Revision: Class 11 >> States of Matter: Gases and Liquids NEET (UG) States of Matter: Gases and Liquids

• Matter is defined based on its physical and chemical structure. It occupies space and has mass, particularly as opposed to energy.
• Atoms and molecules are the building blocks of matter, consisting of positively charged protons, neutral neutrons, and negatively charged electrons, respectively.

Atom — An atom is the smallest part of an element that takes place in a chemical reaction.

Matter—Anything that has mass and occupies space is called matter.

Chemical properties of matter tell us how a substance changes when it interacts with other substances. These properties describe how matter reacts and forms new substances. When a chemical change happens, the matter changes into something new. The atoms in a substance rearrange themselves, and a new substance is formed. For example:

• When wood burns, it turns into ash and smoke.
• When iron is exposed to air and water, it forms rust.

Examples of Chemical Properties:

• Flammability: This describes if a substance can burn. For example, wood is flammable because it can catch fire.
• Reactivity: This tells us how a substance reacts when mixed with others. For example, if you mix vinegar with baking soda, they react to create bubbles.
• Rusting: Some metals, like iron, will form rust when they come in contact with water and air.
• Acidity and Basicity: Some substances are acidic (like lemon juice), while others are basic (like soap). Acids and bases can react with each other to form new substances.

Anything that has mass and occupies space is called matter.

An emulsion is a colloid in which minute droplets of one liquid are dispersed in another liquid which is not miscible with it. Examples are milk and butter.

The matter is defined as anything that has mass and takes up space. The matter is found in solid, liquid and gas.

The process by which matter changes from one state to another and back to the original state, without any change in its chemical composition.

Anything that has mass and occupies space is called matter.

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.

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).

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.

The pressure exerted by saturated water vapour is called aqueous tension.

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 V_2​, pressure from P_1​ 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.”

OR

The relation between the three properties of a gas - pressure (P), volume (V), and temperature (T) - expressed as PV = nRT, is called the ideal gas equation.

\[\frac{P_1V_1}{T_1}=\frac{P_2V_2}{T_2}\]

The total pressure of a gaseous mixture equals the sum of the partial pressures of all individual gases.

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

\[\frac{r_1}{r_2}=\sqrt{\frac{M_2}{M_1}}\]

\[\text{Rate of diffusion}=\frac{\text{Volume of gas diffused}}{\text{Time required for diffusion}}\]

Boyle's Law (Pressure–Volume Relationship)

At constant temperature (T) and number of moles (n), the pressure of a gas is inversely proportional to its volume.

\[P\propto\frac{1}{V}\quad\Rightarrow\quad P_1V_1=P_2V_2\]

The p–V curve at constant temperature is called an isotherm.

or

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.

Charles' Law (Temperature–Volume Relationship)

At constant pressure (P) and number of moles (n), the volume of a gas is directly proportional to its absolute temperature.

\[V\propto T\quad\Rightarrow\quad\frac{V_1}{T_1}=\frac{V_2}{T_2}\]

The V–T curve at constant pressure is called an isobar

Absolute zero = 0 K = –273.15°C — the temperature at which gas volume theoretically becomes zero. It cannot be attained in practice (temperatures of ~0.000001 K have been achieved in labs)

or

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).

Intermolecular forces are attractive (and repulsive) forces acting between neighbouring molecules. They are weaker than covalent or ionic bonds but determine the physical state of matter.

As intermolecular forces increase: Gas → Liquid → Solid (thermal energy decreases in the same direction).

Types of Intermolecular Forces:

1. A gas consists of an extremely large number of tiny, discrete molecules whose actual volume is negligible compared to the total volume of the gas
2. Gas molecules are in constant, random motion moving in straight lines; they change direction upon collisions with other molecules or container walls
3. Intermolecular forces are negligible — molecules neither attract nor repel each other
4. Effect of gravity on molecules is negligible
5. All molecular collisions are perfectly elastic — total kinetic energy is conserved (though energy can be redistributed)
6. Gas pressure is caused by molecular bombardment against the walls of the container
7. Different molecules have different kinetic energies, but the average KE is directly proportional to absolute temperature: Average KE ∝ T

In the liquid state, molecules are held close together but can execute random motion through the spaces between them. Most physical properties are governed by the strength of intermolecular forces.

Properties of Liquids:

(i) Vapour Pressure

• In a closed vessel, liquid and its vapour establish a dynamic equilibrium; the pressure at equilibrium is called saturated vapour pressure
• Vapour pressure is a kinetic phenomenon — depends on temperature and nature of liquid
• Vapour pressure ∝ Temperature (increases with rise in temperature)
• Vapour pressure ∝ 1 / Intermolecular forces (weaker forces → higher vapour pressure)
• Unit: mm Hg or torr

(ii) Viscosity

• The property that determines the ease with which a fluid flows (resistance to flow)
• Arises due to internal friction between layers of fluid in motion
• Viscosity ∝ 1 / Temperature (viscosity decreases as temperature increases)
• Viscosity ∝ Intermolecular forces (stronger forces → more viscous)
• SI unit of viscosity coefficient: N m⁻² s (pascal-second); CGS unit: poise (g cm⁻¹ s⁻¹)

(iii) Surface Tension

• The force acting along the surface of a liquid at right angles to any line per unit length
• Arises because molecules at the surface experience a net inward pull
• Surface tension ∝ 1 / Temperature (decreases as temperature rises)
• Surface tension ∝ Intermolecular forces (stronger forces → higher surface tension)
• SI unit: N m⁻¹