The liquid state occupies an intermediate position between the gaseous and solid states, liquid having a definite volume but no definite shape.
Like a gas, a liquid can take the shape of any vessel in which it is put, but in contrast to a gas, a definite quantity of liquid is required for filling the vessel. A liquid can not be compressed so much as a gas because its molecules are already close together, large pressure producing small changes in volume.
Increasing the temperature increases the kinetic energy of all molecules.
The change of a liquid into a gaseous or liquid states being dependent upon the kinetic energy of the molecules, which in turn is dependent upon the temperature, there are definite temperature characteristics for most liquids at which these changes occur. They are known as transition temperatures.
If we place one liquid layer carefully on top of a layer of a more dense liquid in which it is soluble, and set the vessel where it won't be disturbed, we shall see that two liquids begin gradually mixing. It is also to be taken into consideration that all liquids do not flow with the same ease, water, alcohol, gasoline flowing easily, while heavy oil glycerine flowing very slowly.
When a liquid flows, layers of molecules begin rubbing over each other, friction being generated by this rubbing of layers of particles. The greater the friction, the slower is the flow. A liquid which resists flowing, or resists the reaction of any other deforming force upon it results in a homogeneous solution. We give this example for illustration that the molecules of a liquid diffuse, though much slowly than do those of a gas.
The molecules of a liquid are much closer together than they are in a gas, because of the greater relative strength of attraction, the density of liquids being much greater. Naturally as the volume of a liquid begins varying with temperature its density will also start varying with temperature.
It should be noted that the closeness of the molecules also is known as viscous, the opposite of viscosity being fluidity. Viscosity diminishes and fluidity increases with temperature.
The molecules within the interior of a liquid have a definite average energy of motion, and thus a definite average velocity at each temperature. Some of them, however, at any given instant, have a velocity sufficiently greater than the average velocity to enable them to break through the surface layer of molecules and escape. Thereafter they are free to wander about in the space above and constitute a vapour namely a gas that can be condensed to a liquid merely by increasing the pressure upon it.
The escape of the molecules from a liquid into its vapour is called evaporation. After a sufficient number of molecules have collected in the space above the liquid, their haphazard wanderings bring them back to the surface as fast as other molecules escape. Thereafter, there is a balance between vaporation and recondensation and thus a constant number of molecules within the closed space at any given moment, and these, by bombardment of the walls of the vessel set up a constant pressure, called the vapour pressure.
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