A1 — States of Matter | CSEC Chemistry Hub

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1. The Particle Theory of Matter

All matter is made of tiny particles in constant, random motion. The kinetic particle theory explains why substances behave differently as solids, liquids, and gases.

Property	Solid	Liquid	Gas
Arrangement	Regular lattice, very close	Random, close together	Completely random, far apart
Movement	Vibrate in fixed positions	Slide and flow past each other	Move rapidly in all directions
Inter-particle forces	Very strong	Moderate	Negligible
Shape	Fixed	Takes shape of container	Fills entire container
Volume	Fixed	Fixed	Not fixed
Compressibility	Not compressible	Barely compressible	Highly compressible
Density	High	High (slightly less than solid)	Very low

🌡️ Temperature and kinetic energy: Temperature measures the average kinetic energy of particles. Higher temperature = particles move faster. At absolute zero (0 K / −273°C), all particle motion theoretically stops.

2. Changes of State

Change	Transition	Energy	Example
Melting	Solid → Liquid	Absorbed (endothermic)	Ice → water at 0°C
Freezing	Liquid → Solid	Released (exothermic)	Water → ice at 0°C
Evaporation / Boiling	Liquid → Gas	Absorbed (endothermic)	Water → steam at 100°C
Condensation	Gas → Liquid	Released (exothermic)	Steam → water
Sublimation	Solid → Gas (direct)	Absorbed	Dry ice, iodine crystals
Deposition	Gas → Solid (direct)	Released	Frost forming on glass

💡 Why temperature stays constant during a change of state: The energy supplied is used to break inter-particle forces, not increase kinetic energy. No KE increase = no temperature rise. This is why heating curves have flat plateaus at melting and boiling points.

⚠️ Evaporation vs Boiling: Evaporation occurs at ANY temperature from the liquid surface. Boiling happens throughout the entire liquid at one fixed temperature (the boiling point). A wet towel dries at room temperature — it evaporates, not boils!

3. Diffusion

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, due to random particle motion.

Happens in gases (fastest) and liquids (slower)
Does NOT occur in solids (particles fixed in place)
Higher temperature → faster diffusion (more kinetic energy)
Lighter molecules → faster diffusion (Graham's Law)

🎯 Graham's Law: rate₁/rate₂ = √(M₂/M₁)

Rate of diffusion is inversely proportional to the square root of the molar mass.

Classic experiment: NH₃ (M=17) and HCl (M=36.5) released from opposite ends of a tube form a white ring of NH₄Cl closer to the HCl end — because NH₃ diffuses ~1.47× faster.

4. The Gas Laws

⚠️ Golden rule: ALWAYS convert temperature to Kelvin before using gas laws! T(K) = T(°C) + 273

Law	Constant	Formula	Relationship
Boyle's Law	Temperature	P₁V₁ = P₂V₂	P and V inversely proportional
Charles' Law	Pressure	V₁/T₁ = V₂/T₂	V and T (K) directly proportional
Gay-Lussac's Law	Volume	P₁/T₁ = P₂/T₂	P and T (K) directly proportional
Combined Gas Law	—	P₁V₁/T₁ = P₂V₂/T₂	All three variables

📝 Boyle's Law Example: Gas at 200 kPa occupies 3.0 dm³. Volume at 400 kPa (same T)?
P₁V₁ = P₂V₂ → 200 × 3.0 = 400 × V₂ → V₂ = 1.5 dm³ (pressure doubled → volume halved ✓)

⚡ Particle State Simulator

Select a state to see how particles behave. The temperature slider changes particle speed!

🌡️ Temperature: Low

📈 Heating Curve — Water

Watch water heat from ice to steam. The two flat plateaus show where changes of state occur — temperature stays constant!

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❄️ Cooling Curve — Water

The reverse of the heating curve. Two flat plateaus appear during condensation (100°C) and freezing (0°C) — energy is released but temperature stays constant.

🔑 Heating vs Cooling curves: Heating plateaus = energy absorbed breaking inter-particle forces. Cooling plateaus = energy released forming inter-particle forces.

🃏 Flashcards — States of Matter

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Answer

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❓ Quiz — States of Matter

🔢 Worked Example — Combined Gas Law

Problem: A gas occupies 600 cm³ at 27°C and 100 kPa. It is compressed to 200 cm³ and its temperature is raised to 127°C. Calculate the new pressure. (Aᵣ not needed — this is a gas law problem!)

Step 1: Convert both temperatures to Kelvin

T₁ = 27 + 273 = ? K. What is T₁?

Step 2: All three variables change — write the Combined Gas Law

P, V and T all change. Type the correct formula.

Step 3: Substitute and solve for P₂

P₁=100, V₁=600, T₁=300, V₂=200, T₂=400. Rearrange: P₂ = P₁V₁T₂/(T₁V₂). Calculate P₂ in kPa.

Step 4: Sanity check — should P₂ be greater or less than 100 kPa?

Volume decreased (pressure ↑) AND temperature increased (pressure ↑). Type "greater" or "less".

🔗 Matching — States of Matter

Click a term, then its matching description. Green = correct!

Term / Law

Description / Formula

📝 CSEC-Style Questions

Q1. Describe, in terms of particles, what happens when a solid is heated until it becomes a gas. Explain why temperature is constant during melting. [6 marks]+

Mark Scheme

1 Particles in the solid vibrate faster as heat is added; kinetic energy increases. ✓

2 At the melting point, particles have enough energy to break free from their fixed positions; solid → liquid; particles slide past each other. ✓✓

3 Further heating increases KE of liquid particles; at the boiling point they overcome all inter-particle forces; liquid → gas. ✓✓

4 During melting, temperature is constant because energy is used to break inter-particle forces (not to increase KE). ✓

Q2. A gas occupies 500 cm³ at 25°C and 150 kPa. The pressure is increased to 300 kPa at constant temperature. Calculate the new volume. [3 marks]+

Mark Scheme

1 Temperature constant → Boyle's Law: P₁V₁ = P₂V₂ ✓

2 150 × 500 = 300 × V₂ → V₂ = 75000 / 300 ✓

3 V₂ = 250 cm³ ✓

Q3. NH₃ gas (M = 17) and HCl gas (M = 36.5) are released simultaneously from opposite ends of a 100 cm tube. Calculate the ratio of their diffusion rates and state where the white ring of NH₄Cl forms. [4 marks]+

Mark Scheme

1 Graham's Law: rate(NH₃)/rate(HCl) = √(M(HCl)/M(NH₃)) ✓

2 = √(36.5/17) = √2.147 ≈ 1.47 ✓

3 NH₃ diffuses 1.47× faster than HCl. ✓

4 Ring forms closer to the HCl end (approximately 59 cm from the NH₃ end, 41 cm from the HCl end). ✓

Q4. A fixed volume of gas at 27°C has a pressure of 200 kPa. It is heated to 127°C. Calculate the new pressure. [3 marks]+

Mark Scheme

1 T₁ = 300 K; T₂ = 400 K ✓ (must convert to Kelvin)

2 Gay-Lussac: P₁/T₁ = P₂/T₂ → P₂ = 200 × 400/300 ✓

3 P₂ = 266.7 kPa ✓

⭐ Key Formulas & Concepts

Boyle's Law

P₁V₁ = P₂V₂ (T constant) P ↑ → V ↓ (inverse)

Charles' Law

V₁/T₁ = V₂/T₂ (P constant) V and T (K) directly proportional

Combined Gas Law

P₁V₁/T₁ = P₂V₂/T₂ T(K) = T(°C) + 273 ⚠️

Graham's Law

rate₁/rate₂ = √(M₂/M₁) Lighter gas diffuses faster

Changes of State

Temp CONSTANT during change Energy breaks forces, not ↑ KE Pure substance = sharp m.p./b.p.

Particle Summary

Solid: regular, vibrate, fixed Liquid: random close, flow Gas: far apart, rapid, fills

📚 Resources

🔬 PhET: States of Matter 🔬 PhET: Gas Properties ▶ YouTube: States of Matter ▶ YouTube: Gas Laws 📖 ChemGuide: Kinetic Theory