CA01 — Basic Concepts of Matter

✈ Chemistry – CA01 · AFCAT General Awareness AFCAT Level

This foundational chapter covers everything about matter — what it is, how it exists, and how we classify it. AFCAT tests the difference between physical and chemical changes, the three states of matter, and the distinction between elements, compounds and mixtures. These are scoring questions requiring pure recall.

📌 AFCAT Focus: Physical vs chemical change (burning = chemical; melting = physical); states of matter and their properties; element vs compound vs mixture. One direct question typically comes from this chapter. Memorise 3–4 examples of each type of change.

PART 1 — STATES OF MATTER

1. Three States of Matter

All matter exists in one of three states depending on temperature and pressure. The differences arise from how tightly particles are packed and how freely they move.

Fig. 1 — Three States of Matter: Particle Arrangement, Properties and Examples

PART 2 — PHYSICAL vs CHEMICAL CHANGES

2. Physical vs Chemical Changes

This is one of the most directly tested topics in AFCAT Chemistry. The key test: does a new substance form? If yes — chemical change. If no — physical change.

Fig. 2 — Physical vs Chemical Change: Key Differences and AFCAT Examples

PART 3 — ELEMENTS, COMPOUNDS & MIXTURES

3. Classification of Matter

Fig. 3 — Elements, Compounds and Mixtures: Definitions and Key Differences

💡 Key Distinction — Compound vs Mixture: In a compound, elements are chemically bonded in a fixed ratio — the properties are completely different from the elements (e.g., H₂O is liquid, not a gas like H₂ or O₂). In a mixture, substances retain their individual properties and can be separated by physical methods. Air is a mixture (not a compound) because its composition varies and can be separated by distillation.

📝 AFCAT PYQs — Basic Concepts of Matter

Q1. Which of the following is a chemical change? AFCAT PYQ

(a) Melting of ice(b) Evaporation of water(c) Burning of coal(d) Dissolving sugar in water

✔ Answer: (c) Burning of coal

Burning coal reacts with oxygen to form CO₂ and ash — new substances with completely different properties. This is an irreversible chemical change. Melting, evaporation, and dissolving are physical changes — the substance (H₂O or sugar) remains chemically the same and can be recovered by cooling or evaporation. This exact question type repeats across AFCAT papers.

Q2. Air is classified as a: AFCAT PYQ

(a) Element(b) Compound(c) Mixture(d) Pure substance

✔ Answer: (c) Mixture

Air is a homogeneous mixture of gases (mainly N₂ ~78%, O₂ ~21%, Ar ~0.9%, CO₂ ~0.04%). Its composition varies by location and altitude. The gases can be separated by fractional distillation of liquid air. Since no chemical bonding occurs between the components, it is a mixture, not a compound.

Q3. Which state of matter has no definite shape but a definite volume? AFCAT PYQ

(a) Solid(b) Liquid(c) Gas(d) Plasma

✔ Answer: (b) Liquid

A liquid has a fixed volume (unlike gas) but takes the shape of its container (unlike solid). Solids have both fixed shape and fixed volume. Gases have neither. This is the most basic states-of-matter question and is tested directly in AFCAT. Plasma is the fourth state, at very high temperatures.

Q4. Rusting of iron is an example of: AFCAT PYQ

(a) Physical change only(b) Reversible physical change(c) Chemical change (slow oxidation)(d) Neither physical nor chemical

✔ Answer: (c) Chemical change

Rusting: 4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃ (iron oxide — a completely new substance). It is irreversible under normal conditions and requires both oxygen AND moisture. This makes it a slow chemical change (slow oxidation/corrosion). A common trap: some students think rusting is physical because it's slow — speed does not determine the type of change.

🧠 Quick Memory Chart — CA01

⚛ States of Matter

Solid: fixed shape & volume
Liquid: fixed volume, no fixed shape
Gas: no fixed shape or volume
Particle motion: solid < liquid < gas
Compressibility: solid < liquid < gas

⚖ Physical vs Chemical

Physical: no new substance; reversible
Chemical: new substance; usually irreversible
Burning, rusting = chemical
Melting, dissolving = physical
Cooking egg = chemical (irreversible)

📌 Classification

Element: one type of atom (H, Fe, O)
Compound: chemically combined, fixed ratio
Mixture: physically combined, variable
Air = mixture; Water = compound
Separated: mixture by physical, compound by chemical

📝 Practice Exercise

E1. Which of the following is a physical change?

(a) Burning of wood(b) Rusting of iron(c) Melting of wax(d) Digestion of food

E2. Water is a compound because:

(a) It exists in three states(b) H and O are combined in a fixed ratio by mass(c) It can be separated by filtration(d) It is transparent

E3. Which of the following is NOT a characteristic of a chemical change?

(a) Formation of precipitate(b) Gas evolution(c) Change in colour(d) Change in size or shape

Answers:
E1 → (c) Melting of wax [wax → liquid wax; same chemical composition; reversible on cooling] | E2 → (b) H and O combined in fixed ratio (H:O = 1:8 by mass; always) | E3 → (d) Change in size/shape [this is the indicator of a physical change; the other three indicate chemical change]

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