⚗️ Science – Chemistry: Elements, Reactions & Properties
ACER Level 5 · Mathematical Reasoning – Science component
📋 What You’ll Learn in This Lesson
- Matter classification: element vs compound vs mixture
- States of matter, changes of state, and the particle model
- Periodic table structure: groups, periods, metals vs non-metals
- Reaction types: exothermic vs endothermic, acids vs bases
- 3 worked ACER-style chemistry questions
🎯 ACER Chemistry: You will not need to balance complex equations from scratch. Focus on classifying matter, understanding reaction types, and reading the periodic table structure.
⚗️ Classifying Matter
| Type |
Composition |
Key property |
Examples |
| Element |
One type of atom only |
Cannot be separated by physical or chemical means |
Gold (Au), Carbon (C), Oxygen (O2) |
| Compound |
Two or more elements chemically bonded |
Fixed ratio. Properties differ from constituent elements. |
Water (H2O), Salt (NaCl), CO2 |
| Mixture |
Two or more substances not chemically bonded |
Variable composition. Can be separated physically. |
Air, saltwater, sand+iron filings |
| Pure substance |
Element or compound with uniform composition |
Fixed melting/boiling point |
Distilled water, pure gold |
🧊 States of Matter and Changes of State
| State |
Properties |
Particle model |
| Solid |
Fixed shape, fixed volume |
Particles close together, vibrating in place |
| Liquid |
Variable shape, fixed volume |
Particles close together but can flow past each other |
| Gas |
Variable shape, variable volume |
Particles far apart, moving freely and rapidly |
| Change of state |
Direction |
Energy |
| Melting |
Solid → Liquid |
Absorbs heat energy (endothermic) |
| Freezing |
Liquid → Solid |
Releases heat energy (exothermic) |
| Evaporation/Boiling |
Liquid → Gas |
Absorbs heat energy (endothermic) |
| Condensation |
Gas → Liquid |
Releases heat energy (exothermic) |
| Sublimation |
Solid → Gas directly |
Dry ice (CO2), iodine |
🗂️ Periodic Table – What ACER Tests
| Group |
Examples |
Properties |
| Group 1 – Alkali metals |
Li, Na, K |
Very reactive. React vigorously with water. Soft metals. |
| Group 2 – Alkaline earth metals |
Mg, Ca |
Reactive, but less so than Group 1. |
| Group 17 – Halogens |
F, Cl, Br, I |
Very reactive non-metals. Form salts with metals. |
| Group 18 – Noble gases |
He, Ne, Ar |
Unreactive (full outer electron shell). Used in lighting. |
| Transition metals |
Fe, Cu, Zn, Au |
Less reactive. Good conductors. Used in construction. |
Period vs Group: Periods = horizontal rows (period number = number of electron shells). Groups = vertical columns (same group = similar chemical properties). ACER often asks: “Which element has similar properties to sodium?” Answer: same Group 1 = Li, K, Rb.
🔥 Chemical Reactions
| Reaction type |
Energy change |
Observable sign |
Examples |
| Exothermic |
Releases energy (heat) to surroundings |
Temperature of surroundings increases |
Combustion (burning), respiration, rusting (slowly) |
| Endothermic |
Absorbs energy from surroundings |
Temperature of surroundings decreases |
Photosynthesis, cooking, dissolving ammonium nitrate |
🧪 Acids, Bases and pH
| Type |
pH |
Properties |
Examples |
| Acid |
pH < 7 |
Sour taste, corrosive, turns litmus red |
Hydrochloric acid, vinegar, lemon juice |
| Neutral |
pH = 7 |
No observable effect on litmus |
Pure water |
| Base/Alkali |
pH > 7 |
Bitter taste, slippery feel, turns litmus blue |
Sodium hydroxide, baking soda, ammonia |
📝 Worked Example — Classify the substance
Question
A substance has a fixed melting point of 100°C, is made of hydrogen and oxygen atoms bonded together in a 2:1 ratio, and is a liquid at room temperature. Classify it.
Fixed melting point
Pure substance (not a mixture)
Two elements bonded together
Compound (not an element)
H and O in fixed ratio (H2O)
Water – this is a compound
✅ Answer: Pure compound (water/H2O)
⭐ ACER Examiner’s Trick: ACER often presents a substance and asks you to classify it. The key questions: (1) Is it one type of atom? If yes – element. (2) Is it multiple types bonded chemically in fixed ratio? Compound. (3) Mixed but separable? Mixture.
📌 Quick Reference Card — Chemistry
| Classification |
1 atom type = element. Bonded ratio = compound. Mixed, separable = mixture. |
| Exothermic |
Releases heat: combustion, respiration, rusting. Temperature of surroundings rises. |
| Endothermic |
Absorbs heat: photosynthesis, cooking. Temperature of surroundings falls. |
| pH scale |
7 = base (blue litmus). |
| Noble gases |
Group 18. Unreactive. Full outer shell. |
| Group = property |
Elements in same group have similar chemical properties. |
