Liquid State

Liquid State

Edited By Shivani Poonia | Updated on Jul 02, 2025 06:26 PM IST

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.

This Story also Contains
  1. Liquid State
  2. Some Solved Examples
  3. Summary
Liquid State
Liquid State

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.
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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, Ea = 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, Ea = 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.

Frequently Asked Questions (FAQs)

1. What is the liquid state of matter?
The liquid state is one of the four fundamental states of matter, characterized by particles that are close together but can move freely relative to each other. Liquids have a definite volume but take the shape of their container.
2. Why do liquids flow?
Liquids flow because their particles have enough energy to move past one another but not enough to overcome the intermolecular forces that keep them close together. This allows liquids to change shape easily while maintaining a constant volume.
3. What is surface tension?
Surface tension is the tendency of liquid surfaces to shrink into the minimum surface area possible. It's caused by the cohesive forces between liquid molecules being stronger at the surface, creating a "skin-like" effect.
4. How does temperature affect the viscosity of liquids?
Generally, as temperature increases, the viscosity of liquids decreases. This is because higher temperatures give liquid molecules more kinetic energy, allowing them to overcome intermolecular forces more easily and flow more freely.
5. What causes capillary action in liquids?
Capillary action is caused by the combination of cohesive forces (attraction between like molecules) and adhesive forces (attraction between unlike molecules). When adhesive forces are stronger than cohesive forces, liquids can climb up narrow spaces, defying gravity.
6. Why do some liquids mix while others don't?
Liquids mix when the intermolecular forces between the different types of molecules are strong enough to overcome the forces between like molecules. This is often summarized by the phrase "like dissolves like," referring to the polarity of molecules.
7. What is vapor pressure?
Vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its liquid phase at a given temperature. It represents the tendency of particles to escape from the liquid surface.
8. How does boiling point relate to atmospheric pressure?
Boiling point is the temperature at which a liquid's vapor pressure equals the atmospheric pressure. As atmospheric pressure decreases (e.g., at higher altitudes), the boiling point of a liquid also decreases.
9. What is the difference between evaporation and boiling?
Evaporation occurs at the surface of a liquid at any temperature, while boiling occurs throughout the liquid when its vapor pressure equals the atmospheric pressure. Boiling requires more energy and occurs at a specific temperature for a given pressure.
10. Why do some liquids feel cold when they evaporate from your skin?
When liquids evaporate, they absorb heat from their surroundings, including your skin. This process, called evaporative cooling, removes thermal energy from your skin, making it feel cold.
11. What is viscosity?
Viscosity is a measure of a fluid's resistance to flow. It describes the internal friction of a moving fluid, with more viscous liquids flowing more slowly than less viscous ones.
12. How does the structure of water molecules contribute to its unique properties as a liquid?
Water molecules form hydrogen bonds between the slightly positive hydrogen atoms and slightly negative oxygen atoms of neighboring molecules. This network of hydrogen bonds gives water high surface tension, high boiling point, and the ability to dissolve many substances.
13. What is supercooling in liquids?
Supercooling is the process of lowering a liquid's temperature below its freezing point without it becoming solid. This metastable state occurs when there are no nucleation sites for crystals to form.
14. How do surfactants affect the properties of liquids?
Surfactants lower the surface tension of liquids by disrupting the cohesive forces between molecules at the surface. This allows liquids to spread more easily and can lead to the formation of bubbles or emulsions.
15. What is the meniscus of a liquid?
The meniscus is the curve formed by a liquid in a container. It can be concave (curved downward) or convex (curved upward) depending on whether the liquid's adhesive forces to the container are stronger or weaker than its cohesive forces.
16. How does pressure affect the melting point of substances?
Generally, increased pressure raises the melting point of substances. This is because higher pressure favors the more compact solid state over the less dense liquid state. However, water is a notable exception to this rule.
17. What is the triple point of a substance?
The triple point is the unique combination of temperature and pressure at which a substance can exist simultaneously in solid, liquid, and gas phases in thermodynamic equilibrium.
18. Why do some liquids form droplets while others spread out?
The formation of droplets or spreading of liquids depends on the balance between cohesive forces (within the liquid) and adhesive forces (between the liquid and the surface). Droplets form when cohesive forces are stronger, while spreading occurs when adhesive forces dominate.
19. How does the polarity of liquids affect their properties?
Polarity affects many liquid properties, including boiling point, surface tension, and solubility. Polar liquids generally have higher boiling points, higher surface tension, and can dissolve other polar substances more easily than non-polar liquids.
20. What is dynamic equilibrium in the context of evaporation?
Dynamic equilibrium in evaporation occurs when the rate of molecules leaving the liquid surface (evaporation) equals the rate of gas molecules returning to the liquid (condensation). This equilibrium maintains a constant vapor pressure above the liquid.
21. How do liquids conduct electricity?
Pure liquids generally don't conduct electricity well. However, when ionic compounds dissolve in liquids, they form electrolyte solutions that can conduct electricity due to the movement of ions.
22. What is the difference between cohesion and adhesion in liquids?
Cohesion refers to the attraction between molecules of the same substance (e.g., water molecules attracting other water molecules). Adhesion is the attraction between molecules of different substances (e.g., water molecules attracting glass molecules).
23. How does surface area to volume ratio affect evaporation rate?
A larger surface area to volume ratio increases the evaporation rate of liquids. This is because more molecules are exposed to the air, allowing them to escape more easily from the liquid surface.
24. What is the heat of vaporization?
The heat of vaporization is the amount of energy required to change a given quantity of a substance from a liquid to a gas at constant temperature. It represents the energy needed to overcome intermolecular forces in the liquid state.
25. How do impurities affect the freezing point of liquids?
Impurities generally lower the freezing point of liquids. This occurs because impurities disrupt the formation of the crystal lattice structure of the solid, requiring a lower temperature for freezing to occur.
26. What is the relationship between intermolecular forces and boiling point?
Stronger intermolecular forces generally lead to higher boiling points. This is because more energy is required to overcome these forces and allow molecules to escape into the gas phase.
27. How does pressure affect the density of liquids?
Increasing pressure typically increases the density of liquids slightly. This occurs because pressure forces the molecules closer together, reducing the volume occupied by the same mass of liquid.
28. What is cavitation in liquids?
Cavitation is the formation of vapor cavities in a liquid when it's subjected to rapid changes of pressure. These cavities collapse quickly, potentially causing damage to surrounding surfaces.
29. How do liquids transmit pressure?
Liquids transmit pressure equally in all directions. This principle, known as Pascal's law, states that pressure applied to an enclosed fluid is transmitted undiminished to every portion of the fluid and the walls of the containing vessel.
30. What is the difference between a solution and a suspension?
A solution is a homogeneous mixture where the solute is completely dissolved in the solvent, while a suspension is a heterogeneous mixture where particles are dispersed throughout the liquid but can settle out over time.
31. How does hydrogen bonding affect the properties of liquids?
Hydrogen bonding increases the boiling point, surface tension, and viscosity of liquids. It also contributes to unique properties like water's ability to expand when freezing and its high specific heat capacity.
32. What is the Mpemba effect?
The Mpemba effect is the counterintuitive phenomenon where, under certain conditions, warm water can freeze faster than cold water. The exact mechanism is still debated, but factors like evaporation, convection, and dissolved gases may play a role.
33. How do liquids respond to changes in temperature compared to solids and gases?
Liquids generally expand when heated and contract when cooled, but less dramatically than gases. They are more compressible than solids but less so than gases. Their response to temperature changes is intermediate between solids and gases.
34. What is the role of vapor pressure in distillation?
In distillation, differences in vapor pressure between components of a liquid mixture are exploited. Substances with higher vapor pressures evaporate more readily, allowing for separation of mixtures based on their boiling points.
35. How does surface tension contribute to the formation of spherical droplets?
Surface tension causes liquids to minimize their surface area. A sphere has the smallest surface area for a given volume, so small amounts of liquid tend to form spherical droplets when free from other forces.
36. What is the relationship between molecular structure and viscosity?
Molecular structure affects viscosity through factors like molecular size, shape, and intermolecular forces. Larger molecules, those with more branches, or those with stronger intermolecular attractions tend to have higher viscosities.
37. How do liquids behave in microgravity environments?
In microgravity, surface tension becomes the dominant force acting on liquids. This can cause liquids to form perfect spheres, adhere strongly to container walls, or exhibit other unusual behaviors not typically seen on Earth.
38. What is the difference between intensive and extensive properties of liquids?
Intensive properties, like density or boiling point, don't depend on the amount of liquid present. Extensive properties, such as volume or mass, do depend on the amount of liquid.
39. How does the concept of free energy apply to phase transitions in liquids?
Free energy determines the spontaneity of phase transitions. A phase change occurs spontaneously when it results in a decrease in the system's free energy. This concept explains why liquids evaporate below their boiling point.
40. What is the role of entropy in the liquid state?
Entropy in the liquid state is higher than in solids but lower than in gases. The increased disorder of liquids compared to solids contributes to their ability to flow, while their lower entropy compared to gases explains why they maintain a definite volume.
41. How do liquids contribute to heat transfer?
Liquids can transfer heat through conduction, convection, and phase changes. Convection currents in liquids can efficiently distribute heat, while processes like evaporation and condensation involve significant heat transfer.
42. What is the significance of critical point in liquids?
The critical point is the temperature and pressure above which a substance no longer has distinct liquid and gas phases. Beyond this point, the substance exists as a supercritical fluid with properties of both liquids and gases.
43. How do liquids behave under extreme pressures?
Under extreme pressures, liquids can exhibit unusual properties. They may become more viscous, change color, or even solidify. Some liquids, like water, can form exotic ice phases under very high pressures.
44. What is the role of liquids in chemical reactions?
Liquids often serve as solvents in chemical reactions, allowing reactants to mix and interact. The properties of the liquid solvent, such as polarity and pH, can significantly influence reaction rates and equilibria.
45. How do nanoscale effects influence liquid properties?
At the nanoscale, surface effects become more prominent, potentially altering properties like viscosity, surface tension, and boiling point. Nanofluidics studies these unique behaviors of liquids in extremely small spaces.
46. What is supercritical fluid and how does it relate to the liquid state?
A supercritical fluid exists above a substance's critical temperature and pressure. It has properties between those of liquids and gases, such as the density of a liquid but the compressibility of a gas.
47. How do liquids interact with porous materials?
Liquids can interact with porous materials through processes like adsorption, capillary action, and wetting. The extent of these interactions depends on factors like pore size, liquid surface tension, and the chemical nature of both the liquid and the material.
48. What is the concept of fugacity in relation to liquids?
Fugacity is a measure of the tendency of a substance to escape from a phase. For liquids, it's related to vapor pressure but accounts for non-ideal behavior, becoming particularly important when dealing with mixtures or high pressures.
49. How do quantum effects manifest in liquids?
While quantum effects are less prominent in liquids than in gases or solids, they can still play a role. Quantum tunneling can affect proton transfer in water, and quantum effects influence the behavior of liquid helium at very low temperatures.
50. What is the relationship between surface tension and Laplace pressure?
Laplace pressure is the pressure difference between the inside and outside of a curved surface, such as a liquid droplet. It's directly related to surface tension and the curvature of the surface, explaining phenomena like why smaller droplets have higher internal pressures.

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