Chemistry: Practice Questions & Explanations

Atomic number = number of protons in the nucleus; uniquely identifies the element. Carbon has 6 protons (atomic number 6); oxygen has 8; iron has 26. The periodic table is organized by atomic number, increasing left-to-right and top-to-bottom. Atoms also contain: (1) Neutrons — in the nucleus, no charge, similar mass to protons; mass number = protons + neutrons; isotopes are atoms of the same element with different neutron counts; (2) Electrons — orbit the nucleus in shells; negative charge; mass much less than protons/neutrons; in a neutral atom, electrons = protons. Periodic table organization: Periods (rows) — elements have same number of electron shells. Groups (columns) — elements have similar chemical properties due to same number of valence (outer-shell) electrons. Group 1: alkali metals (highly reactive). Group 2: alkaline earth metals. Groups 3-12: transition metals. Group 17: halogens (highly reactive nonmetals). Group 18: noble gases (very unreactive). Metals on left side; nonmetals upper right; metalloids on the diagonal between.

Water = H₂O = 2 hydrogen atoms + 1 oxygen atom. Common chemical formulas to know: H₂O (water); CO₂ (carbon dioxide); O₂ (oxygen gas); N₂ (nitrogen gas); H₂ (hydrogen gas); NaCl (sodium chloride/salt); HCl (hydrochloric acid); NaOH (sodium hydroxide/lye); H₂SO₄ (sulfuric acid); NH₃ (ammonia); CH₄ (methane); C₆H₁₂O₆ (glucose); H₂O₂ (hydrogen peroxide); O₃ (ozone); CaCO₃ (calcium carbonate/limestone). Water properties making it essential for life: (1) Polar molecule — oxygen attracts electrons more than hydrogen, creating partial negative on O and partial positive on H; this polarity allows water to dissolve many substances ('universal solvent'); (2) Hydrogen bonding — partial charges attract neighboring water molecules; gives water high surface tension, cohesion, adhesion, high specific heat, high heat of vaporization; (3) Density anomaly — water expands when freezing, so ice floats on liquid water; insulates aquatic life in winter; (4) Three states common at Earth temperatures (solid, liquid, gas).

pH scale measures acidity/alkalinity from 0 to 14: pH < 7 = acidic; pH = 7 = neutral; pH > 7 = basic/alkaline. Pure water has pH 7. Logarithmic scale: each integer change is a 10-fold change in H+ concentration. So pH 4 is 10× more acidic than pH 5, 100× more than pH 6. Common substances and approximate pH: stomach acid 1-2; lemon juice 2; vinegar 2-3; orange juice 3-4; coffee 5; rain (natural) 5.6; milk 6.5; pure water 7; blood 7.4; baking soda solution 9; ammonia 11; bleach 12; lye (sodium hydroxide) 14. Acids: substances that donate H+ ions (Brønsted-Lowry definition) or accept electron pairs (Lewis definition). Strong acids fully dissociate (HCl, H₂SO₄); weak acids partially dissociate. Bases: substances that accept H+ ions or donate hydroxide (OH-). Strong bases fully dissociate (NaOH, KOH); weak bases partially. Acid-base reactions produce salt and water. Buffers resist pH change — blood pH maintained 7.35-7.45 by bicarbonate buffer system. pH affects enzyme function; small changes can disrupt metabolism.

Matter classification: (1) ELEMENT — pure substance of one type of atom, cannot be broken down by chemical means. ~118 known elements (~92 naturally occurring). Examples: gold (Au), oxygen (O₂), carbon (C), iron (Fe), helium (He). Listed on periodic table. (2) COMPOUND — pure substance of two or more elements chemically bonded in fixed proportions. Bonds can be ionic (electron transfer, e.g., NaCl), covalent (electron sharing, e.g., H₂O), or metallic. Can be broken into elements by chemical means. Properties differ from constituent elements (sodium is a reactive metal; chlorine is a toxic gas; together they form table salt). Examples: water (H₂O), salt (NaCl), carbon dioxide (CO₂), glucose (C₆H₁₂O₆). (3) MIXTURE — physical combination of substances retaining their individual properties; variable composition; can be separated physically (filtration, distillation, evaporation). Homogeneous mixtures (solutions): uniform composition (saltwater, air, brass). Heterogeneous mixtures: non-uniform (sand and water, salad, oil and vinegar). Distinctions: physical change (state change, dissolving, mixing) vs chemical change (formation of new substances, color/temperature change, gas production, precipitate). Conservation of mass: in chemical reactions, atoms rearrange but matter is neither created nor destroyed.

Chemical change: new substances are formed with different chemical properties; chemical bonds break and form. Wood burning: cellulose + oxygen → carbon dioxide + water vapor + ash; new substances produced; cannot easily reverse. Signs of chemical change: (1) Color change; (2) Temperature change (exothermic — releases heat, or endothermic — absorbs heat); (3) Gas formation/bubbles (not from boiling); (4) Precipitate formation (solid in solution); (5) Light emission; (6) Odor change; (7) Difficulty reversing. Physical change: substance changes appearance or state but composition remains the same. Examples: melting, freezing, boiling, dissolving, breaking, cutting, mixing. Ice → water → vapor are all the same H₂O molecules, just different arrangements. Distinguishing: chemical change creates NEW substances; physical change just rearranges what's already there. Some changes mix both — dissolving sugar in water is mostly physical (sugar molecules still exist), but dissolving NaCl involves ionic dissociation (slight chemical change). Burning, rusting, fermenting, photosynthesis, digestion are chemical. Melting wax, dissolving salt, boiling water, freezing, evaporation are physical.

The pH SCALE: 0-14 scale measuring hydrogen ion concentration. pH 7 = NEUTRAL (pure water); pH below 7 = ACIDIC (more H+ ions); pH above 7 = BASIC/ALKALINE (more OH- ions). Each unit represents a 10x change: pH 6 is 10x more acidic than pH 7; pH 5 is 100x more acidic. COMMON pH VALUES: Battery acid ~0-1; stomach acid ~1.5-3.5; vinegar ~2.4; coffee ~5; milk ~6.5; water 7; blood ~7.4; baking soda ~8.3; bleach ~12.5; drain cleaner ~14. ACID + BASE → SALT + WATER (neutralisation reaction). The litmus test: red = acid; blue = base.

WATER is H2O — two hydrogen atoms bonded to one oxygen atom. ASVAB General Science includes basic chemistry. Other common formulas: CO2 (carbon dioxide); O2 (oxygen gas); NaCl (sodium chloride/table salt); CH4 (methane). Knowing common chemical formulas and symbols (H = hydrogen, O = oxygen, C = carbon, Na = sodium) is frequently tested. Water (H2O) is the most basic and essential formula to know.

A pH of 7 is NEUTRAL (neither acidic nor basic) — pure water is neutral. ASVAB General Science includes the pH scale. The scale runs 0-14: BELOW 7 = ACIDIC (lower = more acidic, e.g., lemon juice, stomach acid); ABOVE 7 = BASIC/ALKALINE (higher = more basic, e.g., bleach, soap); EXACTLY 7 = NEUTRAL. Knowing that 7 is neutral, below 7 is acidic, and above 7 is basic is a commonly tested chemistry concept.

The three most common states of matter are SOLID, LIQUID, and GAS. ASVAB General Science covers states of matter. SOLID (definite shape and volume — particles tightly packed); LIQUID (definite volume, takes the container's shape — particles flow); GAS (no definite shape or volume — particles spread out). A fourth state, PLASMA, exists at very high energy (like in stars). Matter changes states through heating/cooling (melting, freezing, evaporation, condensation). The three basic states are fundamental, commonly tested chemistry knowledge.

An atom is the smallest unit of an element that still has the properties of that element. Atoms consist of protons and neutrons in a central nucleus, with electrons surrounding it. A molecule is two or more atoms bonded together (which may be the same element or a compound). A compound is a substance made of two or more different elements chemically combined. A cell is a biological unit, not a chemical one. The atom is the basic building block of matter for a given element.

The pH scale runs from 0 to 14. A pH below 7 is acidic, exactly 7 is neutral (like pure water), and above 7 is basic (alkaline). A pH of 3 is well below 7, so the substance is an acid (and a fairly strong one, since lower numbers mean more acidic). Examples of acids in the low pH range include lemon juice and vinegar. A strong base would have a high pH (like 12–13). Remembering that 7 is neutral, lower is acidic, and higher is basic lets you classify substances by their pH quickly.

H₂O is the chemical formula for water. The subscript 2 after H means there are two hydrogen atoms, and the O (with an implied subscript of 1) means one oxygen atom, so each water molecule is two hydrogen atoms bonded to one oxygen atom. Hydrogen peroxide is a different compound, H₂O₂ (two hydrogens and two oxygens). Reading chemical formulas — where subscripts give the number of each atom — is a basic chemistry skill. Water's formula H₂O is one of the most important to recognize.