Liquid State

The liquid state is the intermediary state between the solid and the gaseous state. The state of matter is determined by the spaces between the molecules. It is one of the three principal states of matter. Under normal conditions, liquids have less space between the molecules and are denser than gases. The liquid state has a definite structure since the molecules do not separate and are held together by intermolecular forces. However, the molecules can move past each other. Therefore, liquids can flow. That is why liquids take the shape of the container into which they are poured. This liquid state study material provides all the answers you require for your preparation. 

Physical Properties of Liquid State 

The study material notes on liquid state shall provide a comprehensive account of the physical properties of the liquid state of matter. 

• Vapour Pressure 

The pressure imposed by the vapours on the container’s walls is referred to as vapour pressure. If an empty container is half-filled with liquid, a part of the liquid evaporates. It fills the remainder of the container with vapour. This vapour exerts pressure on the walls of the container. After some time, the pressure becomes constant. The pressure is known as the equilibrium vapour pressure of saturated vapour pressure. 

• Boiling Point 

The condition of free vapourisation throughout the liquid state is called boiling. When a liquid is heated in an open vessel, the molecules are vapourised from the surface. At a point when the vapour pressure becomes equal to the external pressure, vapourisation can occur throughout the liquid and the vapours can expand freely in the surroundings. The temperature at which the vapour pressure becomes equal to the external pressure is the boiling temperature. 

The normal boiling point is the pressure at 1 atm. At 1 bar pressure, the boiling temperature is called the standard boiling point. The standard boiling point is slightly less than the normal boiling point because 1 bar pressure is less than 1 atm pressure. For example, the normal boiling point of water is 100 ℃ (373 K), and the standard boiling point of water is 99.6 ℃ (372.6 K). 

At high altitudes, the atmospheric pressure is low; hence liquids boil at lower temperatures at high altitudes than at sea level. Since water boils at low temperatures in the hills, a pressure cooker cooks food to adjust the pressure. 

Vapourisation cannot occur in a closed vessel. On heating, a clear boundary is visible between the liquid and the gas. The liquid is more dense than gas. As more and more molecules move to the vapour phase, it becomes denser. When the density of the liquid and the vapours become the same, the boundary disappears. This temperature is the critical temperature of the liquid. 

• Surface Tension 

Surface tension is one of the characteristic features of a liquid state of matter. Have you ever wondered why Mercury forms a spherical bead when it is dropped on a surface? Why do soil particles stay separated under the river bed but stick together when taken out? All of the above phenomena occurs because of surface tension. 

The bulk of the liquid, that is, the molecules present inside the liquid, receive equal intermolecular forces from all sides. That is the reason why the molecules experience no net force. For the molecule on the surface of the liquid, the net force is towards the interior of the liquid. It is due to the molecules below it. 

In simpler words, the molecules on the surface experience a net downward force while the bulk of the liquid does not experience any net force at all. Liquids tend to minimise their surface area. Therefore, there is less number of molecules on the surface. Hence, the liquid assumes a shape that minimises the surface area.  

If the liquid’s surface area is increased, the attractive forces must be overcome, which will need the expenditure of energy. The energy required to increase the area by one unit is the surface energy. Surface tension is defined as the force operating per unit length perpendicular to a line drawn on the liquid state’s surface. The Greek letter γ (gamma) denotes surface tension.

• Viscosity 

Viscosity is defined as the resistance to the flow of a liquid caused by internal friction between fluid layers as they slip past one another when the liquid moves. Strong intermolecular forces hold the molecules together. 

When a liquid flows over a fixed surface, the layer of molecules in direct touch with the surface remains stable. The upper-velocity layers grow as the distance between layers grows. The fixed layer grows thicker. This form of inflow has a consistent velocity gradation. It is referred to as going from one layer to the next and called a laminar flow. 

Conclusion 

A liquid state does not have a highly ordered molecular structure as in a solid, and it also does not have large empty spaces between the molecules as in a gas. Understanding of the liquid state, as well as all other states of matter, came with the kinetic molecular theory. It said that matter was built of particles in continual motion and that this motion was the expression of thermal energy. The larger the particle’s thermal energy, the quicker it travels. Hence, the liquid state study material has briefed all the major points regarding the liquid state, its physical properties and the transition between each state.