Properties of liquids: vapor pressure, surface tension, viscosity

Vapor Pressure

1. Vapor pressure is the pressure exerted by the vapor of a liquid in equilibrium with its liquid phase at a given temperature.
2. It depends on the temperature and the nature of the liquid.
3. As the temperature increases, the vapor pressure also increases due to higher kinetic energy of the molecules.
4. A liquid with a higher vapor pressure at a given temperature is considered more volatile.
5. The temperature at which the vapor pressure equals atmospheric pressure is called the boiling point.
6. Liquids with strong intermolecular forces (e.g., hydrogen bonding) have lower vapor pressures.
7. Example: Water has a lower vapor pressure than acetone at room temperature due to hydrogen bonding.
8. Vapor pressure is crucial in processes like evaporation and distillation.
9. It is measured using a manometer or other pressure-measuring devices.
10. Substances with high vapor pressures at room temperature are often called volatile liquids (e.g., alcohol).

Surface Tension

1. Surface tension is the energy required to increase the surface area of a liquid by a unit amount.
2. It arises due to cohesive forces between the molecules of the liquid.
3. Liquids with strong intermolecular forces, like hydrogen bonding, exhibit higher surface tension.
4. Water has high surface tension due to extensive hydrogen bonding.
5. Surface tension causes liquids to form spherical droplets to minimize surface area.
6. Examples of applications: capillary action, floating of small objects (like insects) on water, and detergents reducing surface tension.
7. Surface tension decreases with increasing temperature as molecular forces weaken.
8. Surfactants, like soap and detergents, reduce surface tension by disrupting cohesive forces.
9. It plays a role in biological processes, such as the functioning of alveoli in lungs.
10. The SI unit of surface tension is newton per meter (N/m).

Viscosity

1. Viscosity is the measure of a liquid's resistance to flow.
2. It arises due to internal friction between the layers of the liquid.
3. Liquids with strong intermolecular forces exhibit higher viscosity.
4. Example: Honey has a higher viscosity than water due to stronger cohesive forces.
5. Viscosity decreases with an increase in temperature as molecular mobility increases.
6. The SI unit of viscosity is pascal-second (Pa·s), though it is often measured in poise (1 poise = 0.1 Pa·s).
7. Newtonian fluids have a constant viscosity regardless of applied stress, while non-Newtonian fluids have variable viscosity.
8. Examples of non-Newtonian fluids: ketchup, toothpaste, and oobleck.
9. Viscosity is important in industries like lubrication, polymer production, and fluid mechanics.
10. It is measured using instruments like a viscometer or rheometer.

Key Points for Competitive Exams

1. Vapor pressure increases with temperature and is inversely related to intermolecular forces.
2. A liquid boils when its vapor pressure equals atmospheric pressure.
3. Surface tension depends on cohesive forces and decreases with temperature.
4. Applications of surface tension include capillary action and the action of detergents.
5. Viscosity measures resistance to flow and decreases with increasing temperature.
6. Liquids with strong intermolecular forces have high viscosity and low vapor pressure.
7. Surface tension causes liquids to form minimized surface areas, such as droplets.
8. Viscosity is critical in processes involving lubricants and fluids.
9. Units of measurement: Vapor pressure in torr, surface tension in N/m, and viscosity in Pa·s.
10. All three properties—vapor pressure, surface tension, and viscosity—are influenced by the strength of intermolecular forces.