Definitions [9]
Define matter
Anything that has mass and occupies space is called matter.
Define interconversion of states of matter.
The process by which matter changes from one state to another and back to the original state, without any change in its chemical composition.
Define the term polarizability
Polarizability is defined as the ability of an atom or a molecule to form momentary dipoles, which means, the ability of the atom or molecule to become polar by redistributing its electrons.
Define the term Aqueous tension
The pressure exerted by saturated water vapour is called aqueous tension.
Define the term Hydrogen bond
The electrostatic force of attraction between a positively polarised hydrogen atom of one molecule and a highly electronegative atom (which may be negatively charged) of another molecule is called a hydrogen bond.
Define the term Dipole moment
Dipole moment (μ) is the product of the magnitude of the charge (Q) and the distance between the centres of positive and negative charge (r). It is designated by a Greek Letter (μ) and its unit is Debye (D).
A temperature scale with absolute zero (zero kelvin) as the starting point is called the absolute scale or the kelvin scale.
The volume of a given mass of a dry gas varies inversely as the pressure and directly as the absolute temperature.
V ∝ \[\frac {1}{P}\] × T or \[\frac {PV}{T}\] = k (constant)
If volume changes from V1 to V2, pressure from P1 to P2, and temperature from T1 to T2, then:
\[\frac {P_1V_1}{T_1}\] = \[\frac {P_2V_2}{T_2}\] = k (constant)
“The relation between three properties of a gas, i.e., pressure, volume and temperature, is called the ideal gas equation.”
Formulae [1]
\[\frac{P_1V_1}{T_1}=\frac{P_2V_2}{T_2}\]
Theorems and Laws [5]
It states that the volume of a given mass of dry gas is inversely proportional to its pressure at a constant temperature.
P1V1 = P2V2 = k at constant temperature
It states that volume of a given mass of a dry gas is directly proportional to its absolute (kelvin) temperature, if the pressure is kept constant.
OR
The pressure remaining constant, the volume of a given mass of a dry gas increases or decreases by 1/273 of its volume for each 1°C increase or decrease in temperature respectively.
\[\frac {V_1}{T_1}\] = \[\frac {V_2}{T_2}\] = k at constant pressure
Statement:
The pressure of a fixed mass of gas is directly proportional to its absolute temperature if volume is kept constant.
Mathematically, P ∝ T ⇒ \[\frac {P}{T}\] = constant
This means: heating a gas in a sealed (rigid) container increases its pressure.
Graph: P vs T (Isochore)
P vs T graph at constant volume — a straight line through the origin (Kelvin scale).
Statement:
For a fixed mass of gas at constant temperature, the pressure is inversely proportional to the volume.
Mathematically, P ∝ \[\frac {1}{V}\] ⇒ PV = constant
Graph: P vs V (Isotherm)
This means: squeezing a gas into a smaller space increases its pressure. Doubling the pressure halves the volume.
Statement:
The volume of a fixed mass of gas is directly proportional to its absolute temperature if the pressure is kept constant.
Mathematically, V ∝ T ⇒ \[\frac {V}{T}\] = constant
Graph: V vs T (Isobar)
A straight line through the origin when using Kelvin. All lines converge at 0 K (absolute zero).
Key Points
- An ideal gas has point-mass molecules, no intermolecular forces, and perfectly elastic collisions.
- Boyle's Law (constant T): PV = constant — pressure and volume are inversely proportional.
- Charles' Law (constant P): V/T = constant — volume and temperature are directly proportional.
- Gay-Lussac's Law (constant V): P/T = constant — pressure and temperature are directly proportional.
- The Ideal Gas Equation, PV = nRT, combines all three laws into a single universal relationship.
- The Universal Gas Constant R = 8.314 J mol⁻¹ K⁻¹ is the same for all ideal gases.
- Real gases approximate ideal behaviour at low pressure and high temperature.
- Always use absolute temperature (Kelvin) in gas law calculations. T(K) = T(°C) + 273.15
