Notes:Liquid State

States of Mattre II(Liquid State)
S.No	SLO’s
		liquid state Behaviors of Liquids:                 The liquids show the following behaviors or properties by which they are distinguished from other substances. 1. Diffusibility:                                 Liquids can diffuse into one another, they mix with each other to form a homogeneous mixture e.g. if a drop of ink is added in water it spreads out in all direction, till a homogenous colour mixture is formed. But the rates of diffusion are much lesser than those of gases, because the liquids molecules have inter molecular attraction and are not free to move like gases. 2. Compressibility:                                 Unlike gases, liquids are normally incompressible. However, at very high pressure the volume of a liquid is reduced very slightly. This behavior of liquids is due to the close packing of their molecules. The molecules of liquids are so close to each other that the repulsions of electron clouds resist all attempts at bringing tem further closer. 3. Expansion & Contraction:                                 Some of the liquids show expansion on heating or they show increase in their volumes. The temperature increases the K.E. of the liquid molecules increase due to this they move apart, causing increase in volume or the liquid show expansion.                 On, the other hand on cooling liquids show decrease in their volumes, i.e. the show contraction. It is due to cooling process, where thermal energy of molecules is removed. This causes decrease in Kinetic energy of the molecules and decreases in inter – spaces, and the liquid is contracted. Viscosity:                 It is common observation that some liquids flow more readily than the other. For example water moves over a glass plate more quickly than glycerine. Similarly, honey and mobil oil flow more slowly than water. Hence, liquids which flow easily are called “MOBILE” & such liquids which do not flow easily are known as “Viscous”. The resistance of a liquid to flow is expressed in terms of viscosity, which may be defined as, “The internal resistance to the flow of a liquid is called its viscosity” Viscosity is represented by ‘h’ and its unit is “POLSE”. Normally smaller units “CENTIPOISE” or “MILLIPOISE” are used. 1 POISE =             1 gm/cm & 1 POISE            =             100 CENTIPOISE                                 =             1000 MILLIPOISE Explanation:                 Imagine a liquid flowing through a tube and consists of concentric layers. The layers in contact with the walls of the tube remain almost stationary, whereas the layers in the centre have the highest velocity and the intermediate layers move with a gradation of velocities. Hence each layer exerts a drag on the next layer which causes resistance to the flow.   max velocity   min velocity     The liquid whose layers offer more resistance to its flow is more viscous than the liquids whose layer offer less resistance. Therefore glycerine and honey are more viscous than water, ether & alcohol. Factors affecting Viscosity:                 On the following factors affect the viscosity of a liquid. a) Molecular Size:                 Viscosity increases with increase in molecular size, because it is difficult for the layer molecules to slide over another and to go from one layer to the other. b) Molecular Shape:                 An irregular shape of molecules also causes the molecules to offer more resistance than the molecules of regular shape. Thus the                nonlinear molecules have greater viscosity than linear ones. c) Inter-Molecular Attraction:                 Greater the inter – molecular attraction in a liquid, greater will be force to resist the flow. Thus the viscosity will also be higher. d) Temperature:                 Viscosities of the liquids decrease with the increasing the temperature and vice-versa. This is due to the increase of average K.E. of the molecules at higher temperature. Surface Tension:                 “The inter-molecular force that drawn the molecules on the surface of a liquid together causing the surface to act like a thin elastic skin, this phenomenon is called SURFACE TENSION”.                         OR “The force per unit length or energy per unit area of the surface of a liquid is called SURFACE TENSION”. Surface tension of the liquid is represented by  g    OR   s, and its units are dynes / cm   OR      erg / cm2. Explanation:                 We consider a molecule ‘A’ at the surface and ‘B’ inside the liquid. The resultant force on ‘B’ is zero, because, it is attracted equally in all direction. On the other hand, molecule ‘A’ is attracted laterally by neighbouring molecules with equal forces. The molecule ‘A’ is also attracted downward at right angle by the molecules underneath it. As there is no liquid on its to balance the downward attractive forces, therefore, therefore the molecules ‘A’ is pulled inside the liquid. A similar pull is also experienced by other molecules on the surface of the liquid. However, the inward movement of these molecules is not possible, because of the lateral forces of neighbouring molecules. This creates a constant tension in the molecules of the surface of the liquid, called “SURFACE TENSION”.                 The surface of liquid thus appears like a stretched membrane. It is so strong that a needle or a shaving blade can float on it.                Factors Effecting Surface Tension:                 The surface tension of a liquid depends upon two factors: a) Inter-Molecular Attraction:                 Stronger the inter-molecular attractive forces, greater is the surface tension, and vise versa. For example, water possesses higher surface tension than most of the organic solvents. This is because of strong inter – molecular forces in water due to hydrogen bonding. b) Temperature:                 Surface tension of a liquid also depends on temperature, it decreases with the increase of temperature and vise – versa. Vapours Pressure: “The Pressure exerted by the vapours of a liquid, in equilibrium state with the pure liquid itself at a given temperature is called VAPOURS PRESSURE” of a liquid”. Explanation: Consider a volatile liquid in a closed container. Due to evaporation, the vapours are accumulated in the space above the surface of the liquid. During their motion, vapours lose a part of K.E. and are condensed again. After sometime, the space above the surface of the liquid is saturated with vapours. At this stage the rate of condensation becomes equal to the rate of evaporation. This is called the “Equilibrium State”.                                                      Liquid                     Vapours                 The vapours due to their continuous state of random motion exert pressure on the surface of the liquid. This pressure of vapours at the equilibrium is called “Vapours Pressure”. Boiling Point:                                 The vapours pressure of a liquid increases with the increase in its temperature. A certain temperature is reached when the vapours pressure of the liquid becomes equal to the atmospheric pressure. At this temperature, the gas bubbles can be seen coming out of the liquid. It is called the Boiling of the Liquid & the temperature is called Boiling Point, so it can be defined as “Boiling Point is the temperature at which the vapours pressure of a liquid becomes equal to the atmospheric pressure”.

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