Unit 3: Liquid State
Physical Properties of Liquids
Liquids have properties intermediate between gases (disorder) and solids (order). They have a definite volume but no definite shape, and they possess strong intermolecular forces.
Vapour Pressure
Definition: In a closed container, it is the pressure exerted by the vapour in dynamic equilibrium with its liquid at a constant temperature.
- Dynamic Equilibrium: Rate of evaporation = Rate of condensation.
- Factors affecting Vapour Pressure:
- Nature of Liquid: Liquids with weak intermolecular forces (e.g., ether) are volatile and have high vapour pressure. Liquids with strong forces (e.g., water, glycerol) are non-volatile and have low vapour pressure.
- Temperature: Vapour pressure increases with temperature (as more molecules gain enough KE to escape).
- Determination: A simple method is the barometric method, where the liquid is injected into the vacuum at the top of a mercury barometer, and the depression of the mercury column is measured. A more accurate method uses an isoteniscope.
Surface Tension (γ)
Definition: The tendency of a liquid to shrink into the minimum possible surface area. It is defined as the force acting per unit length perpendicular to an imaginary line drawn on the surface (Unit: N · m-1) or as the energy required to increase the surface area by one unit (Unit: J · m-2).
- Cause: Molecules in the bulk of the liquid are attracted equally in all directions. Molecules at the surface experience a net inward pull, which causes the surface to contract.
- Determination (Stalagmometer Method):
- This "drop-count" or "drop-weight" method compares the number of drops (n) formed by a fixed volume of a test liquid (1) and a reference liquid (2, usually water).
- The weight of a falling drop is balanced by the surface tension: mg = 2π rγ.
- For two liquids: (m1g) / (m2g) = (γ1) / (γ2) (assuming r is constant).
- Since m = V × d, and we use the *same* volume for n drops: V1 = n1 Vdrop1 and m1 = n1 Vdrop1 d1. This is complex.
- The simpler relation used is that the weight of one drop is proportional to γ/n: m \propto γ. And total mass M = n × m. Or m1 = M1/n1.
- The standard working formula is:
Formula: (γ1) / (γ2) = (n2 d1) / (n1 d2)
Where γ1, d1, n1 are the surface tension, density, and drop count for liquid 1, and γ2, d2, n2 are for the reference liquid 2.
Coefficient of Viscosity (η)
Definition: The measure of a fluid's internal resistance to flow. (e.g., Honey has high η, Water has low η).
- Cause: Arises from intermolecular forces that make it difficult for one layer of liquid to slide past another.
- Determination (Ostwald Viscometer):
- This method compares the time (t) it takes for a fixed volume of liquid to flow through a narrow capillary tube under its own weight.
- The flow rate is governed by Poiseuille's equation.
- By comparing a test liquid (1) to a reference liquid (2, water), we get a simple relation:
Formula: (η1) / (η2) = (d1 t1) / (d2 t2)
Where η1, d1, t1 are the viscosity, density, and flow time for liquid 1, and η2, d2, t2 are for the reference liquid 2.
Effect of Solutes on Properties
Effect of Addition of Solutes on Surface Tension
- Inorganic Solutes (e.g., NaCl, KNO3): These strong electrolytes generally increase the surface tension of water. The strong ion-dipole forces pull water molecules more strongly into the bulk, increasing the net inward pull on the surface.
- Organic Solutes (e.g., soaps, alcohols): These solutes (which are often surfactants) decrease the surface tension of water. They are less attracted to the bulk water and tend to accumulate at the surface, which disrupts the strong hydrogen bonding of the water surface and reduces the inward pull.
Effect of Addition of Solutes on Viscosity
The effect is complex, but in general:
- Dissolving a solid (like sugar or salt) in a liquid increases the viscosity. The dissolved particles disrupt the flow of the solvent molecules.
- Mixing two liquids can result in a viscosity that is higher or lower than the average, depending on the new intermolecular forces formed.
Surface Active Agents (Surfactants)
Interfacial Tension
This is the "surface tension" that exists at the interface (boundary) between two immiscible liquids (e.g., oil and water). It's the energy required to create a new area of this interface.
Surface Active Agent (Surfactant)
Definition: A substance that, when added in low concentrations, significantly reduces the surface tension of a solvent (like water) and the interfacial tension between two liquids.
Surfactants are amphiphilic: they have two distinct parts:
- Hydrophilic ("water-loving") Head: A polar or ionic group (e.g., -COO-Na+, -SO3-Na+, -N(CH3)3+).
- Hydrophobic ("water-fearing") Tail: A long non-polar hydrocarbon chain (e.g., C17H35-).
Explanation of Cleansing Action of Detergents
Detergents (a type of surfactant) clean grease and oil from surfaces (like fabric) through a process called emulsification.
- Wetting: The detergent solution lowers the surface tension of water, allowing it to "wet" the fabric and the greasy dirt more effectively.
- Micelle Formation: The surfactant molecules surround the non-polar grease droplet.
- The hydrophobic tails dissolve into the grease.
- The hydrophilic heads remain on the outside, facing the water.
- Emulsification: This cluster of surfactant and grease is called a micelle. The micelle has a stable, water-soluble outer surface (all the negative heads), so the entire grease droplet is "solubilized" and can be lifted off the fabric and washed away by the water.
Temperature Variation of Viscosity
Comparison of Liquids and Gases
The effect of temperature on viscosity is opposite for liquids and gases.
Viscosity of Liquids
- Trend: Viscosity of liquids DECREASES as temperature INCREASES.
- Reason: Viscosity in liquids is primarily due to strong intermolecular forces. Increasing the temperature gives the molecules more kinetic energy, which allows them to overcome these attractive forces and slide past each other more easily, thus reducing the resistance to flow.
Viscosity of Gases (Recap from Unit 1)
- Trend: Viscosity of gases INCREASES as temperature INCREASES.
- Reason: Viscosity in gases is due to momentum transfer between layers. Increasing the temperature makes the molecules move faster (v \propto √(T)). These faster molecules transfer more momentum when they cross between layers, resulting in *more* friction, not less.
Exam Question: "Compare and contrast the effect of temperature on the viscosity of liquids and gases."
Answer: Viscosity of liquids *decreases* with temperature because higher KE overcomes intermolecular forces. Viscosity of gases *increases* with temperature because higher KE leads to greater momentum transfer between layers.