Liquid State

The liquid state is one of the primary states of matter characterized by having a definite volume with no fixed shape. In essence, liquids can be flow and acquire the shape of their container while maintaining a constant volume. Some examples of liquids encountered in everyday life include water, milk, and oil.

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This Story also Contains

1. Liquid State
2. Some Solved Examples
3. Summary

Liquid State

Intermolecular forces are stronger in the liquid state than in the gaseous state. Molecules in liquids are so close that there is very little empty space between them and under normal conditions liquids are denser than gases. Molecules of liquids are held together by attractive intermolecular forces. Liquids have a definite volume because molecules do not separate from each other. However, molecules of liquids can move past one another freely, therefore, liquids can flow, can be poured, and can assume the shape of the container in which these are stored. In the following sections, we will look into some of the physical properties of liquids such as vapor pressure, surface tension, and viscosity.

Vapor Pressure

At a particular temperature, it is the pressure exerted by vapors over the liquid surface when vapors are in equilibrium with the liquid.

• Vapor pressure increases with the increase in temperature.
• The variation of the vapor pressure of liquid with temperature is given as
  $\log \mathrm{P}=-\frac{\mathrm{A}}{\mathrm{T}}+\mathrm{B}$
  Here, A, B = constant, P = Vapour pressure of liquid, T = Temperature
• The plot of log P vs 1/T will be in a straight line.
• The vapor pressure of H₂O at 373 K is 76 cm.
• At the critical temperature, the meniscus between liquid and vapor disappears.
• The amount of heat needed to convert one gram of a liquid into its vapor at its B.P is known as heat or enthalpy or latent heat of vaporization.

Surface Tension

It is the force at right angles to the surface of a liquid along one cm or one-meter length of the surface.

• Units: Newton metre^-1 or Nm^-1, dyne cm^-1
• Due to surface tension, the surface area of the liquid decreases up to a minimum. example. , Falling drops are spherical which is the minimum surface area for a given volume.
• Due to surface tension, a liquid rises in the capillary tube, water moves upward In the soil, and the walking of insects over water's surface.
• As Temperature increases, surface tension decreases.
• At the critical temperatures, surface tension is zero.

Viscosity

• Viscosity
  It is the internal resistance of a liquid to flow which exists due to the relative motion between two layers. It decreases with the increase of temperature. It is calculated as the force per unit area needed to maintain a velocity difference of unity between two parallel layers of liquid unit distance apart.
  
• Laminar Flow
  The liquid is considered to be consisting of molecular layers arranged one over the other. When the liquid flows over a glass surface then the layer of molecules immediately in contact with the glass surface is stationary with zero velocity. But layer immediately above it is not stationary but flows with some velocity. Further, the next layer above it flows still faster and this continues and the topmost layer of molecules flow with maximum velocity. So, this type of flow in which there is a gradual gradation in the velocities on passing from one layer to another is called laminar flow.
  
  
  $
  \mathrm{F} \propto \mathrm{A} \cdot \frac{\mathrm{dv}}{\mathrm{dz}}
  $
  Where $\mathrm{A}=$ Area
  $
  \begin{aligned}
  & \frac{d v}{d z}=\text { velocity gradient which is change of velocity with distance. } \\
  & F=\eta A \cdot \frac{d v}{d z}
  \end{aligned}
  $
  where $\eta$ is the proportionality constant $\eta$ is a Greek letter (eta.)
• Viscosity Coefficient
  It is the force of friction needed to maintain a velocity difference of 1 cm/sec between any two parallel layers of 1 cm² area which are 1 cm apart.
  $\begin{aligned} & \eta=\frac{f \cdot x}{A \cdot v}=\frac{\text { dymes } \times \mathrm{cm}}{c m^2 \times \mathrm{sec}^{-1}}=\text { dyne } \mathrm{cm}^2 \mathrm{sec} \\ & \eta=1 \text { poise } \\ & \text { Here } \mathrm{f}=\text { Force, } \mathrm{a}=\text { Area, } \mathrm{v}=\text { Velocity Difference, } \mathrm{x}=\text { Distance between two layers } \\ & \text { 1 Poise }=1 \mathrm{gm} \mathrm{cm}^{-1} \mathrm{sec}^{-1} \\ & \text { since dyne }=\mathrm{gm} \times \mathrm{cm} \times \mathrm{sec}^{-2} \\ & \text { 1 Poise }=1 / 10 \mathrm{Newton}^2 \mathrm{metre}^2 \mathrm{sec}^{-1} \\ & \text { or Pas or } \mathrm{Kgm}^{-1} \mathrm{~s}^{-1}\end{aligned}$
• Effect of Temperature on Viscosity
  On increasing temperature, viscosity decreases as the average thermal energy of molecules increases hence the effect of intermolecular attraction forces decreases.
  It can be shown by Arrhenius's equation as follows:
  $\eta=\mathrm{Ae}^{\mathrm{E}_{\mathrm{a}} / \mathrm{RT}}$
  Here T = Temperature, R = Universal gas constant, E_a = Activation energy
• Fluidity
  It is the reciprocal of the viscosity coefficient of a liquid denoted by ?.$\phi=\frac{1}{\eta}$

Recommended topic video on (Liquid state)

Some Solved Examples

Example 1: With the increase in temperature, how does the surface tension of a liquid change?

1)Increases

2) Decreases

3)Remain same

4)Nothing can be predicted

Solution

With the increase in temperature, the intermolecular forces of attraction decrease. Because of this temperature increase, the molecules' kinetic energy also increases, and thus, the surface tension decreases.
Hence, the answer is the option (2).

Example 2: An increase in kinetic energy can overcome intermolecular forces of attraction. How will the viscosity of liquid be affected by the increase in temperature?

1)No effect

2) Decrease

3)Increase

4)Nothing can be predicted

Solution

Effect of Temperature on Viscosity

With increasing temperature, viscosity decreases as the average thermal energy of molecules increases; hence, the effect of intermolecular attraction forces decreases.
It can be shown by the Arrhenius equation as follows:
$\eta=\mathrm{Ae}^{\mathrm{E}_{\mathrm{a}} / \mathrm{RT}}$
Here, T = Temperature, R = Universal gas constant, E_a = Activation energy.

With the increase in kinetic energy, intermolecular forces of attraction decrease. Thus, due to this decrease, the viscosity also decreases.
Hence, the answer is the option (2).

Example 3: Soap helps in cleaning clothes, because

1)Chemicals of soap change

2)It increases the surface tension of the solution

3)It absorbs the dirt

4) It lowers the surface tension of the solution

Solution

As we learned in Surface tension due to surface tension, liquids tend to minimize their surface area.
Thus, Soap helps to lower the surface tension of the solution, thus soap sticks to the dust particles and grease, and these are removed by action of water.

Hence, the answer is the option (4).

Example 4: On increasing temperature, the viscosity of liquid:

1)Increases

2) Decreases

3)Remain same

4)None of these

Solution

Viscosity
As we learned in Viscosity - The viscosity of liquids decreases as the temperature rises.

Viscosity of liquid $\alpha \frac{1}{\text { temperature }}$

Hence, the answer is the option (2).

Summary

The liquid state of matter has two features: it maintains a definite volume but takes a shape that will fit its container. Examples are water, milk, and oil—all products in use every day. For instance, one drinks, cooks, and cleans with these liquids. Water is, however, very important for life as it helps the body to hydrate, transport nutrients, and control the body temperature.