8.2 Matter, Chemical Changes, Acids and Bases

8.2 Matter, Chemical Changes, Acids and Bases

Why This Matters for the PUPCET

This is the most scenario-based part of PUPCET chemistry. Instead of asking for a definition, the exam shows you an everyday event — ice candy melting, an egg frying, metal rusting on a tricycle — and asks you to classify it. Master the logic below and these items become some of the fastest points on the test.

States of Matter and Phase Changes

Matter is anything that has mass and occupies space. Its three common states differ in how their particles behave:

Heating gives particles energy to move apart; cooling does the opposite. The six phase changes, with examples you actually see:

• Melting — solid to liquid (ice candy softening at recess)
• Freezing — liquid to solid (water hardening in the freezer)
• Evaporation (vaporization) — liquid to gas (wet clothes drying on the sampayan)
• Condensation — gas to liquid (droplets forming on a cold bottle of soft drink)
• Sublimation — solid directly to gas (mothballs or naphthalene slowly disappearing in a cabinet; dry ice producing fog)
• Deposition — gas directly to solid (frost forming on cold surfaces)

Every phase change is a physical change: the particles rearrange, but the substance itself — water, for example — never becomes something else.

Pure Substances vs Mixtures

Matter divides into two big families:

• Pure substance — has a fixed composition. Either an element (one kind of atom: oxygen gas, copper wire) or a compound (two or more elements chemically joined in a fixed ratio: water H₂O, table salt NaCl, carbon dioxide CO₂).
• Mixture — substances physically combined, each keeping its own properties. A homogeneous mixture looks uniform throughout, like vinegar, brewed coffee, or air; you cannot see the parts. A heterogeneous mixture has visibly different parts, like halo-halo, a fruit salad, or oil floating on water.

Common trap: compounds and homogeneous mixtures both look uniform, but a compound can only be separated by a chemical reaction, while a mixture separates by physical means — filtration, evaporation, or a magnet.

Physical vs Chemical Change

A physical change alters appearance — size, shape, state — without creating a new substance. Tearing paper, dissolving sugar in juice, melting candle wax: all physical, and usually reversible. A chemical change creates at least one new substance. Watch for these clues:

1. Bubbles of a new gas form (batter fizzing with baking soda)
2. A color change not from mixing (banana peel browning, iron rusting)
3. Heat or light given off (burning firewood, fireworks on New Year's Eve)
4. A precipitate — a new solid — forms in a liquid
5. The change is hard to reverse (a fried egg cannot be unfried)

Practice identifications: boiling rice involves both — water evaporating is physical, the starch in the rice cooking is chemical. Chopping onions is physical; the tears from the onion's released compounds reacting involve chemistry. Rust forming on a jeepney's roof is the classic chemical change: iron plus oxygen becomes iron oxide, a genuinely new substance.

Acids, Bases, and the pH Scale

Acids taste sour (never taste anything in a lab — this is about common knowledge), turn blue litmus paper red, react with metals to release hydrogen gas, and feel like nothing unusual on skin until they burn. Bases taste bitter, feel slippery like soap, and turn red litmus paper blue. Mix an acid and a base in the right amounts and they undergo neutralization, producing salt and water.

The pH scale measures acidity from 0 to 14:

Lower pH means more acidic; each step down is ten times more acidic, so pH 3 is ten times more acidic than pH 4.

Balancing Simple Chemical Equations

A balanced equation has the same number of atoms of each element on both sides, because matter is neither created nor destroyed. The one rule: adjust coefficients only, never subscripts.

Worked example. Balance H₂ + O₂ → H₂O.

1. Count atoms: left has 2 H and 2 O; right has 2 H and 1 O.
2. Put a coefficient of 2 before H₂O: H₂ + O₂ → 2H₂O. Now oxygen balances (2 and 2), but hydrogen is 2 versus 4.
3. Put a coefficient of 2 before H₂: 2H₂ + O₂ → 2H₂O. Hydrogen is 4 and 4; oxygen is 2 and 2. Balanced.

Try the same method on CH₄ + O₂ → CO₂ + H₂O and you will get CH₄ + 2O₂ → CO₂ + 2H₂O.

Lab Safety Symbols

Recognize the four most-tested hazard pictograms: the flammable symbol (a flame — keep away from heat, as with alcohol and LPG), the corrosive symbol (liquid dripping onto a hand and a surface — acids and bleach that burn skin), the toxic symbol (skull and crossbones — can poison or kill, like some pesticides), and the irritant/harmful symbol (an exclamation mark — less severe but avoid contact). In any lab: wear goggles, never smell chemicals directly, and always add acid to water, never the reverse.

Exam Tips

• Ask one question to classify a change: is a new substance formed? Yes means chemical, no means physical.
• Uniform appearance is not enough — decide element, compound, or mixture by asking whether the parts are chemically bonded.
• pH questions are really direction questions: small numbers are acidic, big numbers are basic, 7 is neutral.