soda (pH 9) neutralizes excess acid!
8. Key Takeaways ✅
✔️ Matter consists of atoms, molecules, and compounds.
✔️ States of matter change with temperature & energy levels.
✔️ Atomic structure determines element properties (protons, neutrons, electrons).
✔️ Chemical reactions form new substances (e.g., combustion, oxidation).
✔️ pH scale measures acidity & basicity (Acid <7, Base >7).