States of Matter: Gases and Liquids
1. Introduction to States of Matter
Matter exists in three common physical states: solid, liquid, and gas.
The difference between these states arises due to variation in:
- Intermolecular forces
- Intermolecular distances
- Kinetic energy of particles
2. Gaseous State – Characteristics
Gases have:
- No fixed shape
- No fixed volume
- High compressibility
- Negligible intermolecular forces
3. Gas Laws
(a) Boyle’s Law
At constant temperature, pressure is inversely proportional to volume.
$P \propto \frac{1}{V} \quad \Rightarrow \quad PV = \text{constant}$
(b) Charles’ Law
At constant pressure, volume is directly proportional to absolute temperature.
$\frac{V}{T} = \text{constant}$
(c) Gay-Lussac’s Law
$\frac{P}{T} = \text{constant}$
(d) Avogadro’s Law
$V \propto n \quad \Rightarrow \quad \frac{V}{n} = \text{constant}$
4. Ideal Gas Equation
$PV = nRT$
| Symbol | Meaning |
|---|---|
| $P$ | Pressure |
| $V$ | Volume |
| $n$ | Number of moles |
| $R$ | Gas constant |
| $T$ | Temperature (K) |
5. Kinetic Theory of Gases
According to kinetic theory:
- Gas particles are in continuous random motion
- Collisions are perfectly elastic
- Average kinetic energy depends only on temperature
$\text{Average kinetic energy} = \frac{3}{2}kT$
6. Root Mean Square (RMS) Speed
$u_{rms} = \sqrt{\frac{3RT}{M}}$
Lighter gases move faster than heavier gases at the same temperature.
7. Real Gases
Real gases deviate from ideal behavior due to:
- Finite molecular volume
- Intermolecular attractions
8. van der Waals Equation
$\left(P + \frac{a n^2}{V^2}\right)(V - nb) = nRT$
$a$ accounts for intermolecular attraction
$b$ accounts for molecular volume
9. Critical Temperature, Pressure, and Volume
- Critical Temperature: Above this, gas cannot be liquefied
- Critical Pressure: Minimum pressure required to liquefy gas
- Critical Volume: Volume at critical point
10. Liquefaction of Gases
Gases can be liquefied by:
- Lowering temperature
- Increasing pressure
11. Liquid State – Characteristics
Liquids have:
- Fixed volume
- No fixed shape
- Moderate intermolecular forces
12. Vapour Pressure
Vapour pressure is the pressure exerted by vapour in equilibrium with its liquid.
Higher vapour pressure → higher volatility
13. Surface Tension
$\text{Surface tension} = \frac{\text{Force}}{\text{Length}}$
14. Viscosity
Viscosity is resistance to flow of liquid.
Viscosity decreases with increase in temperature.
15. Capillary Action
$h = \frac{2T \cos \theta}{r d g}$
16. Differences Between Gases and Liquids
| Property | Gas | Liquid |
|---|---|---|
| Volume | Not fixed | Fixed |
| Compressibility | High | Low |
| Intermolecular force | Very weak | Moderate |
17. Important Exam Points
- Always convert temperature to Kelvin
- Use van der Waals equation for real gases
- Remember critical constants
- Viscosity decreases with temperature in liquids
18. Summary
This chapter explains the behavior of gases and liquids using:
- Gas laws
- Kinetic theory
- Real gas corrections
- Properties of liquids