Liquid State

In a liquid state of matter, particles are less tightly packed as compared to solids and takes the shape of the container in which they are kept.

Properties of liquid state of matter

1. The liquid state has a definite volume but not size.
2. They show very poor compressibility compared to gas.
3. They have definite boiling and freezing point.
4. They have definite surface tension and viscosity.
5. They have definite vapour pressure at a given temperature.

Evaporation

The spontaneous escaping of molecules at any temperature from the liquid state to a vapour state due to its higher kinetic energy than the intermolecular force of attraction. It is a spontaneous and endothermic process. It causes cooling. It is a surface phenomenon.

Factors affecting evaporation

1. Nature of liquid:

• Greater the strength of the intermolecular force of attraction, less is the evaporation.
• Presence of hydrogen bonding decreases the rate of evaporation.
• More volatile liquids escape faster than less volatile liquids.

2. Temperature: The rate of evaporation increases with an increase in temperature.
3. Surface area: Greater the surface area, the greater will be the rate of evaporation.

Vapour pressure

The pressure exerted by the vapour of liquid when the rate of evaporation is equal to the rate of condensation at a given temperature in a closed container.

Factors affecting vapour pressure

1. Nature of liquid: Greater the strength of the intermolecular force of attraction, less is the vapour pressure.
2. Temperature: Vapour pressure increases with an increase in temperature.

Boiling point

The temperature at which the vapour pressure of a liquid becomes equal to the atmospheric pressure is called the boiling point. Boiling is the rapid vaporization of liquid which occurs when a liquid is heated to its boiling point.

Difference between boiling and evaporation

Surface tension

Due to an imbalance of one-sided downward force, the surface molecules are under tension. This effect is known as surface tension. Thus, surface tension is defined as the force per unit length acting perpendicular to the tangential line on the surface. Its unit is dyne per cm. it governs the physical properties of the liquid.

Factors affecting surface tension

1. Intermolecular force: Greater the intermolecular force of attraction, the greater will be the surface tension.
2. Temperature: Increase in temperature decreases in surface tension.

Effects of surface tension

1. The liquid drops have a spherical shape: The effect of surface tension is to reduce the surface area of the liquid to a minimum. Since a sphere has the minimum surface area for a given volume of liquid, the inward pull or the surface tension is responsible for the spherical shape of liquid drops.

2. The rise or fall of a liquid in a capillary tube: If a capillary tube is dipped into a liquid, there is either rise or fall in liquid. Two types of force are responsible for this. One is the intermolecular force of attraction between a similar type of liquid called cohesive force and another is the force of attraction between the dissimilar type of liquid called adhesive force. When adhesive force is greater than cohesive force, the liquid rises within the capillary tube. This happens when the capillary tube is dipped in water. If cohesive force is greater than adhesive force, liquid falls within the capillary tube. This happens when a capillary tube is dipped into mercury.

Viscosity

The internal resistance to the flow of liquid that one layer offers to another layer trying to pass through it. The liquid flowing slowly (eg. glycerine, honey) are high viscous whereas those flowing easily (eg. water, alcohol) are low viscous.

Factors affecting viscosity

1. Nature of liquid: If the intermolecular force of liquid is large, the viscosity will be high. Glycerine is more viscous than water because its intermolecular force of attraction (hydrogen bond) is high.
2. Temperature: Viscosity decreases with an increase in temperature.

Liquid crystal

The phase which is intermediate of solid and liquid and which has properties in between true crystalline solid and clear liquid is called liquid crystal. eg. protein, cell membrane, solution of soap and detergent.

Solid\ \underset{cool}{\stackrel{heat}{\rightleftharpoons}}\ Liquid\ crystals\ \underset{cool}{\stackrel{heat}{\rightleftharpoons}}\ Liquid

The first liquid crystal was discovered by the Austrian botanist, Friedrich Reinitzer in 1888 when he observed that cholesteryl benzoate exhibit two melting points.

Cholesteryl\ benzoate\ \underset{}{\stackrel{145^{o}C}{\rightleftharpoons}}\ Turbid\ liquid\ \underset{}{\stackrel{178^{o}C}{\rightleftharpoons}}\ Clear\ solution

Types of liquid crystals

Based on the structure or symmetry of the phase, it is classified into three types:

1. Nematic liquid crystal: The least order liquid crystal phase having thread-like molecule is nematic liquid crystal. They have parallel molecule to each other, low viscosity and therefore free to slide or roll individually. They have orientational order but not positional order.

2. Smectic liquid crystal: The molecules in this crystal are also parallel but these are arranged in layers. They have both orientational and positional order.

3. Cholesteric liquid crystal: The molecules are parallel but arranged in layers. They do not have orientational and positional order. They can change the colour at different temperature.

Properties of liquid crystals

Solid like properties:

• Optical activity
• Ordered arrangement

Liquid like properties:

• Fluidity
• Viscosity
• Surface tension

Uses of liquid crystals

• It is used in temperature sensor.
• It is used to locate the blockage of arteries, veins, etc.
• It is used in the display of digital watches, computer, calculator, etc.

Some Important Questions

1. Why is boiling point of water lower at high altitude?

2. Give reasons why?

• Water drops have a spherical shape.
• Alcohol can flow easily but honey cant.
• Evaporation takes place from the surface of the liquid.
• Cooking food takes more time at high altitude.
• Liquid like acetone is stored in a cold place.

3. Define boiling point and vapour pressure.
4. What is a liquid crystal? Write any two properties and uses.