Properties of liquids – Vapour Pressure, Viscosity

What is a Liquid?

The liquid is a state of matter that is a nearly incompressible fluid that adapts to the shape of its container while retaining a (nearly) constant volume independent of pressure. The volume is fixed if the temperature and pressure stay constant. When a solid is heated beyond its melting point, and the pressure exceeds the triple point of the material, it becomes liquid. While intermolecular forces remain the same, the molecules have enough energy to move about, and the structure is mobile. This suggests that a liquid’s shape is governed by its container rather than the liquid itself. The volume is often higher than the comparable solid, with the most well-known exception being water (H2O). A liquid’s critical temperature is the highest temperature it can survive. The liquid state of matter is a transitional condition between the solid and gaseous forms of matter. 

Water exists in three states: 

• ice (solid)
• liquid as water and 
• gas as vapour.

Nature and Properties of Liquids:

1. A liquid has no definite shape and takes on the shape of the container in which it is stored.
2. Because the intermolecular force of attraction is weaker in liquids than in solids, they have a definite volume.
3. They can move from a higher to a lower level.
4. A liquid is compressible because the distance between neighbouring molecules is greater than that of a solid but less than that of a gas.
5. They are less dense.
6. A liquid can diffuse into another liquid because molecules move faster in liquids but slower in gases.
7. Compression has little effect on liquids.
8. Liquid molecules are very similar to one another.
9. Although liquids have a constant volume, they do not have a constant form.
10. The flow of liquids from one stage to the next.
11. Under normal conditions, liquids have boiling points higher than room temperature.

Change in Matter’s State

It is known as a change of state of matter. In which one state may be transformed into another, and so on. The following are some examples:

Condensation is the process of changing from a gaseous to a liquid form.

Turning a liquid into a gaseous state is known as evaporation.

Solidification is the process of transforming a liquid into a solid-state.

Sublimation is the process of turning a solid to a gaseous state without passing through the liquid stage.

Mechanical Properties of Fluid

Fluids are substances that can flow, such as liquids and gases. It doesn’t have a definite shape. When an item is submerged at rest in a liquid, the fluid typically exerts a force on its surface. It’s referred to as liquid thrust. The push felt per unit area of a liquid’s surface at rest is described as pressure. Depth governs pressure at every point in liquid under the surface, liquid density, and gravity acceleration. 

Pascal’s Law: According to Pascal’s Law, pressure placed on a confined liquid is conveyed in its totality to every segment of liquid as well as the walls of the enclosing vessel. A body loses weight when partly or entirely immersed in a liquid. The weight of the body lost in the liquid equals the weight of the liquid displaced by the body’s submerged part. 

The body floats in a liquid if the weight of the liquid displaced by the submerged part of the body matches the weight of the body.

Viscosity: It is a fluid property that generates an internal frictional force when the fluid moves in the shape of layers with relative motion. It operates against numerous layers’ relative motion.

Viscosity is also known as fluid friction. The rearward dragging force exerted on a small spherical body of radius r travelling with velocity v in a viscous medium with a coefficient of 

Stoke’s law: Stoke’s Law is an equation that expresses the settling velocities of small spherical particles in a fluid medium. The law is established by taking into account the forces acting on a certain particle as it falls through a liquid column with respect to gravity. 

According to the same, 𝐹=6π𝑟η𝑣

The force of surface tension is defined as the force operating per unit length of a line drawn parallel to the liquid surface. The dimensional formula for surface tension is MT-2 and is measured in Nm-1 units.

What exactly is Vapour Pressure?

Vapour pressure is the pressure formed by the vapour of a liquid (or solid) over the liquid’s surface. This pressure is created in a closed container at a specific temperature while in a thermodynamic equilibrium state. The equilibrium Vapour pressure determines the rate of evaporation of a liquid. When the temperature rises, vapour pressure also increases. The boiling point of a liquid is the point at which the pressure exerted by the surrounding environment equals the pressure exerted by the vapour. A liquid solution is created by dissolving a solid, liquid, or gas in a specific liquid solvent. The amount of pressure that the vapours exert on the liquid solvent when they are in equilibrium and at a particular temperature is called liquid solution pressure. The Vapour pressure of a liquid varies with its temperature and the nature of the liquid. Let’s look at some of the most important aspects of vapour pressure on liquid solutions.

Characteristics of Vapour Pressure

1. A pure liquid has a higher vapour pressure than a liquid solution.
2. It is inversely proportional to the forces of attraction that exist between liquid molecules.
3. It increases as the temperature rises. This is due to the molecules gaining kinetic energy and thus rapidly vapourising.

Roult’s Law

According to Raoult’s law the vapour pressure of a solution is proportional to the mole fraction of a solute added to the solution.

Raoult’s Law is expressed by the formula:

P0, solvent is the vapour pressure of the pure solvent

If more than one solute is introduced to the solution, the component of each solvent is added to the total pressure. Raoult’s law is similar to the ideal gas law, except it applies to solution characteristics. The ideal gas law posits perfect behaviour, in which intermolecular interactions between different molecules equal forces between identical molecules. 

Raoult’s law is based on the assumption that the physical characteristics of the components of a chemical solution are equivalent.

Conclusion:

A liquid adapts to its container while retaining a (nearly) constant volume independent of pressure. The volume is fixed if the temperature and pressure stay constant. A liquid’s critical temperature is the highest temperature it can survive at which it becomes liquid. Vapour pressure is created in a closed container at a specific temperature. Equilibrium vapour pressure determines the rate of evaporation of a liquid. A liquid solution is created by dissolving a solid, liquid, or gas in a specific liquid solvent. The amount of pressure that vapours exert on the liquid solvent is liquid solution pressure.