Properties of Liquids — Revision Notes

⚡ 30-Second Revision

Vapour Pressure ($P_{vap}$) — Pressure of vapour in equilibrium with liquid. $P_{vap} \uparrow$ with $T \uparrow$, $P_{vap} \downarrow$ with IMF $\uparrow$.
Boiling Point ($T_b$) — $T$ where $P_{vap} = P_{atm}$. $T_b \uparrow$ with IMF $\uparrow$, $T_b \downarrow$ with $P_{atm} \downarrow$.
Surface Tension ($\gamma$) — Force/length at surface. $\gamma \downarrow$ with $T \uparrow$, $\gamma \uparrow$ with IMF $\uparrow$. Detergents $\downarrow \gamma$.
Viscosity ($\eta$) — Resistance to flow. $\eta \downarrow$ with $T \uparrow$, $\eta \uparrow$ with IMF $\uparrow$. Unit: Pa\cdot s.
IMFs — London Dispersion < Dipole-Dipole < Hydrogen Bonding.

2-Minute Revision

The properties of liquids are governed by intermolecular forces (IMFs). Vapour pressure is the pressure exerted by vapour in equilibrium with its liquid. It increases with temperature because more molecules gain enough kinetic energy to escape.

Stronger IMFs lead to lower vapour pressure. The boiling point is the temperature at which vapour pressure equals external pressure; it's higher for liquids with stronger IMFs. Surface tension is the inward pull on surface molecules, causing the liquid to minimize its surface area (e.

g., spherical drops). It decreases with increasing temperature and is reduced by surfactants like detergents. Viscosity is a liquid's resistance to flow, essentially internal friction. It increases with stronger IMFs and decreases significantly with increasing temperature, as molecules can overcome attractions more easily.

Remember, hydrogen bonding is a particularly strong IMF, leading to high boiling points, low vapour pressures, high surface tensions, and high viscosities.

5-Minute Revision

Liquids exhibit unique properties due to the intermediate strength of intermolecular forces (IMFs) between their molecules. These IMFs include London Dispersion Forces (weakest), Dipole-Dipole forces, and Hydrogen Bonding (strongest). The stronger the IMFs, the more energy is required to separate molecules, influencing all liquid properties.

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Vapour Pressure — This is the pressure exerted by the vapour in dynamic equilibrium with its liquid phase in a closed container. It's a measure of a liquid's volatility. Factors: It increases significantly with temperature (more kinetic energy to escape) and decreases with stronger IMFs (harder to escape). For example, diethyl ether (weak IMFs) has a higher vapour pressure than water (strong H-bonding).

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Boiling Point — The temperature at which a liquid's vapour pressure equals the external atmospheric pressure. At this point, bulk vaporization occurs. Relationship: Liquids with higher vapour pressures at a given temperature will have lower boiling points. Stronger IMFs lead to higher boiling points (e.g., water boils at 100°C, while ether boils at 34.6°C).

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Surface Tension ($\gamma$) — The force per unit length acting on the liquid surface, causing it to contract and minimize its area. It arises from the net inward pull on surface molecules. Factors: It decreases with increasing temperature (weakens IMFs) and is significantly reduced by surfactants (like detergents) which disrupt IMFs at the surface. Liquids with stronger IMFs have higher surface tension (e.g., water has high surface tension).

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Viscosity ($\eta$) — A measure of a liquid's resistance to flow, representing internal friction between layers. Factors: It increases with stronger IMFs and molecular size/shape (more entanglement). It decreases sharply with increasing temperature (molecules overcome IMFs more easily). Glycerol, with extensive hydrogen bonding, is highly viscous. Water is less viscous than glycerol but more than acetone.

Key Takeaway: Always relate the property back to the strength of intermolecular forces and the effect of temperature on molecular kinetic energy. Strong IMFs generally mean higher boiling point, lower vapour pressure, higher surface tension, and higher viscosity.

Prelims Revision Notes

Properties of Liquids: NEET Revision Notes

1. Intermolecular Forces (IMFs):

Definition — Attractive forces between molecules.
Types (increasing strength) — London Dispersion Forces (LDFs) < Dipole-Dipole Forces < Hydrogen Bonding.
Impact — Stronger IMFs lead to higher boiling points, lower vapour pressures, higher surface tensions, and higher viscosities.

2. Vapour Pressure ($P_{vap}$):

Definition — Pressure exerted by vapour in equilibrium with liquid in a closed system.
Volatility — Higher $P_{vap}$ means more volatile (evaporates easily).
Factors

* Temperature (T): $P_{vap} \uparrow$ with $T \uparrow$ (more kinetic energy, more escape). * Nature of Liquid (IMFs): $P_{vap} \downarrow$ with IMF $\uparrow$ (harder to escape).

Example — Diethyl ether > Ethanol > Water (in terms of $P_{vap}$ at same T).

3. Boiling Point ($T_b$):

Definition — Temperature at which $P_{vap}$ equals external atmospheric pressure ($P_{atm}$).
Relationship with $P_{vap}$ — Lower $P_{vap}$ at a given T implies higher $T_b$.
Factors

* IMFs: $T_b \uparrow$ with IMF $\uparrow$ (more energy to overcome forces). * External Pressure: $T_b \downarrow$ with $P_{atm} \downarrow$ (e.g., at high altitudes).

Example — Glycerol > Water > Ethanol > Diethyl ether (in terms of $T_b$).

4. Surface Tension ($\gamma$):

Definition — Force per unit length (N/m) or energy per unit area (J/m$^2$) at liquid surface.
Origin — Net inward pull on surface molecules due to unbalanced IMFs.
Consequences — Liquids minimize surface area (spherical drops), capillary action, wetting/non-wetting.
Factors

* Temperature (T): $\gamma \downarrow$ with $T \uparrow$ (IMFs weakened). * IMFs: $\gamma \uparrow$ with IMF $\uparrow$. * Impurities: Surfactants (detergents) $\downarrow \gamma$ by disrupting IMFs.

5. Viscosity ($\eta$):

Definition — Resistance to flow (internal friction between layers).
Unit — Pascal-second (Pa\cdot s) or poise (1 Pa\cdot s = 10 P).
Factors

* Temperature (T): $\eta \downarrow$ with $T \uparrow$ (molecules overcome IMFs more easily). * IMFs: $\eta \uparrow$ with IMF $\uparrow$. * Molecular Size/Shape: Larger/more entangled molecules $\uparrow \eta$.

Example — Glycerol > Water > Acetone > n-Pentane (in terms of $\eta$).

Quick Check: Stronger IMFs $\implies$ Higher $T_b$, Lower $P_{vap}$, Higher $\gamma$, Higher $\eta$.

Vyyuha Quick Recall

Very Strong Intermolecular Forces Lower Vapour Pressure, Increase Boiling Point, Surface Tension, and Viscosity.